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Chiang WT, Chang YK, Hui WH, Chang SW, Liao CY, Chang YC, Chen CJ, Wang WC, Lai CC, Wang CH, Luo SY, Huang YP, Chou SH, Horng TL, Hou MH, Muench SP, Chen RS, Tsai MD, Hu NJ. Structural basis and synergism of ATP and Na + activation in bacterial K + uptake system KtrAB. Nat Commun 2024; 15:3850. [PMID: 38719864 PMCID: PMC11078986 DOI: 10.1038/s41467-024-48057-y] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2023] [Accepted: 04/17/2024] [Indexed: 05/12/2024] Open
Abstract
The K+ uptake system KtrAB is essential for bacterial survival in low K+ environments. The activity of KtrAB is regulated by nucleotides and Na+. Previous studies proposed a putative gating mechanism of KtrB regulated by KtrA upon binding to ATP or ADP. However, how Na+ activates KtrAB and the Na+ binding site remain unknown. Here we present the cryo-EM structures of ATP- and ADP-bound KtrAB from Bacillus subtilis (BsKtrAB) both solved at 2.8 Å. A cryo-EM density at the intra-dimer interface of ATP-KtrA was identified as Na+, as supported by X-ray crystallography and ICP-MS. Thermostability assays and functional studies demonstrated that Na+ binding stabilizes the ATP-bound BsKtrAB complex and enhances its K+ flux activity. Comparing ATP- and ADP-BsKtrAB structures suggests that BsKtrB Arg417 and Phe91 serve as a channel gate. The synergism of ATP and Na+ in activating BsKtrAB is likely applicable to Na+-activated K+ channels in central nervous system.
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Affiliation(s)
- Wesley Tien Chiang
- Graduate Institute of Biochemistry, National Chung Hsing University, Taichung, 402202, Taiwan
| | - Yao-Kai Chang
- Institute of Biological Chemistry, Academia Sinica, Taipei, 115201, Taiwan
| | - Wei-Han Hui
- Department of Civil Engineering, National Taiwan University, Taipei, 106319, Taiwan
| | - Shu-Wei Chang
- Department of Civil Engineering, National Taiwan University, Taipei, 106319, Taiwan
- Department of Biomedical Engineering, National Taiwan University, Taipei, 10663, Taiwan
| | - Chen-Yi Liao
- Graduate Institute of Biochemistry, National Chung Hsing University, Taichung, 402202, Taiwan
| | - Yi-Chuan Chang
- Graduate Institute of Biochemistry, National Chung Hsing University, Taichung, 402202, Taiwan
| | - Chun-Jung Chen
- Life Science Group, Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu, 30092, Taiwan
| | - Wei-Chen Wang
- Institute of Molecular Biology, National Chung Hsing University, Taichung, 402202, Taiwan
| | - Chien-Chen Lai
- Institute of Molecular Biology, National Chung Hsing University, Taichung, 402202, Taiwan
- Graduate Institute of Chinese Medical Science, China Medical University, Taichung, 406040, Taiwan
| | - Chun-Hsiung Wang
- Institute of Biological Chemistry, Academia Sinica, Taipei, 115201, Taiwan
| | - Siou-Ying Luo
- Institute of Biological Chemistry, Academia Sinica, Taipei, 115201, Taiwan
| | - Ya-Ping Huang
- Institute of Biological Chemistry, Academia Sinica, Taipei, 115201, Taiwan
| | - Shan-Ho Chou
- Graduate Institute of Biochemistry, National Chung Hsing University, Taichung, 402202, Taiwan
| | - Tzyy-Leng Horng
- Department of Applied Mathematics, Feng Chia University, Taichung, 407102, Taiwan
| | - Ming-Hon Hou
- Institute of Genomics and Bioinformatics, National Chung Hsing University, Taichung, 402202, Taiwan
| | - Stephen P Muench
- School of Biomedical Sciences, Faculty of Biological Sciences and the Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK
| | - Ren-Shiang Chen
- Department of Life Science, Tunghai University, Taichung, 407224, Taiwan
| | - Ming-Daw Tsai
- Institute of Biological Chemistry, Academia Sinica, Taipei, 115201, Taiwan.
- Institute of Biochemical Sciences, National Taiwan University, Taipei, 106319, Taiwan.
| | - Nien-Jen Hu
- Graduate Institute of Biochemistry, National Chung Hsing University, Taichung, 402202, Taiwan.
- Ph.D Program in Translational Medicine, National Chung Hsing University, Taichung, 402202, Taiwan.
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2
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Wang J, Luo Y, Ye F, Ding ZJ, Zheng SJ, Qiao S, Wang Y, Guo J, Yang W, Su N. Structures and ion transport mechanisms of plant high-affinity potassium transporters. MOLECULAR PLANT 2024; 17:409-422. [PMID: 38335958 DOI: 10.1016/j.molp.2024.01.007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/01/2023] [Revised: 01/11/2024] [Accepted: 01/19/2024] [Indexed: 02/12/2024]
Abstract
Plant high-affinity K+ transporters (HKTs) mediate Na+ and K+ uptake, maintain Na+/K+ homeostasis, and therefore play crucial roles in plant salt tolerance. In this study, we present cryoelectron microscopy structures of HKTs from two classes, class I HKT1;1 from Arabidopsis thaliana (AtHKT1;1) and class II HKT2;1 from Triticum aestivum (TaHKT2;1), in both Na+- and K+-bound states at 2.6- to 3.0-Å resolutions. Both AtHKT1;1 and TaHKT2;1 function as homodimers. Each HKT subunit consists of four tandem domain units (D1-D4) with a repeated K+-channel-like M-P-M topology. In each subunit, D1-D4 assemble into an ion conduction pore with a pseudo-four-fold symmetry. Although both TaHKT2;1 and AtHKT1;1 have only one putative Na+ ion bound in the selectivity filter with a similar coordination pattern, the two HKTs display different K+ binding modes in the filter. TaHKT2;1 has three K+ ions bound in the selectivity filter, but AtHKT1;1 has only two K+ ions bound in the filter, which has a narrowed external entrance due to the presence of a Ser residue in the first filter motif. These structures, along with computational, mutational, and electrophysiological analyses, enable us to pinpoint key residues that are critical for the ion selectivity of HKTs. The findings provide new insights into the ion selectivity and ion transport mechanisms of plant HKTs and improve our understanding about how HKTs mediate plant salt tolerance and enhance crop growth.
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Affiliation(s)
- Jiangqin Wang
- International Institutes of Medicine, the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Yiwu, Zhejiang 322000, China
| | - Yanping Luo
- Department of Biophysics and Department of Neurology of the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, Zhejiang 310058, China
| | - Fan Ye
- Department of Biophysics and Department of Neurology of the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, Zhejiang 310058, China
| | - Zhong Jie Ding
- State Key Laboratory of Plant Environmental Resilience, College of Life Sciences, Zhejiang University, Hangzhou, Zhejiang 310058, China
| | - Shao Jian Zheng
- State Key Laboratory of Plant Environmental Resilience, College of Life Sciences, Zhejiang University, Hangzhou, Zhejiang 310058, China
| | - Shuai Qiao
- International Institutes of Medicine, the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Yiwu, Zhejiang 322000, China
| | - Yong Wang
- College of Life Sciences, Zhejiang University, Hangzhou, Zhejiang 310058, China
| | - Jiangtao Guo
- Department of Biophysics and Department of Neurology of the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, Zhejiang 310058, China; State Key Laboratory of Plant Environmental Resilience, College of Life Sciences, Zhejiang University, Hangzhou, Zhejiang 310058, China; NHC and CAMS Key Laboratory of Medical Neurobiology, MOE Frontier Science Center for Brain Science and Brain-machine Integration, School of Brain Science and Brain Medicine, Zhejiang University, Hangzhou, Zhejiang 310058, China; Nanhu Brain-computer Interface Institute, Hangzhou, Zhejiang 311100, China.
| | - Wei Yang
- Department of Biophysics and Department of Neurology of the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, Zhejiang 310058, China; NHC and CAMS Key Laboratory of Medical Neurobiology, MOE Frontier Science Center for Brain Science and Brain-machine Integration, School of Brain Science and Brain Medicine, Zhejiang University, Hangzhou, Zhejiang 310058, China.
| | - Nannan Su
- International Institutes of Medicine, the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Yiwu, Zhejiang 322000, China.
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Yang M, Chen S, Huang Z, Gao S, Yu T, Du T, Zhang H, Li X, Liu CM, Chen S, Li H. Deep learning-enabled discovery and characterization of HKT genes in Spartina alterniflora. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2023; 116:690-705. [PMID: 37494542 DOI: 10.1111/tpj.16397] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/10/2023] [Revised: 07/03/2023] [Accepted: 07/11/2023] [Indexed: 07/28/2023]
Abstract
Spartina alterniflora is a halophyte that can survive in high-salinity environments, and it is phylogenetically close to important cereal crops, such as maize and rice. It is of scientific interest to understand why S. alterniflora can live under such extremely stressful conditions. The molecular mechanism underlying its high-saline tolerance is still largely unknown. Here we investigated the possibility that high-affinity K+ transporters (HKTs), which function in salt tolerance and maintenance of ion homeostasis in plants, are responsible for salt tolerance in S. alterniflora. To overcome the imprecision and unstable of the gene screening method caused by the conventional sequence alignment, we used a deep learning method, DeepGOPlus, to automatically extract sequence and protein characteristics from our newly assemble S. alterniflora genome to identify SaHKTs. Results showed that a total of 16 HKT genes were identified. The number of S. alterniflora HKTs (SaHKTs) is larger than that in all other investigated plant species except wheat. Phylogenetically related SaHKT members had similar gene structures, conserved protein domains and cis-elements. Expression profiling showed that most SaHKT genes are expressed in specific tissues and are differentially expressed under salt stress. Yeast complementation expression analysis showed that type I members SaHKT1;2, SaHKT1;3 and SaHKT1;8 and type II members SaHKT2;1, SaHKT2;3 and SaHKT2;4 had low-affinity K+ uptake ability and that type II members showed stronger K+ affinity than rice and Arabidopsis HKTs, as well as most SaHKTs showed preference for Na+ transport. We believe the deep learning-based methods are powerful approaches to uncovering new functional genes, and the SaHKT genes identified are important resources for breeding new varieties of salt-tolerant crops.
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Affiliation(s)
- Maogeng Yang
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
- Key Laboratory of Plant Molecular & Developmental Biology, College of Life Sciences, Yantai University, Yantai, Shandong, China
| | - Shoukun Chen
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
- Hainan Yazhou Bay Seed Laboratory, Sanya, Hainan, China
| | - Zhangping Huang
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
| | - Shang Gao
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
| | - Tingxi Yu
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
| | - Tingting Du
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
| | - Hao Zhang
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
| | - Xiang Li
- State Key Laboratory of Plant Genomics and National Center for Plant Gene Research, Institute of Genetics and Developmental Biology, Innovation Academy for Seed Design, Chinese Academy of Sciences, Beijing, China
| | - Chun-Ming Liu
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Key Laboratory of Plant Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing, China
- College of Life Sciences, University of Chinese Academy of Sciences, Beijing, China
- School of Advanced Agricultural Sciences, Peking University, Beijing, China
| | - Shihua Chen
- Key Laboratory of Plant Molecular & Developmental Biology, College of Life Sciences, Yantai University, Yantai, Shandong, China
| | - Huihui Li
- State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences (CAAS), Beijing, China
- Nanfan Research Institute, CAAS, Sanya, Hainan, China
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4
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Tanudjaja E, Hoshi N, Yamamoto K, Ihara K, Furuta T, Tsujii M, Ishimaru Y, Uozumi N. Two Trk/Ktr/HKT-type potassium transporters, TrkG and TrkH, perform distinct functions in Escherichia coli K-12. J Biol Chem 2022; 299:102846. [PMID: 36586436 PMCID: PMC9898762 DOI: 10.1016/j.jbc.2022.102846] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2022] [Revised: 12/19/2022] [Accepted: 12/23/2022] [Indexed: 12/29/2022] Open
Abstract
Escherichia coli K-12 possesses two versions of Trk/Ktr/HKT-type potassium ion (K+) transporters, TrkG and TrkH. The current paradigm is that TrkG and TrkH have largely identical characteristics, and little information is available regarding their functional differences. Here, we show using cation uptake experiments with K+ transporter knockout mutants that TrkG and TrkH have distinct ion transport activities and physiological roles. K+-transport by TrkG required Na+, whereas TrkH-mediated K+ uptake was not affected by Na+. An aspartic acid located five residues away from a critical glycine in the third pore-forming region might be involved in regulation of Na+-dependent activation of TrkG. In addition, we found that TrkG but not TrkH had Na+ uptake activity. Our analysis of K+ transport mutants revealed that TrkH supported cell growth more than TrkG; however, TrkG was able to complement loss of TrkH-mediated K+ uptake in E. coli. Furthermore, we determined that transcription of trkG in E. coli was downregulated but not completely silenced by the xenogeneic silencing factor H-NS (histone-like nucleoid structuring protein or heat-stable nucleoid-structuring protein). Taken together, the transport function of TrkG is clearly distinct from that of TrkH, and TrkG seems to have been accepted by E. coli during evolution as a K+ uptake system that coexists with TrkH.
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Affiliation(s)
- Ellen Tanudjaja
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai, Japan
| | - Naomi Hoshi
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai, Japan
| | | | - Kunio Ihara
- Center for Gene Research, Nagoya University, Nagoya, Japan
| | - Tadaomi Furuta
- School of Life Science and Technology, Tokyo Institute of Technology, Yokohama, Japan
| | - Masaru Tsujii
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai, Japan
| | - Yasuhiro Ishimaru
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai, Japan
| | - Nobuyuki Uozumi
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai, Japan.
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5
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Unappreciated Roles for K+ Channels in Bacterial Physiology. Trends Microbiol 2021; 29:942-950. [DOI: 10.1016/j.tim.2020.11.005] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2020] [Revised: 11/07/2020] [Accepted: 11/09/2020] [Indexed: 01/03/2023]
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6
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Deciphering ion transport and ATPase coupling in the intersubunit tunnel of KdpFABC. Nat Commun 2021; 12:5098. [PMID: 34429416 PMCID: PMC8385062 DOI: 10.1038/s41467-021-25242-x] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2021] [Accepted: 07/29/2021] [Indexed: 02/07/2023] Open
Abstract
KdpFABC, a high-affinity K+ pump, combines the ion channel KdpA and the P-type ATPase KdpB to secure survival at K+ limitation. Here, we apply a combination of cryo-EM, biochemical assays, and MD simulations to illuminate the mechanisms underlying transport and the coupling to ATP hydrolysis. We show that ions are transported via an intersubunit tunnel through KdpA and KdpB. At the subunit interface, the tunnel is constricted by a phenylalanine, which, by polarized cation-π stacking, controls K+ entry into the canonical substrate binding site (CBS) of KdpB. Within the CBS, ATPase coupling is mediated by the charge distribution between an aspartate and a lysine. Interestingly, individual elements of the ion translocation mechanism of KdpFABC identified here are conserved among a wide variety of P-type ATPases from different families. This leads us to the hypothesis that KdpB might represent an early descendant of a common ancestor of cation pumps.
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7
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Stautz J, Hellmich Y, Fuss MF, Silberberg JM, Devlin JR, Stockbridge RB, Hänelt I. Molecular Mechanisms for Bacterial Potassium Homeostasis. J Mol Biol 2021; 433:166968. [PMID: 33798529 DOI: 10.1016/j.jmb.2021.166968] [Citation(s) in RCA: 54] [Impact Index Per Article: 18.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2021] [Revised: 03/11/2021] [Accepted: 03/22/2021] [Indexed: 10/21/2022]
Abstract
Potassium ion homeostasis is essential for bacterial survival, playing roles in osmoregulation, pH homeostasis, regulation of protein synthesis, enzyme activation, membrane potential adjustment and electrical signaling. To accomplish such diverse physiological tasks, it is not surprising that a single bacterium typically encodes several potassium uptake and release systems. To understand the role each individual protein fulfills and how these proteins work in concert, it is important to identify the molecular details of their function. One needs to understand whether the systems transport ions actively or passively, and what mechanisms or ligands lead to the activation or inactivation of individual systems. Combining mechanistic information with knowledge about the physiology under different stress situations, such as osmostress, pH stress or nutrient limitation, one can identify the task of each system and deduce how they are coordinated with each other. By reviewing the general principles of bacterial membrane physiology and describing the molecular architecture and function of several bacterial K+-transporting systems, we aim to provide a framework for microbiologists studying bacterial potassium homeostasis and the many K+-translocating systems that are still poorly understood.
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Affiliation(s)
- Janina Stautz
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt am Main, Germany
| | - Yvonne Hellmich
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt am Main, Germany
| | - Michael F Fuss
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt am Main, Germany
| | - Jakob M Silberberg
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt am Main, Germany
| | - Jason R Devlin
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, United States
| | - Randy B Stockbridge
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, United States.
| | - Inga Hänelt
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt am Main, Germany.
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8
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Plant HKT Channels: An Updated View on Structure, Function and Gene Regulation. Int J Mol Sci 2021; 22:ijms22041892. [PMID: 33672907 PMCID: PMC7918770 DOI: 10.3390/ijms22041892] [Citation(s) in RCA: 30] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2021] [Revised: 01/29/2021] [Accepted: 02/09/2021] [Indexed: 12/28/2022] Open
Abstract
HKT channels are a plant protein family involved in sodium (Na+) and potassium (K+) uptake and Na+-K+ homeostasis. Some HKTs underlie salt tolerance responses in plants, while others provide a mechanism to cope with short-term K+ shortage by allowing increased Na+ uptake under K+ starvation conditions. HKT channels present a functionally versatile family divided into two classes, mainly based on a sequence polymorphism found in the sequences underlying the selectivity filter of the first pore loop. Physiologically, most class I members function as sodium uniporters, and class II members as Na+/K+ symporters. Nevertheless, even within these two classes, there is a high functional diversity that, to date, cannot be explained at the molecular level. The high complexity is also reflected at the regulatory level. HKT expression is modulated at the level of transcription, translation, and functionality of the protein. Here, we summarize and discuss the structure and conservation of the HKT channel family from algae to angiosperms. We also outline the latest findings on gene expression and the regulation of HKT channels.
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9
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Two Ways To Convert a Low-Affinity Potassium Channel to High Affinity: Control of Bacillus subtilis KtrCD by Glutamate. J Bacteriol 2020; 202:JB.00138-20. [PMID: 32253343 DOI: 10.1128/jb.00138-20] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2020] [Accepted: 04/01/2020] [Indexed: 02/06/2023] Open
Abstract
Potassium and glutamate are the major cation and anion, respectively, in every living cell. Due to the high concentrations of both ions, the cytoplasm of all cells can be regarded as a potassium glutamate solution. This implies that the concentrations of both ions need to be balanced. While the control of potassium uptake by glutamate is well established for eukaryotic cells, much less is known about the mechanisms that link potassium homeostasis to glutamate availability in bacteria. Here, we have discovered that the availability of glutamate strongly decreases the minimal external potassium concentration required for the highly abundant Bacillus subtilis potassium channel KtrCD to accumulate potassium. In contrast, the inducible KtrAB and KimA potassium uptake systems have high apparent affinities for potassium even in the absence of glutamate. Experiments with mutant strains revealed that the KtrD subunit responds to the presence of glutamate. For full activity, KtrD synergistically requires the presence of the regulatory subunit KtrC and of glutamate. The analysis of suppressor mutants of a strain that has KtrCD as the only potassium uptake system and that experiences severe potassium starvation identified a mutation in the ion selectivity filter of KtrD (Gly282 to Val) that similarly results in a strongly glutamate-independent increase of the apparent affinity for potassium. Thus, this work has identified two conditions that increase the apparent affinity of KtrCD for potassium, i.e., external glutamate and the acquisition of a single point mutation in KtrD.IMPORTANCE In each living cell, potassium is required for maintaining the intracellular pH and for the activity of essential enzymes. Like most other bacteria, Bacillus subtilis possesses multiple low- and high-affinity potassium uptake systems. Their activity is regulated by the second messenger cyclic di-AMP. Moreover, the pools of the most abundant ions potassium and glutamate must be balanced. We report two conditions under which the low-affinity potassium channel KtrCD is able to mediate potassium uptake at low external potassium concentrations: physiologically, the presence of glutamate results in a severely increased potassium uptake. Moreover, this is achieved by a mutation affecting the selectivity filter of the KtrD channel. These results highlight the integration between potassium and glutamate homeostasis in bacteria.
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10
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Raddatz N, Morales de los Ríos L, Lindahl M, Quintero FJ, Pardo JM. Coordinated Transport of Nitrate, Potassium, and Sodium. FRONTIERS IN PLANT SCIENCE 2020; 11:247. [PMID: 32211003 PMCID: PMC7067972 DOI: 10.3389/fpls.2020.00247] [Citation(s) in RCA: 55] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/23/2019] [Accepted: 02/18/2020] [Indexed: 05/19/2023]
Abstract
Potassium (K+) and nitrogen (N) are essential nutrients, and their absorption and distribution within the plant must be coordinated for optimal growth and development. Potassium is involved in charge balance of inorganic and organic anions and macromolecules, control of membrane electrical potential, pH homeostasis and the regulation of cell osmotic pressure, whereas nitrogen is an essential component of amino acids, proteins, and nucleic acids. Nitrate (NO3 -) is often the primary nitrogen source, but it also serves as a signaling molecule to the plant. Nitrate regulates root architecture, stimulates shoot growth, delays flowering, regulates abscisic acid-independent stomata opening, and relieves seed dormancy. Plants can sense K+/NO3 - levels in soils and adjust accordingly the uptake and root-to-shoot transport to balance the distribution of these ions between organs. On the other hand, in small amounts sodium (Na+) is categorized as a "beneficial element" for plants, mainly as a "cheap" osmolyte. However, at high concentrations in the soil, Na+ can inhibit various physiological processes impairing plant growth. Hence, plants have developed specific mechanisms to transport, sense, and respond to a variety of Na+ conditions. Sodium is taken up by many K+ transporters, and a large proportion of Na+ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K+-NO3 -; Na+-NO3 -, and K+-Na+ transports, and the regulators controlling their uptake and allocation.
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Affiliation(s)
| | | | | | | | - José M. Pardo
- Institute of Plant Biochemistry and Photosynthesis, Consejo Superior de Investigaciones Científicas and Universidad de Sevilla, Seville, Spain
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11
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Schrecker M, Wunnicke D, Hänelt I. How RCK domains regulate gating of K+ channels. Biol Chem 2020; 400:1303-1322. [PMID: 31361596 DOI: 10.1515/hsz-2019-0153] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2019] [Accepted: 07/02/2019] [Indexed: 11/15/2022]
Abstract
Potassium channels play a crucial role in the physiology of all living organisms. They maintain the membrane potential and are involved in electrical signaling, pH homeostasis, cell-cell communication and survival under osmotic stress. Many prokaryotic potassium channels and members of the eukaryotic Slo channels are regulated by tethered cytoplasmic domains or associated soluble proteins, which belong to the family of regulator of potassium conductance (RCK). RCK domains and subunits form octameric rings, which control ion gating. For years, a common regulatory mechanism was suggested: ligand-induced conformational changes in the octameric ring would pull open a gate in the pore via flexible linkers. Consistently, ligand-dependent conformational changes were described for various RCK gating rings. Yet, recent structural and functional data of complete ion channels uncovered that the following signal transduction to the pore domains is divers. The different RCK-regulated ion channels show remarkably heterogeneous mechanisms with neither the connection from the RCK domain to the pore nor the gate being conserved. Some channels even lack the flexible linkers, while in others the gate cannot easily be assigned. In this review we compare available structures of RCK-gated potassium channels, highlight the similarities and differences of channel gating, and delineate existing inconsistencies.
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Affiliation(s)
- Marina Schrecker
- Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Str. 9, D-60438 Frankfurt Main, Germany
| | - Dorith Wunnicke
- Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Str. 9, D-60438 Frankfurt Main, Germany
| | - Inga Hänelt
- Institute of Biochemistry, Biocenter, Goethe University Frankfurt, Max-von-Laue Str. 9, D-60438 Frankfurt Main, Germany
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12
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Mikušević V, Schrecker M, Kolesova N, Patiño-Ruiz M, Fendler K, Hänelt I. A channel profile report of the unusual K + channel KtrB. J Gen Physiol 2019; 151:1357-1368. [PMID: 31624134 PMCID: PMC6888753 DOI: 10.1085/jgp.201912384] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2019] [Revised: 09/03/2019] [Accepted: 09/27/2019] [Indexed: 11/20/2022] Open
Abstract
KtrAB is a key player in bacterial K+ uptake required for K+ homeostasis and osmoadaptation. The system is unique in structure and function. It consists of the K+-translocating channel subunit KtrB, which forms a dimer in the membrane, and the soluble regulatory subunit KtrA, which attaches to the cytoplasmic side of the dimer as an octameric ring conferring Na+ and ATP dependency to the system. Unlike most K+ channels, KtrB lacks the highly conserved T(X)GYG selectivity filter sequence. Instead, only a single glycine residue is found in each pore loop, which raises the question of how selective the ion channel is. Here, we characterized the KtrB subunit from the Gram-negative pathogen Vibrio alginolyticus by isothermal titration calorimetry, solid-supported membrane-based electrophysiology, whole-cell K+ uptake, and ACMA-based transport assays. We found that, despite its simple selectivity filter, KtrB selectively binds K+ with micromolar affinity. Rb+ and Cs+ bind with millimolar affinities. However, only K+ and the poorly binding Na+ are efficiently translocated, based on size exclusion by the gating loop. Importantly, the physiologically required K+ over Na+ selectivity is provided by the channel's high affinity for potassium, which interestingly results from the presence of the sodium ions themselves. In the presence of the KtrA subunit, sodium ions further decrease the Michaelis-Menten constant for K+ uptake from milli- to micromolar concentrations and increase the Vmax, suggesting that Na+ also facilitates channel gating. In conclusion, high binding affinity and facilitated K+ gating allow KtrAB to function as a selective K+ channel.
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Affiliation(s)
- Vedrana Mikušević
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt, Germany
| | - Marina Schrecker
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt, Germany
| | - Natalie Kolesova
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt, Germany
| | - Miyer Patiño-Ruiz
- Department of Biophysical Chemistry, Max Planck Institute for Biophysics, Frankfurt, Germany
| | - Klaus Fendler
- Department of Biophysical Chemistry, Max Planck Institute for Biophysics, Frankfurt, Germany
| | - Inga Hänelt
- Institute of Biochemistry, Goethe University Frankfurt, Frankfurt, Germany
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13
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Riedelsberger J, Vergara-Jaque A, Piñeros M, Dreyer I, González W. An extracellular cation coordination site influences ion conduction of OsHKT2;2. BMC PLANT BIOLOGY 2019; 19:316. [PMID: 31307394 PMCID: PMC6632200 DOI: 10.1186/s12870-019-1909-5] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/14/2019] [Accepted: 06/27/2019] [Indexed: 05/17/2023]
Abstract
BACKGROUND HKT channels mediate sodium uniport or sodium and potassium symport in plants. Monocotyledons express a higher number of HKT proteins than dicotyledons, and it is only within this clade of HKT channels that cation symport mechanisms are found. The prevailing ion composition in the extracellular medium affects the transport abilities of various HKT channels by changing their selectivity or ion transport rates. How this mutual effect is achieved at the molecular level is still unknown. Here, we built a homology model of the monocotyledonous OsHKT2;2, which shows sodium and potassium symport activity. We performed molecular dynamics simulations in the presence of sodium and potassium ions to investigate the mutual effect of cation species. RESULTS By analyzing ion-protein interactions, we identified a cation coordination site on the extracellular protein surface, which is formed by residues P71, D75, D501 and K504. Proline and the two aspartate residues coordinate cations, while K504 forms salt bridges with D75 and D501 and may be involved in the forwarding of cations towards the pore entrance. Functional validation via electrophysiological experiments confirmed the biological relevance of the predicted ion coordination site and identified K504 as a central key residue. Mutation of the cation coordinating residues affected the functionality of HKT only slightly. Additional in silico mutants and simulations of K504 supported experimental results. CONCLUSION We identified an extracellular cation coordination site, which is involved in ion coordination and influences the conduction of OsHKT2;2. This finding proposes a new viewpoint in the discussion of how the mutual effect of variable ion species may be achieved in HKT channels.
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Affiliation(s)
- Janin Riedelsberger
- Centro de Bioinformática y Simulación Molecular, Facultad de Ingeniería, Universidad de Talca, Talca, Chile
| | - Ariela Vergara-Jaque
- Centro de Bioinformática y Simulación Molecular, Facultad de Ingeniería, Universidad de Talca, Talca, Chile
- Millennium Nucleus of Ion Channels-Associated Diseases (MiNICAD), Santiago, Chile
| | - Miguel Piñeros
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY USA
| | - Ingo Dreyer
- Centro de Bioinformática y Simulación Molecular, Facultad de Ingeniería, Universidad de Talca, Talca, Chile
| | - Wendy González
- Centro de Bioinformática y Simulación Molecular, Facultad de Ingeniería, Universidad de Talca, Talca, Chile
- Millennium Nucleus of Ion Channels-Associated Diseases (MiNICAD), Santiago, Chile
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14
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Breuer M, Earnest TM, Merryman C, Wise KS, Sun L, Lynott MR, Hutchison CA, Smith HO, Lapek JD, Gonzalez DJ, de Crécy-Lagard V, Haas D, Hanson AD, Labhsetwar P, Glass JI, Luthey-Schulten Z. Essential metabolism for a minimal cell. eLife 2019; 8:36842. [PMID: 30657448 PMCID: PMC6609329 DOI: 10.7554/elife.36842] [Citation(s) in RCA: 78] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2018] [Accepted: 01/17/2019] [Indexed: 11/29/2022] Open
Abstract
JCVI-syn3A, a robust minimal cell with a 543 kbp genome and 493 genes, provides a versatile platform to study the basics of life. Using the vast amount of experimental information available on its precursor, Mycoplasma mycoides capri, we assembled a near-complete metabolic network with 98% of enzymatic reactions supported by annotation or experiment. The model agrees well with genome-scale in vivo transposon mutagenesis experiments, showing a Matthews correlation coefficient of 0.59. The genes in the reconstruction have a high in vivo essentiality or quasi-essentiality of 92% (68% essential), compared to 79% in silico essentiality. This coherent model of the minimal metabolism in JCVI-syn3A at the same time also points toward specific open questions regarding the minimal genome of JCVI-syn3A, which still contains many genes of generic or completely unclear function. In particular, the model, its comparison to in vivo essentiality and proteomics data yield specific hypotheses on gene functions and metabolic capabilities; and provide suggestions for several further gene removals. In this way, the model and its accompanying data guide future investigations of the minimal cell. Finally, the identification of 30 essential genes with unclear function will motivate the search for new biological mechanisms beyond metabolism. One way that researchers can test whether they understand a biological system is to see if they can accurately recreate it as a computer model. The more they learn about living things, the more the researchers can improve their models and the closer the models become to simulating the original. In this approach, it is best to start by trying to model a simple system. Biologists have previously succeeded in creating ‘minimal bacterial cells’. These synthetic cells contain fewer genes than almost all other living things and they are believed to be among the simplest possible forms of life that can grow on their own. The minimal cells can produce all the chemicals that they need to survive – in other words, they have a metabolism. Accurately recreating one of these cells in a computer is a key first step towards simulating a complete living system. Breuer et al. have developed a computer model to simulate the network of the biochemical reactions going on inside a minimal cell with just 493 genes. By altering the parameters of their model and comparing the results to experimental data, Breuer et al. explored the accuracy of their model. Overall, the model reproduces experimental results, but it is not yet perfect. The differences between the model and the experiments suggest new questions and tests that could advance our understanding of biology. In particular, Breuer et al. identified 30 genes that are essential for life in these cells but that currently have no known purpose. Continuing to develop and expand models like these to reproduce more complex living systems provides a tool to test current knowledge of biology. These models may become so advanced that they could predict how living things will respond to changing situations. This would allow scientists to test ideas sooner and make much faster progress in understanding life on Earth. Ultimately, these models could one day help to accelerate medical and industrial processes to save lives and enhance productivity.
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Affiliation(s)
- Marian Breuer
- Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, United States
| | - Tyler M Earnest
- Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, United States
| | | | - Kim S Wise
- J Craig Venter Institute, La Jolla, United States
| | - Lijie Sun
- J Craig Venter Institute, La Jolla, United States
| | | | | | | | - John D Lapek
- Department of Pharmacology and School of Pharmacy, University of California at San Diego, La Jolla, United States
| | - David J Gonzalez
- Department of Pharmacology and School of Pharmacy, University of California at San Diego, La Jolla, United States
| | - Valérie de Crécy-Lagard
- Department of Microbiology and Cell Science, University of Florida, Gainesville, United States
| | - Drago Haas
- Department of Microbiology and Cell Science, University of Florida, Gainesville, United States
| | - Andrew D Hanson
- Horticultural Sciences Department, University of Florida, Gainesville, United States
| | - Piyush Labhsetwar
- Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, United States
| | - John I Glass
- J Craig Venter Institute, La Jolla, United States
| | - Zaida Luthey-Schulten
- Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, United States
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15
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Cryo-EM structures of KdpFABC suggest a K + transport mechanism via two inter-subunit half-channels. Nat Commun 2018; 9:4971. [PMID: 30478378 PMCID: PMC6255902 DOI: 10.1038/s41467-018-07319-2] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2018] [Accepted: 10/24/2018] [Indexed: 12/16/2022] Open
Abstract
P-type ATPases ubiquitously pump cations across biological membranes to maintain vital ion gradients. Among those, the chimeric K+ uptake system KdpFABC is unique. While ATP hydrolysis is accomplished by the P-type ATPase subunit KdpB, K+ has been assumed to be transported by the channel-like subunit KdpA. A first crystal structure uncovered its overall topology, suggesting such a spatial separation of energizing and transporting units. Here, we report two cryo-EM structures of the 157 kDa, asymmetric KdpFABC complex at 3.7 Å and 4.0 Å resolution in an E1 and an E2 state, respectively. Unexpectedly, the structures suggest a translocation pathway through two half-channels along KdpA and KdpB, uniting the alternating-access mechanism of actively pumping P-type ATPases with the high affinity and selectivity of K+ channels. This way, KdpFABC would function as a true chimeric complex, synergizing the best features of otherwise separately evolved transport mechanisms.
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16
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Nishijima T, Furuhashi M, Sakaoka S, Morikami A, Tsukagoshi H. Ectopic expression of Mesembryanthemum crystallinum sodium transporter McHKT2 provides salt stress tolerance in Arabidopsis thaliana. Biosci Biotechnol Biochem 2017; 81:2139-2144. [PMID: 29017432 DOI: 10.1080/09168451.2017.1383847] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Most plants do not tolerate highly saline environments; the development of salt stress tolerance is crucial for improving crop yield. An efficient way of finding genes involved in salt tolerance is to study and use data from halophytes. In this study, we used the Mesembryanthemum crystallinum (ice plant) expression data-set and selected for further study the gene McHKT2, which encodes for the Arabidopsis sodium transporter ortholog AtHKT1. In comparison with the HKT1 amino acid sequences from other plants, McHKT2 has several unique features. It seems to be localized to the plasma membrane, and its overexpression confers strong salt tolerance in Arabidopsis thaliana. Our results indicate that McHKT2 is a suitable candidate protein that can induce salt tolerance in non-halophytes. Like McHKT2, using transcriptome data-sets from halophytes such as ice plant give us an efficiency way to obtain new gene resources that might involve in plant salt tolerance.
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Affiliation(s)
- Taiga Nishijima
- a Faculty of Agriculture , Meijo University , Tempaku-ku, Nagoya , Japan
| | - Megumi Furuhashi
- a Faculty of Agriculture , Meijo University , Tempaku-ku, Nagoya , Japan
| | - Satomi Sakaoka
- a Faculty of Agriculture , Meijo University , Tempaku-ku, Nagoya , Japan
| | - Atsushi Morikami
- a Faculty of Agriculture , Meijo University , Tempaku-ku, Nagoya , Japan
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17
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Diskowski M, Mehdipour AR, Wunnicke D, Mills DJ, Mikusevic V, Bärland N, Hoffmann J, Morgner N, Steinhoff HJ, Hummer G, Vonck J, Hänelt I. Helical jackknives control the gates of the double-pore K + uptake system KtrAB. eLife 2017; 6. [PMID: 28504641 PMCID: PMC5449183 DOI: 10.7554/elife.24303] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2016] [Accepted: 05/14/2017] [Indexed: 12/27/2022] Open
Abstract
Ion channel gating is essential for cellular homeostasis and is tightly controlled. In some eukaryotic and most bacterial ligand-gated K+ channels, RCK domains regulate ion fluxes. Until now, a single regulatory mechanism has been proposed for all RCK-regulated channels, involving signal transduction from the RCK domain to the gating area. Here, we present an inactive ADP-bound structure of KtrAB from Vibrio alginolyticus, determined by cryo-electron microscopy, which, combined with EPR spectroscopy and molecular dynamics simulations, uncovers a novel regulatory mechanism for ligand-induced action at a distance. Exchange of activating ATP to inactivating ADP triggers short helical segments in the K+-translocating KtrB dimer to organize into two long helices that penetrate deeply into the regulatory RCK domains, thus connecting nucleotide-binding sites and ion gates. As KtrAB and its homolog TrkAH have been implicated as bacterial pathogenicity factors, the discovery of this functionally relevant inactive conformation may advance structure-guided drug development. DOI:http://dx.doi.org/10.7554/eLife.24303.001
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Affiliation(s)
- Marina Diskowski
- Institute of Biochemistry, Goethe-University, Frankfurt, Germany
| | - Ahmad Reza Mehdipour
- Department of Theoretical Biophysics, Max Planck Institute of Biophysics, Frankfurt, Germany
| | - Dorith Wunnicke
- Institute of Biochemistry, Goethe-University, Frankfurt, Germany
| | - Deryck J Mills
- Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt, Germany
| | | | - Natalie Bärland
- Institute of Biochemistry, Goethe-University, Frankfurt, Germany.,Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt, Germany
| | - Jan Hoffmann
- Institute for Physical and Theoretical Chemistry, Goethe-University, Frankfurt, Germany
| | - Nina Morgner
- Institute for Physical and Theoretical Chemistry, Goethe-University, Frankfurt, Germany
| | | | - Gerhard Hummer
- Department of Theoretical Biophysics, Max Planck Institute of Biophysics, Frankfurt, Germany.,Institute of Biophysics, Goethe-University, Frankfurt, Germany
| | - Janet Vonck
- Department of Structural Biology, Max Planck Institute of Biophysics, Frankfurt, Germany
| | - Inga Hänelt
- Institute of Biochemistry, Goethe-University, Frankfurt, Germany
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18
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Ariyarathna HACK, Francki MG. Phylogenetic relationships and protein modelling revealed two distinct subfamilies of group II HKT genes between crop and model grasses. Genome 2016; 59:509-17. [PMID: 27203707 DOI: 10.1139/gen-2016-0035] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Molecular evolution of large protein families in closely related species can provide useful insights on structural functional relationships. Phylogenetic analysis of the grass-specific group II HKT genes identified two distinct subfamilies, I and II. Subfamily II was represented in all species, whereas subfamily I was identified only in the small grain cereals and possibly originated from an ancestral gene duplication post divergence from the coarse grain cereal lineage. The core protein structures were highly analogous despite there being no more than 58% amino acid identity between members of the two subfamilies. Distinctly variable regions in known functional domains, however, indicated functional divergence of the two subfamilies. The subsets of codons residing external to known functional domains predicted signatures of positive Darwinian selection potentially identifying new domains of functional divergence and providing new insights on the structural function and relationships between protein members of the two subfamilies.
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Affiliation(s)
- H A Chandima K Ariyarathna
- a School of Plant Biology and Institute of Agriculture, The University of Western Australia, Crawley WA 6009, Australia.,b State Agricultural Biotechnology Centre, Murdoch University, Murdoch WA 6150, Australia
| | - Michael G Francki
- b State Agricultural Biotechnology Centre, Murdoch University, Murdoch WA 6150, Australia.,c Department of Agriculture and Food Western Australia, 3 Baron Hay Ct, South Perth WA 6151, Australia
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19
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Böhm J, Scherzer S, Shabala S, Krol E, Neher E, Mueller TD, Hedrich R. Venus Flytrap HKT1-Type Channel Provides for Prey Sodium Uptake into Carnivorous Plant Without Conflicting with Electrical Excitability. MOLECULAR PLANT 2016; 9:428-436. [PMID: 26455461 PMCID: PMC4791408 DOI: 10.1016/j.molp.2015.09.017] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/18/2015] [Revised: 09/24/2015] [Accepted: 09/24/2015] [Indexed: 05/05/2023]
Abstract
The animal diet of the carnivorous Venus flytrap, Dionaea muscipula, contains a sodium load that enters the capture organ via an HKT1-type sodium channel, expressed in special epithelia cells on the inner trap lobe surface. DmHKT1 expression and sodium uptake activity is induced upon prey contact. Here, we analyzed the HKT1 properties required for prey sodium osmolyte management of carnivorous Dionaea. Analyses were based on homology modeling, generation of model-derived point mutants, and their functional testing in Xenopus oocytes. We showed that the wild-type HKT1 and its Na(+)- and K(+)-permeable mutants function as ion channels rather than K(+) transporters driven by proton or sodium gradients. These structural and biophysical features of a high-capacity, Na(+)-selective ion channel enable Dionaea glands to manage prey-derived sodium loads without confounding the action potential-based information management of the flytrap.
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Affiliation(s)
- J Böhm
- Julius-von-Sachs Institute, Department for Molecular Plant Physiology and Biophysics, University of Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany
| | - S Scherzer
- Julius-von-Sachs Institute, Department for Molecular Plant Physiology and Biophysics, University of Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany
| | - S Shabala
- School of Land and Food, University of Tasmania, Hobart TAS 7001, Australia
| | - E Krol
- Julius-von-Sachs Institute, Department for Molecular Plant Physiology and Biophysics, University of Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany
| | - E Neher
- Zoology Department, College of Science, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia; Department for Membrane Biophysics, Max Planck Institute for Biophysical Chemistry, 37077 Goettingen, Germany
| | - T D Mueller
- Julius-von-Sachs Institute, Department for Molecular Plant Physiology and Biophysics, University of Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany.
| | - R Hedrich
- Julius-von-Sachs Institute, Department for Molecular Plant Physiology and Biophysics, University of Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany.
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20
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Moscoso JA, Schramke H, Zhang Y, Tosi T, Dehbi A, Jung K, Gründling A. Binding of Cyclic Di-AMP to the Staphylococcus aureus Sensor Kinase KdpD Occurs via the Universal Stress Protein Domain and Downregulates the Expression of the Kdp Potassium Transporter. J Bacteriol 2016; 198:98-110. [PMID: 26195599 PMCID: PMC4686210 DOI: 10.1128/jb.00480-15] [Citation(s) in RCA: 83] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2015] [Accepted: 07/14/2015] [Indexed: 02/06/2023] Open
Abstract
UNLABELLED Nucleotide signaling molecules are important intracellular messengers that regulate a wide range of biological functions. The human pathogen Staphylococcus aureus produces the signaling nucleotide cyclic di-AMP (c-di-AMP). This molecule is common among Gram-positive bacteria and in many organisms is essential for survival under standard laboratory growth conditions. In this study, we investigated the interaction of c-di-AMP with the S. aureus KdpD protein. The sensor kinase KdpD forms a two-component signaling system with the response regulator KdpE and regulates the expression of the kdpDE genes and the kdpFABC operon coding for the Kdp potassium transporter components. Here we show that the S. aureus KdpD protein binds c-di-AMP specifically and with an affinity in the micromolar range through its universal stress protein (USP) domain. This domain is located within the N-terminal cytoplasmic region of KdpD, and amino acids of a conserved SXS-X20-FTAXY motif are important for this binding. We further show that KdpD2, a second KdpD protein found in some S. aureus strains, also binds c-di-AMP, and our bioinformatics analysis indicates that a subclass of KdpD proteins in c-di-AMP-producing bacteria has evolved to bind this signaling nucleotide. Finally, we show that c-di-AMP binding to KdpD inhibits the upregulation of the kdpFABC operon under salt stress, thus indicating that c-di-AMP is a negative regulator of potassium uptake in S. aureus. IMPORTANCE Staphylococcus aureus is an important human pathogen and a major cause of food poisoning in Western countries. A common method for food preservation is the use of salt to drive dehydration. This study sheds light on the regulation of potassium uptake in Staphylococcus aureus, an important aspect of this bacterium's ability to tolerate high levels of salt. We show that the signaling nucleotide c-di-AMP binds to a regulatory component of the Kdp potassium uptake system and that this binding has an inhibitory effect on the expression of the kdp genes encoding a potassium transporter. c-di-AMP binds to the USP domain of KdpD, thus providing for the first time evidence for the ability of such a domain to bind a cyclic dinucleotide.
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Affiliation(s)
- Joana A Moscoso
- Section of Microbiology and MRC Centre for Molecular Bacteriology and Infection (CMBI), Imperial College London, London, United Kingdom
| | - Hannah Schramke
- Center for Integrated Protein Science (CiPSM), Department of Biology I, Microbiology, Ludwig-Maximilians-Universität München, Martinsried, Germany
| | - Yong Zhang
- Section of Microbiology and MRC Centre for Molecular Bacteriology and Infection (CMBI), Imperial College London, London, United Kingdom
| | - Tommaso Tosi
- Section of Microbiology and MRC Centre for Molecular Bacteriology and Infection (CMBI), Imperial College London, London, United Kingdom
| | - Amina Dehbi
- Section of Microbiology and MRC Centre for Molecular Bacteriology and Infection (CMBI), Imperial College London, London, United Kingdom
| | - Kirsten Jung
- Center for Integrated Protein Science (CiPSM), Department of Biology I, Microbiology, Ludwig-Maximilians-Universität München, Martinsried, Germany
| | - Angelika Gründling
- Section of Microbiology and MRC Centre for Molecular Bacteriology and Infection (CMBI), Imperial College London, London, United Kingdom
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21
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Suzuki K, Costa A, Nakayama H, Katsuhara M, Shinmyo A, Horie T. OsHKT2;2/1-mediated Na(+) influx over K(+) uptake in roots potentially increases toxic Na(+) accumulation in a salt-tolerant landrace of rice Nona Bokra upon salinity stress. JOURNAL OF PLANT RESEARCH 2016; 129:67-77. [PMID: 26578190 DOI: 10.1007/s10265-015-0764-1] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/17/2015] [Accepted: 07/18/2015] [Indexed: 05/10/2023]
Abstract
HKT transporters are Na(+)-permeable membrane proteins, which mediate Na(+) and K(+) homeostasis in K(+)-depleted and saline environments in plants. Class II HKT transporters, a distinct subgroup found predominantly in monocots, are known to mediate Na(+)-K(+) co-transport in principle. Here we report features of ion transport functions of No-OsHKT2;2/1, a class II transporter identified in a salt tolerant landrace of indica rice, Nona Bokra. We profiled No-OsHKT2;2/1 expression in organs of Nona Bokra plants with or without salinity stress. Dominant accumulation of the No-OsHKT2;2/1 transcript in K(+)-starved roots of Nona Bokra plants largely disappeared in response to 50 mM NaCl. We found that No-OsHKT2;2/1 expressed in the high-affinity K(+) uptake deficient mutant of Saccharomyces cerevisiae and Xenopus laevis oocytes shows robust K(+) selectivity even in the presence of a large amount of NaCl as reported previously. However, No-OsHKT2;2/1-expressing yeast cells exhibited Na(+) hypersensitive growth under various concentrations of K(+) and Na(+) as the cells expressing Po-OsHKT2;2, a similar class II transporter from another salt tolerant indica rice Pokkali, when compared with the growth of cells harboring empty vector or cells expressing OsHKT2;4. The OsHKT2;4 protein expressed in Xenopus oocytes showed strong K(+) selectivity in the presence of 50 mM NaCl in comparison with No-OsHKT2;2/1 and Po-OsHKT2;2. Together with apparent plasma membrane-localization of No-OsHKT2;2/1, these results point to possibilities that No-OsHKT2;2/1 could mediate destructive Na(+) influx over K(+) uptake in Nona Bokra plants upon salinity stress, and that a predominant physiological function of No-OsHKT2;2/1 might be the acquisition of Na(+) and K(+) in K(+)-limited environments.
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Affiliation(s)
- Kei Suzuki
- Division of Applied Biology, Faculty of Textile Science and Technology, Shinshu University, 3-15-1, Tokida, Ueda, Nagano, 386-8567, Japan
| | - Alex Costa
- Department of Biosciences, University of Milan, Via G. Celoria 26, 20133, Milan, Italy
- Institute of Biophysics, Consiglio Nazionale delle Ricerche, Via G. Celoria 26, 20133, Milan, Italy
| | - Hideki Nakayama
- Institute of Environmental Studies, Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki, 852-8521, Japan
| | - Maki Katsuhara
- Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, 710-0046, Japan
| | - Atsuhiko Shinmyo
- Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara, 630-0192, Japan
| | - Tomoaki Horie
- Division of Applied Biology, Faculty of Textile Science and Technology, Shinshu University, 3-15-1, Tokida, Ueda, Nagano, 386-8567, Japan.
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22
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Shahzad K, Rauf M, Ahmed M, Malik ZA, Habib I, Ahmed Z, Mahmood K, Ali R, Masmoudi K, Lemtiri-Chlieh F, Gehring C, Berkowitz GA, Saeed NA. Functional characterisation of an intron retaining K(+) transporter of barley reveals intron-mediated alternate splicing. PLANT BIOLOGY (STUTTGART, GERMANY) 2015; 17:840-51. [PMID: 25631371 DOI: 10.1111/plb.12290] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/21/2014] [Accepted: 11/19/2014] [Indexed: 06/04/2023]
Abstract
Intron retention in transcripts and the presence of 5' and 3' splice sites within these introns mediate alternate splicing, which is widely observed in animals and plants. Here, functional characterisation of the K(+) transporter, HvHKT2;1, with stably retained introns from barley (Hordeum vulgare) in yeast (Saccharomyces cerevisiae), and transcript profiling in yeast and transgenic tobacco (Nicotiana tabacum) is presented. Expression of intron-retaining HvHKT2;1 cDNA (HvHKT2;1-i) in trk1, trk2 yeast strain defective in K(+) uptake restored growth in medium containing hygromycin in the presence of different concentrations of K(+) and mediated hypersensitivity to Na(+) . HvHKT2;1-i produces multiple transcripts via alternate splicing of two regular introns and three exons in different compositions. HKT isoforms with retained introns and exon skipping variants were detected in relative expression analysis of (i) HvHKT2;1-i in barley under native conditions, (ii) in transgenic tobacco plants constitutively expressing HvHKT2;1-i, and (iii) in trk1, trk2 yeast expressing HvHKT2;1-i under control of an inducible promoter. Mixed proportions of three HKT transcripts: HvHKT2;1-e (first exon region), HvHKT2;1-i1 (first intron) and HvHKT2;1-i2 (second intron) were observed. The variation in transcript accumulation in response to changing K(+) and Na(+) concentrations was observed in both heterologous and plant systems. These findings suggest a link between intron-retaining transcripts and different splice variants to ion homeostasis, and their possible role in salt stress.
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Affiliation(s)
- K Shahzad
- National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
- Pakistan Institute of Engineering and Applied Sciences, Nilore, Islamabad, Pakistan
| | - M Rauf
- National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
- Pakistan Institute of Engineering and Applied Sciences, Nilore, Islamabad, Pakistan
| | - M Ahmed
- National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
| | - Z A Malik
- National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
| | - I Habib
- National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
| | - Z Ahmed
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON, Canada
| | - K Mahmood
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON, Canada
| | - R Ali
- Agricultural Biotechnology Laboratory, Department of Plant Science, University of Connecticut, Storrs, CT, USA
| | - K Masmoudi
- International Centre for Biosaline Agriculture (ICBA), Dubai, UAE
| | - F Lemtiri-Chlieh
- Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
| | - C Gehring
- Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
| | - G A Berkowitz
- Agricultural Biotechnology Laboratory, Department of Plant Science, University of Connecticut, Storrs, CT, USA
| | - N A Saeed
- National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
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Zayats V, Stockner T, Pandey SK, Wörz K, Ettrich R, Ludwig J. A refined atomic scale model of the Saccharomyces cerevisiae K+-translocation protein Trk1p combined with experimental evidence confirms the role of selectivity filter glycines and other key residues. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2015; 1848:1183-95. [DOI: 10.1016/j.bbamem.2015.02.007] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/29/2014] [Revised: 02/04/2015] [Accepted: 02/08/2015] [Indexed: 11/25/2022]
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Hamamoto S, Horie T, Hauser F, Deinlein U, Schroeder JI, Uozumi N. HKT transporters mediate salt stress resistance in plants: from structure and function to the field. Curr Opin Biotechnol 2015; 32:113-120. [DOI: 10.1016/j.copbio.2014.11.025] [Citation(s) in RCA: 110] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2014] [Accepted: 11/28/2014] [Indexed: 10/24/2022]
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Comparative analysis of kdp and ktr mutants reveals distinct roles of the potassium transporters in the model cyanobacterium Synechocystis sp. strain PCC 6803. J Bacteriol 2014; 197:676-87. [PMID: 25313394 DOI: 10.1128/jb.02276-14] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Photoautotrophic bacteria have developed mechanisms to maintain K(+) homeostasis under conditions of changing ionic concentrations in the environment. Synechocystis sp. strain PCC 6803 contains genes encoding a well-characterized Ktr-type K(+) uptake transporter (Ktr) and a putative ATP-dependent transporter specific for K(+) (Kdp). The contributions of each of these K(+) transport systems to cellular K(+) homeostasis have not yet been defined conclusively. To verify the functionality of Kdp, kdp genes were expressed in Escherichia coli, where Kdp conferred K(+) uptake, albeit with lower rates than were conferred by Ktr. An on-chip microfluidic device enabled monitoring of the biphasic initial volume recovery of single Synechocystis cells after hyperosmotic shock. Here, Ktr functioned as the primary K(+) uptake system during the first recovery phase, whereas Kdp did not contribute significantly. The expression of the kdp operon in Synechocystis was induced by extracellular K(+) depletion. Correspondingly, Kdp-mediated K(+) uptake supported Synechocystis cell growth with trace amounts of external potassium. This induction of kdp expression depended on two adjacent genes, hik20 and rre19, encoding a putative two-component system. The circadian expression of kdp and ktr peaked at subjective dawn, which may support the acquisition of K(+) required for the regular diurnal photosynthetic metabolism. These results indicate that Kdp contributes to the maintenance of a basal intracellular K(+) concentration under conditions of limited K(+) in natural environments, whereas Ktr mediates fast potassium movements in the presence of greater K(+) availability. Through their distinct activities, both Ktr and Kdp coordinate the responses of Synechocystis to changes in K(+) levels under fluctuating environmental conditions.
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Levin EJ, Zhou M. Recent progress on the structure and function of the TrkH/KtrB ion channel. Curr Opin Struct Biol 2014; 27:95-101. [PMID: 25011047 DOI: 10.1016/j.sbi.2014.06.004] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2014] [Accepted: 06/12/2014] [Indexed: 12/29/2022]
Abstract
Members of the Superfamily of K(+) Transporters (SKT) are integral membrane proteins that mediate the uptake of ions into non-animal cells. Although these proteins are homologous to the well-characterized K(+) channel family, relatively little was known about their transport and gating mechanisms until the recent determination of crystal structures for two SKT proteins, TrkH and KtrB. These structures reveal that the SKT proteins are channels, containing a flexible loop in the middle of the permeation pathway that may act as a gate. Two different conformational changes have been observed for the associated gating rings, suggesting different mechanisms of regulation by the binding of nucleotides.
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Affiliation(s)
- Elena J Levin
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA
| | - Ming Zhou
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA.
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27
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Benito B, Haro R, Amtmann A, Cuin TA, Dreyer I. The twins K+ and Na+ in plants. JOURNAL OF PLANT PHYSIOLOGY 2014; 171:723-31. [PMID: 24810769 DOI: 10.1016/j.jplph.2013.10.014] [Citation(s) in RCA: 127] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/26/2013] [Revised: 10/01/2013] [Accepted: 10/02/2013] [Indexed: 05/02/2023]
Abstract
In the earth's crust and in seawater, K(+) and Na(+) are by far the most available monovalent inorganic cations. Physico-chemically, K(+) and Na(+) are very similar, but K(+) is widely used by plants whereas Na(+) can easily reach toxic levels. Indeed, salinity is one of the major and growing threats to agricultural production. In this article, we outline the fundamental bases for the differences between Na(+) and K(+). We present the foundation of transporter selectivity and summarize findings on transporters of the HKT type, which are reported to transport Na(+) and/or Na(+) and K(+), and may play a central role in Na(+) utilization and detoxification in plants. Based on the structural differences in the hydration shells of K(+) and Na(+), and by comparison with sodium channels, we present an ad hoc mechanistic model that can account for ion permeation through HKTs.
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Affiliation(s)
- Begoña Benito
- Centro de Biotecnología y Genómica de Plantas, Universidad Politécnica de Madrid, Madrid, Spain
| | - Rosario Haro
- Centro de Biotecnología y Genómica de Plantas, Universidad Politécnica de Madrid, Madrid, Spain
| | - Anna Amtmann
- Institute of Molecular, Cellular and Systems Biology (MCSB), College of Medical, Veterinary and Life Sciences (MVLS), University of Glasgow, Glasgow, UK
| | - Tracey Ann Cuin
- Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, Montpellier, France
| | - Ingo Dreyer
- Centro de Biotecnología y Genómica de Plantas, Universidad Politécnica de Madrid, Madrid, Spain.
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28
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Sato Y, Nanatani K, Hamamoto S, Shimizu M, Takahashi M, Tabuchi-Kobayashi M, Mizutani A, Schroeder JI, Souma S, Uozumi N. Defining membrane spanning domains and crucial membrane-localized acidic amino acid residues for K+ transport of a Kup/HAK/KT-type Escherichia coli potassium transporter. ACTA ACUST UNITED AC 2014; 155:315-23. [DOI: 10.1093/jb/mvu007] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022]
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29
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Yamaguchi T, Hamamoto S, Uozumi N. Sodium transport system in plant cells. FRONTIERS IN PLANT SCIENCE 2013; 4:410. [PMID: 24146669 PMCID: PMC3797977 DOI: 10.3389/fpls.2013.00410] [Citation(s) in RCA: 94] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/20/2013] [Accepted: 09/27/2013] [Indexed: 05/18/2023]
Abstract
Since sodium, Na, is a non-essential element for the plant growth, the molecular mechanism of Na(+) transport system in plants has remained elusive for the last two decades. The accumulation of Na(+) in soil through irrigation for sustainable agricultural crop production, particularly in arid land, and by changes in environmental and climate conditions leads to the buildup of toxic level of salts in the soil. Since the latter half of the twentieth century, extensive molecular research has identified several classes of Na(+) transporters that play major roles in the alleviation of ionic stress by excluding toxic Na(+) from the cytosol or preventing Na(+) transport to the photosynthetic organs, and also in osmotic stress by modulating intra/extracellular osmotic balance. In this review, we summarize the current knowledge of three major Na(+) transporters, namely NHX, SOS1, and HKT transporters, including recently revealed characteristics of these transporters.
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Affiliation(s)
- Toshio Yamaguchi
- Department of Microbiology, Faculty of Pharmacy, Niigata University of Pharmacy and Applied Life SciencesNiigata, Japan
| | - Shin Hamamoto
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku UniversitySendai, Japan
| | - Nobuyuki Uozumi
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku UniversitySendai, Japan
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30
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Gries CM, Bose JL, Nuxoll AS, Fey PD, Bayles KW. The Ktr potassium transport system in Staphylococcus aureus and its role in cell physiology, antimicrobial resistance and pathogenesis. Mol Microbiol 2013; 89:760-73. [PMID: 23815639 DOI: 10.1111/mmi.12312] [Citation(s) in RCA: 52] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/27/2013] [Indexed: 02/05/2023]
Abstract
Potassium (K(+) ) plays a vital role in bacterial physiology, including regulation of cytoplasmic pH, turgor pressure and transmembrane electrical potential. Here, we examine the Staphylococcus aureus Ktr system uniquely comprised of two ion-conducting proteins (KtrB and KtrD) and only one regulator (KtrA). Growth of Ktr system mutants was severely inhibited under K(+) limitation, yet detectable after an extended lag phase, indicating the presence of a secondary K(+) transporter. Disruption of both ktrA and the Kdp-ATPase system, important for K(+) uptake in other organisms, eliminated regrowth in 0.1 mM K(+) , demonstrating a compensatory role for Kdp to the Ktr system. Consistent with K(+) transport mutations, S. aureus devoid of the Ktr system became sensitive to hyperosmotic conditions, exhibited a hyperpolarized plasma membrane, and increased susceptibility to aminoglycoside antibiotics and cationic antimicrobials. In contrast to other organisms, the S. aureus Ktr system was shown to be important for low-K(+) growth under alkaline conditions, but played only a minor role in neutral and acidic conditions. In a mouse competitive index model of bacteraemia, the ktrA mutant was significantly outcompeted by the parental strain. Combined, these results demonstrate a primary mechanism of K(+) uptake in S. aureus and a role for this system in pathogenesis.
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Affiliation(s)
- Casey M Gries
- Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, NE, 68198, USA
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31
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Francis BR. Evolution of the genetic code by incorporation of amino acids that improved or changed protein function. J Mol Evol 2013; 77:134-58. [PMID: 23743924 DOI: 10.1007/s00239-013-9567-y] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2013] [Accepted: 05/25/2013] [Indexed: 12/31/2022]
Abstract
Fifty years have passed since the genetic code was deciphered, but how the genetic code came into being has not been satisfactorily addressed. It is now widely accepted that the earliest genetic code did not encode all 20 amino acids found in the universal genetic code as some amino acids have complex biosynthetic pathways and likely were not available from the environment. Therefore, the genetic code evolved as pathways for synthesis of new amino acids became available. One hypothesis proposes that early in the evolution of the genetic code four amino acids-valine, alanine, aspartic acid, and glycine-were coded by GNC codons (N = any base) with the remaining codons being nonsense codons. The other sixteen amino acids were subsequently added to the genetic code by changing nonsense codons into sense codons for these amino acids. Improvement in protein function is presumed to be the driving force behind the evolution of the code, but how improved function was achieved by adding amino acids has not been examined. Based on an analysis of amino acid function in proteins, an evolutionary mechanism for expansion of the genetic code is described in which individual coded amino acids were replaced by new amino acids that used nonsense codons differing by one base change from the sense codons previously used. The improved or altered protein function afforded by the changes in amino acid function provided the selective advantage underlying the expansion of the genetic code. Analysis of amino acid properties and functions explains why amino acids are found in their respective positions in the genetic code.
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Affiliation(s)
- Brian R Francis
- Department of Molecular Biology, University of Wyoming, Laramie, WY, 82071-3944, USA,
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32
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The structure of the KtrAB potassium transporter. Nature 2013; 496:323-8. [PMID: 23598340 DOI: 10.1038/nature12055] [Citation(s) in RCA: 85] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2012] [Accepted: 03/05/2013] [Indexed: 12/23/2022]
Abstract
In bacteria, archaea, fungi and plants the Trk, Ktr and HKT ion transporters are key components of osmotic regulation, pH homeostasis and resistance to drought and high salinity. These ion transporters are functionally diverse: they can function as Na(+) or K(+) channels and possibly as cation/K(+) symporters. They are closely related to potassium channels both at the level of the membrane protein and at the level of the cytosolic regulatory domains. Here we describe the crystal structure of a Ktr K(+) transporter, the KtrAB complex from Bacillus subtilis. The structure shows the dimeric membrane protein KtrB assembled with a cytosolic octameric KtrA ring bound to ATP, an activating ligand. A comparison between the structure of KtrAB-ATP and the structures of the isolated full-length KtrA protein with ATP or ADP reveals a ligand-dependent conformational change in the octameric ring, raising new ideas about the mechanism of activation in these transporters.
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33
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Gating of the TrkH ion channel by its associated RCK protein TrkA. Nature 2013; 496:317-22. [PMID: 23598339 PMCID: PMC3726529 DOI: 10.1038/nature12056] [Citation(s) in RCA: 73] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2012] [Accepted: 03/05/2013] [Indexed: 11/09/2022]
Abstract
TrkH belongs to a superfamily of K(+) transport proteins required for growth of bacteria in low external K(+) concentrations. The crystal structure of TrkH from Vibrio parahaemolyticus showed that TrkH resembles a K(+) channel and may have a gating mechanism substantially different from K(+) channels. TrkH assembles with TrkA, a cytosolic protein comprising two RCK (regulate the conductance of K(+)) domains, which are found in certain K(+) channels and control their gating. However, fundamental questions on whether TrkH is an ion channel and how it is regulated by TrkA remain unresolved. Here we show single-channel activity of TrkH that is upregulated by ATP via TrkA. We report two structures of the tetrameric TrkA ring, one in complex with TrkH and one in isolation, in which the ring assumes two markedly different conformations. These results suggest a mechanism for how ATP increases TrkH activity by inducing conformational changes in TrkA.
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34
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Jimenez V, Docampo R. Molecular and electrophysiological characterization of a novel cation channel of Trypanosoma cruzi. PLoS Pathog 2012; 8:e1002750. [PMID: 22685407 PMCID: PMC3369953 DOI: 10.1371/journal.ppat.1002750] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2012] [Accepted: 04/27/2012] [Indexed: 01/27/2023] Open
Abstract
We report the identification, functional expression, purification, reconstitution and electrophysiological characterization of a novel cation channel (TcCat) from Trypanosoma cruzi, the etiologic agent of Chagas disease. This channel is potassium permeable and shows inward rectification in the presence of magnesium. Western blot analyses with specific antibodies indicated that the protein is expressed in the three main life cycle stages of the parasite. Surprisingly, the parasites have the unprecedented ability to rapidly change the localization of the channel when they are exposed to different environmental stresses. TcCat rapidly translocates to the tip of the flagellum when trypomastigotes are submitted to acidic pH, to the plasma membrane when epimastigotes are submitted to hyperosmotic stress, and to the cell surface when amastigotes are released to the extracellular medium. Pharmacological block of TcCat activity also resulted in alterations in the trypomastigotes ability to respond to hyperosmotic stress. We also demonstrate the feasibility of purifying and reconstituting a functional ion channel from T. cruzi after recombinant expression in bacteria. The peculiar characteristics of TcCat could be important for the development of specific inhibitors with therapeutic potential against trypanosomes. The use of high-resolution electrophysiological techniques to study ion channels has provided a large amount of information on functional aspects of these important membrane proteins. However, the study of ion channels in unicellular eukaryotes has been limited to detection of ion conductances in large cells, gene identification studies, and pharmacological treatments to investigate the potential presence of different ion channels. In this paper we report the first identification, functional expression, purification, reconstitution, and electrophysiological characterization with single-molecule resolution of a novel cation channel (TcCat) from Trypanosoma cruzi. This is a novel channel that shares little sequence and functional similarities to other potassium channels and its peculiar characteristics could be important for the development of specific inhibitors with therapeutic potential against trypanosomiasis. Surprisingly, the parasites have the unprecedented ability to rapidly change the localization of the channel when they are exposed to different environmental stresses. We demonstrated the feasibility of purifying and reconstituting a functional ion channel from T. cruzi after recombinant expression in bacteria. In addition, we obtained yeast mutants that will provide a useful genetic system for studies of the assembly and composition of the channel.
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Affiliation(s)
- Veronica Jimenez
- Center for Tropical and Emerging Global Diseases and Department of Cellular Biology, University of Georgia, Athens, Georgia, United States of America
- * E-mail: (VJ); (RD)
| | - Roberto Docampo
- Center for Tropical and Emerging Global Diseases and Department of Cellular Biology, University of Georgia, Athens, Georgia, United States of America
- * E-mail: (VJ); (RD)
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35
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Sciara G, Mancia F. Highlights from recently determined structures of membrane proteins: a focus on channels and transporters. Curr Opin Struct Biol 2012; 22:476-81. [PMID: 22472602 DOI: 10.1016/j.sbi.2012.03.005] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2012] [Revised: 03/09/2012] [Accepted: 03/14/2012] [Indexed: 12/23/2022]
Abstract
After decades of absent or lackluster growth, recent years have at long last witnessed an exponential growth in the number of novel membrane protein structures determined. Every single achievement has had a tremendous impact on the scientific community, providing an unprecedented wealth of information that typically only an atomic resolution structure can contribute to our molecular understanding of how a protein functions. Presented here is a review of some of the most exciting novel structures of channels and transporters determined by X-ray crystallography in the last two years, and a discussion of their analogies, differences and mechanistic implications.
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Affiliation(s)
- Giuliano Sciara
- Department of Physiology and Cellular Biophysics, Columbia University, New York, NY, USA
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36
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AtHKT1;1 mediates nernstian sodium channel transport properties in Arabidopsis root stelar cells. PLoS One 2011; 6:e24725. [PMID: 21931830 PMCID: PMC3170383 DOI: 10.1371/journal.pone.0024725] [Citation(s) in RCA: 55] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2011] [Accepted: 08/19/2011] [Indexed: 11/19/2022] Open
Abstract
The Arabidopsis AtHKT1;1 protein was identified as a sodium (Na+) transporter by heterologous expression in Xenopus laevis oocytes and Saccharomyces cerevisiae. However, direct comparative in vivo electrophysiological analyses of a plant HKT transporter in wild-type and hkt loss-of-function mutants has not yet been reported and it has been recently argued that heterologous expression systems may alter properties of plant transporters, including HKT transporters. In this report, we analyze several key functions of AtHKT1;1-mediated ion currents in their native root stelar cells, including Na+ and K+ conductances, AtHKT1;1-mediated outward currents, and shifts in reversal potentials in the presence of defined intracellular and extracellular salt concentrations. Enhancer trap Arabidopsis plants with GFP-labeled root stelar cells were used to investigate AtHKT1;1-dependent ion transport properties using patch clamp electrophysiology in wild-type and athkt1;1 mutant plants. AtHKT1;1-dependent currents were carried by sodium ions and these currents were not observed in athkt1;1 mutant stelar cells. However, K+ currents in wild-type and athkt1;1 root stelar cell protoplasts were indistinguishable correlating with the Na+ over K+ selectivity of AtHKT1;1-mediated transport. Moreover, AtHKT1;1-mediated currents did not show a strong voltage dependence in vivo. Unexpectedly, removal of extracellular Na+ caused a reduction in AtHKT1;1-mediated outward currents in Columbia root stelar cells and Xenopus oocytes, indicating a role for external Na+ in regulation of AtHKT1;1 activity. Shifting the NaCl gradient in root stelar cells showed a Nernstian shift in the reversal potential providing biophysical evidence for the model that AtHKT1;1 mediates passive Na+ channel transport properties.
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37
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Hänelt I, Tholema N, Kröning N, Vor der Brüggen M, Wunnicke D, Bakker EP. KtrB, a member of the superfamily of K+ transporters. Eur J Cell Biol 2011; 90:696-704. [DOI: 10.1016/j.ejcb.2011.04.010] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022] Open
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38
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Horie T, Brodsky DE, Costa A, Kaneko T, Lo Schiavo F, Katsuhara M, Schroeder JI. K+ transport by the OsHKT2;4 transporter from rice with atypical Na+ transport properties and competition in permeation of K+ over Mg2+ and Ca2+ ions. PLANT PHYSIOLOGY 2011; 156:1493-507. [PMID: 21610181 PMCID: PMC3135959 DOI: 10.1104/pp.110.168047] [Citation(s) in RCA: 90] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/25/2010] [Accepted: 05/20/2011] [Indexed: 05/18/2023]
Abstract
Members of class II of the HKT transporters, which have thus far only been isolated from grasses, were found to mediate Na(+)-K(+) cotransport and at high Na(+) concentrations preferred Na(+)-selective transport, depending on the ionic conditions. But the physiological functions of this K(+)-transporting class II of HKT transporters remain unknown in plants, with the exception of the unique class II Na(+) transporter OsHKT2;1. The genetically tractable rice (Oryza sativa; background Nipponbare) possesses two predicted K(+)-transporting class II HKT transporter genes, OsHKT2;3 and OsHKT2;4. In this study, we have characterized the ion selectivity of the class II rice HKT transporter OsHKT2;4 in yeast and Xenopus laevis oocytes. OsHKT2;4 rescued the growth defect of a K(+) uptake-deficient yeast mutant. Green fluorescent protein-OsHKT2;4 is targeted to the plasma membrane in transgenic plant cells. OsHKT2;4-expressing oocytes exhibited strong K(+) permeability. Interestingly, however, K(+) influx in OsHKT2;4-expressing oocytes did not require stimulation by extracellular Na(+), in contrast to other class II HKT transporters. Furthermore, OsHKT2;4-mediated currents exhibited permeabilities to both Mg(2+) and Ca(2+) in the absence of competing K(+) ions. Comparative analyses of Ca(2+) and Mg(2+) permeabilities in several HKT transporters, including Arabidopsis thaliana HKT1;1 (AtHKT1;1), Triticum aestivum HKT2;1 (TaHKT2;1), OsHKT2;1, OsHKT2;2, and OsHKT2;4, revealed that only OsHKT2;4 and to a lesser degree TaHKT2;1 mediate Mg(2+) transport. Interestingly, cation competition analyses demonstrate that the selectivity of both of these class II HKT transporters for K(+) is dominant over divalent cations, suggesting that Mg(2+) and Ca(2+) transport via OsHKT2;4 may be small and would depend on competing K(+) concentrations in plants.
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Abstract
The TrkH/TrkG/KtrB proteins mediate K+ uptake in bacteria and likely evolved from simple K+ channels by multiple gene duplications or fusions. Here we present the crystal structure of a TrkH from Vibrio parahaemolyticus. TrkH is a homodimer, and each protomer contains an ion permeation pathway. A selectivity filter, similar in architecture to those of K+ channels but significantly shorter, is lined by backbone and side chain oxygen atoms. Functional studies showed that the TrkH allows permeation of K+ and Rb+ but not smaller ions such as Na+ or Li+. Immediately intracellular to the selectivity filter are an intramembrane loop and an arginine residue, both highly conserved, which constrict the permeation pathway. Substituting the arginine with an alanine significantly increases the rate of K+ flux. These results reveal the molecular basis of K+ selectivity and suggest a novel gating mechanism by this large and important family of membrane transport proteins.
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Ochrombel I, Ott L, Krämer R, Burkovski A, Marin K. Impact of improved potassium accumulation on pH homeostasis, membrane potential adjustment and survival of Corynebacterium glutamicum. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2011; 1807:444-50. [PMID: 21295539 DOI: 10.1016/j.bbabio.2011.01.008] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/22/2010] [Revised: 01/26/2011] [Accepted: 01/28/2011] [Indexed: 11/16/2022]
Abstract
Metal ion uptake is crucial for all living cells and an essential part of cellular bioenergetic homeostasis. In this study the uptake and the impact of the most abundant internal cation, potassium, were investigated in Actinobacteria, a group of high G+C Gram-positives with a number of prominent biotechnologically and medically important members. Genome analyses revealed a variety of different potassium uptake systems in this monophyletic group ranging from potassium channels common in virtually all Actinobacteria to different active carriers that were present predominantly in pathogenic members able to cope with various stress conditions. By applying Corynebacterium glutamicum as model system we provide experimental evidence that under optimal conditions a potassium channel is sufficient in bacteria for the maintenance of internal pH and membrane potential ensuring survival of cells under stress conditions. Under potassium limitation, however, viability of C. glutamicum was increased under acidic stress or during desiccation when a functional KtrAB potassium transporter from the pathogen Corynebacterium jeikeium was heterologously expressed. We provide experimental evidence that the KtrAB mediated enhanced potassium accumulation improved maintenance of internal pH and membrane potential. The results indicate that the occurrence of active potassium transport systems correlates with an improved potassium-dependent bioenergetic homeostasis and survival of bacterial cells under stress conditions.
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Affiliation(s)
- Ines Ochrombel
- Institute of Biochemistry, University of Cologne, 50674 Cologne, Germany
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Corratgé-Faillie C, Jabnoune M, Zimmermann S, Véry AA, Fizames C, Sentenac H. Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family. Cell Mol Life Sci 2010; 67:2511-32. [PMID: 20333436 PMCID: PMC11115768 DOI: 10.1007/s00018-010-0317-7] [Citation(s) in RCA: 150] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2009] [Revised: 02/10/2010] [Accepted: 02/15/2010] [Indexed: 02/06/2023]
Abstract
Bacterial Trk and Ktr, fungal Trk and plant HKT form a family of membrane transporters permeable to K(+) and/or Na(+) and characterized by a common structure probably derived from an ancestral K(+) channel subunit. This transporter family, specific of non-animal cells, displays a large diversity in terms of ionic permeability, affinity and energetic coupling (H(+)-K(+) or Na(+)-K(+) symport, K(+) or Na(+) uniport), which might reflect a high need for adaptation in organisms living in fluctuating or dilute environments. Trk/Ktr/HKT transporters are involved in diverse functions, from K(+) or Na(+) uptake to membrane potential control, adaptation to osmotic or salt stress, or Na(+) recirculation from shoots to roots in plants. Structural analyses of bacterial Ktr point to multimeric structures physically interacting with regulatory subunits. Elucidation of Trk/Ktr/HKT protein structures along with characterization of mutated transporters could highlight functional and evolutionary relationships between ion channels and transporters displaying channel-like features.
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Affiliation(s)
- C. Corratgé-Faillie
- Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, 2 Place Viala, 34060 Montpellier Cedex 2, France
| | - M. Jabnoune
- Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, 2 Place Viala, 34060 Montpellier Cedex 2, France
- Present Address: Plant Biotechnology Laboratory, DBMV, University of Lausanne, 1015 Lausanne, Switzerland
| | - S. Zimmermann
- Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, 2 Place Viala, 34060 Montpellier Cedex 2, France
| | - A.-A. Véry
- Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, 2 Place Viala, 34060 Montpellier Cedex 2, France
| | - C. Fizames
- Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, 2 Place Viala, 34060 Montpellier Cedex 2, France
| | - H. Sentenac
- Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, 2 Place Viala, 34060 Montpellier Cedex 2, France
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The KtrA and KtrE subunits are required for Na+-dependent K+ uptake by KtrB across the plasma membrane in Synechocystis sp. strain PCC 6803. J Bacteriol 2010; 192:5063-70. [PMID: 20656904 DOI: 10.1128/jb.00569-10] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The Na+-dependent K+ uptake KtrABE system is essential for the adaptation of Synechocystis to salinity stress and high osmolality. While KtrB forms the K+-translocating pore, the role of the subunits KtrA and KtrE for Ktr function remains elusive. Here, we characterized the role of KtrA and KtrE in Ktr-mediated K+ uptake and in modulating Na+ dependency. Expression of KtrB alone in a K+ uptake-deficient Escherichia coli strain conferred low K+ uptake activity that was not stimulated by Na+. Coexpression of both KtrA and KtrE with KtrB increased the K+ transport activity in a Na+-dependent manner. KtrA and KtrE were found to be localized to the plasma membrane in Synechocystis. Site-directed mutagenesis was used to analyze the role of single charged residues in KtrB for Ktr function. Replacing negatively charged residues facing the extracellular space with residues of the opposite charge increased the apparent Km for K+ in all cases. However, none of the mutations eliminated the Na+ dependency of Ktr-mediated K+ transport. Mutations of residues on the cytoplasmic side had larger effects on K+ uptake activity than those of residues on the extracellular side. Further analysis revealed that replacement of R262, which is well conserved among Ktr/Trk/HKT transporters in the third extracellular loop, by Glu abolished transport activity. The atomic-scale homology model indicated that R262 might interact with E247 and D261. Based on these data, interaction of KtrA and KtrE with KtrB increased the K+ uptake rate and conferred Na+ dependency.
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Hänelt I, Wunnicke D, Müller-Trimbusch M, Vor der Brüggen M, Kraus I, Bakker EP, Steinhoff HJ. Membrane region M2C2 in subunit KtrB of the K+ uptake system KtrAB from Vibrio alginolyticus forms a flexible gate controlling K+ flux: an electron paramagnetic resonance study. J Biol Chem 2010; 285:28210-9. [PMID: 20573964 DOI: 10.1074/jbc.m110.139311] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Transmembrane stretch M(2C) from the bacterial K(+)-translocating protein KtrB is unusually long. In its middle part, termed M(2C2), it contains several small and polar amino acids. This region is flanked by the two alpha-helices M(2C1) and M(2C3) and may form a flexible gate at the cytoplasmic side of the membrane controlling K(+) translocation. In this study, we provide experimental evidence for this notion by using continuous wave and pulse EPR measurements of single and double spin-labeled cysteine variants of KtrB. Most of the spin-labeled residues in M(2C2) were shown to be immobile, pointing to a compact structure. However, the high polarity revealed for the microenvironment of residue positions 317, 318, and 327 indicated the existence of a water-accessible cavity. Upon the addition of K(+) ions, M(2C2) residue Thr-318R1 (R1 indicates the bound spin label) moved with respect to M(2B) residue Asp-222R1 and M(2C3) residue Val-331R1 but not with respect to M(2C1) residue Met-311R1. Based on distances determined between spin-labeled residues of double-labeled variants of KtrB in the presence and absence of K(+) ions, structural models of the open and closed conformations were developed.
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Affiliation(s)
- Inga Hänelt
- Department of Microbiology, University of Osnabrück, D-49076 Osnabrück, Germany.
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Hauser F, Horie T. A conserved primary salt tolerance mechanism mediated by HKT transporters: a mechanism for sodium exclusion and maintenance of high K(+)/Na(+) ratio in leaves during salinity stress. PLANT, CELL & ENVIRONMENT 2010; 33:552-65. [PMID: 19895406 DOI: 10.1111/j.1365-3040.2009.02056.x] [Citation(s) in RCA: 264] [Impact Index Per Article: 18.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
Increasing soil salinity is a serious threat to agricultural productions worldwide in the 21st century. Several essential Na(+) transporters such as AtNHX1 and AtSOS1 function in Na(+) tolerance under salinity stress in plants. Recently, evidence for a new primary salt tolerance mechanism has been reported, which is mediated by a class of HKT transporters both in dicots such as Arabidopsis and monocot crops such as rice and wheat. Here we present a review on vital physiological functions of HKT transporters including AtHKT1;1 and OsHKT1;5 in preventing shoot Na(+) over-accumulation by mediating Na(+) exclusion from xylem vessels in the presence of a large amount of Na(+) thereby protecting leaves from salinity stress. Findings of the HKT2 transporter sub-family are also updated in this review. Subjects regarding function and regulation of HKT transporters, which need to be elucidated in future research, are discussed.
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Affiliation(s)
- Felix Hauser
- Center for Molecular Genetics, University of California, San Diego, La Jolla, 92093-0116, USA
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Abstract
The molecular mechanisms of K(+) homeostasis are only poorly understood for protozoan parasites. Trypanosoma brucei subsp. parasites, the causative agents of human sleeping sickness and nagana, are strictly extracellular and need to actively concentrate K(+) from their hosts' body fluids. The T. brucei genome contains two putative K(+) channel genes, yet the trypanosomes are insensitive to K(+) antagonists and K(+) channel-blocking agents, and they do not spontaneously depolarize in response to high extracellular K(+) concentrations. However, the trypanosomes are extremely sensitive to K(+) ionophores such as valinomycin. Surprisingly, T. brucei possesses a member of the Trk/HKT superfamily of monovalent cation permeases which so far had only been known from bacteria, archaea, fungi, and plants. The protein was named TbHKT1 and functions as a Na(+)-independent K(+) transporter when expressed in Escherichia coli, Saccharomyces cerevisiae, or Xenopus laevis oocytes. In trypanosomes, TbHKT1 is expressed in both the mammalian bloodstream stage and the Tsetse fly midgut stage; however, RNA interference (RNAi)-mediated silencing of TbHKT1 expression did not produce a growth phenotype in either stage. The presence of HKT genes in trypanosomatids adds a further piece to the enigmatic phylogeny of the Trk/HKT superfamily of K(+) transporters. Parsimonial analysis suggests that the transporters were present in the first eukaryotes but subsequently lost in several of the major eukaryotic lineages, in at least four independent events.
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Hänelt I, Löchte S, Sundermann L, Elbers K, Vor der Brüggen M, Bakker EP. Gain of function mutations in membrane region M2C2 of KtrB open a gate controlling K+ transport by the KtrAB system from Vibrio alginolyticus. J Biol Chem 2010; 285:10318-27. [PMID: 20097755 DOI: 10.1074/jbc.m109.089870] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
KtrB, the K(+)-translocating subunit of the Na(+)-dependent bacterial K(+) uptake system KtrAB, consists of four M(1)PM(2) domains, in which M(1) and M(2) are transmembrane helices and P indicates a p-loop that folds back from the external medium into the cell membrane. The transmembrane stretch M(2C) is, with its 40 residues, unusually long. It consists of three parts, the hydrophobic helices M(2C1) and M(2C3), which are connected by a nonhelical M(2C2) region, containing conserved glycine, alanine, serine, threonine, and lysine residues. Several point mutations in M(2C2) led to a huge gain of function of K(+) uptake by KtrB from the bacterium Vibrio alginolyticus. This effect was exclusively due to an increase in V(max) for K(+) transport. Na(+) translocation by KtrB was not affected. Partial to complete deletions of M(2C2) also led to enhanced V(max) values for K(+) uptake via KtrB. However, several deletion variants also exhibited higher K(m) values for K(+) uptake and at least one deletion variant, KtrB(Delta326-328), also transported Na(+) faster. The presence of KtrA did not suppress any of these effects. For the deletion variants, this was due to a diminished binding of KtrA to KtrB. PhoA studies indicated that M(2C2) forms a flexible structure within the membrane allowing M(2C3) to be directed either to the cytoplasm or (artificially) to the periplasm. These data are interpreted to mean (i) that region M(2C2) forms a flexible gate controlling K(+) translocation at the cytoplasmic side of KtrB, and (ii) that M(2C2) is required for the interaction between KtrA and KtrB.
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Affiliation(s)
- Inga Hänelt
- Abteilung Mikrobiologie, University of Osnabrück, D-49076 Osnabrück, Germany.
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Ion Channels and Plant Stress: Past, Present, and Future. ION CHANNELS AND PLANT STRESS RESPONSES 2010. [DOI: 10.1007/978-3-642-10494-7_1] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
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Yao X, Horie T, Xue S, Leung HY, Katsuhara M, Brodsky DE, Wu Y, Schroeder JI. Differential sodium and potassium transport selectivities of the rice OsHKT2;1 and OsHKT2;2 transporters in plant cells. PLANT PHYSIOLOGY 2010; 152:341-55. [PMID: 19889878 PMCID: PMC2799368 DOI: 10.1104/pp.109.145722] [Citation(s) in RCA: 95] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2009] [Accepted: 10/26/2009] [Indexed: 05/18/2023]
Abstract
Na(+) and K(+) homeostasis are crucial for plant growth and development. Two HKT transporter/channel classes have been characterized that mediate either Na(+) transport or Na(+) and K(+) transport when expressed in Xenopus laevis oocytes and yeast. However, the Na(+)/K(+) selectivities of the K(+)-permeable HKT transporters have not yet been studied in plant cells. One study expressing 5' untranslated region-modified HKT constructs in yeast has questioned the relevance of cation selectivities found in heterologous systems for selectivity predictions in plant cells. Therefore, here we analyze two highly homologous rice (Oryza sativa) HKT transporters in plant cells, OsHKT2;1 and OsHKT2;2, that show differential K(+) permeabilities in heterologous systems. Upon stable expression in cultured tobacco (Nicotiana tabacum) Bright-Yellow 2 cells, OsHKT2;1 mediated Na(+) uptake, but little Rb(+) uptake, consistent with earlier studies and new findings presented here in oocytes. In contrast, OsHKT2;2 mediated Na(+)-K(+) cotransport in plant cells such that extracellular K(+) stimulated OsHKT2;2-mediated Na(+) influx and vice versa. Furthermore, at millimolar Na(+) concentrations, OsHKT2;2 mediated Na(+) influx into plant cells without adding extracellular K(+). This study shows that the Na(+)/K(+) selectivities of these HKT transporters in plant cells coincide closely with the selectivities in oocytes and yeast. In addition, the presence of external K(+) and Ca(2+) down-regulated OsHKT2;1-mediated Na(+) influx in two plant systems, Bright-Yellow 2 cells and intact rice roots, and also in Xenopus oocytes. Moreover, OsHKT transporter selectivities in plant cells are shown to depend on the imposed cationic conditions, supporting the model that HKT transporters are multi-ion pores.
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Affiliation(s)
| | | | | | | | | | | | | | - Julian I. Schroeder
- Division of Biological Sciences, Cell and Developmental Biology Section, and Center for Molecular Genetics, University of California, San Diego, La Jolla, California 92093–0116 (X.Y., T.H., S.X., H.-Y.L., D.E.B., J.I.S.); Key Laboratory of Ministry of Education for Plant Developmental Biology, College of Life Sciences, Wuhan University, Wuhan, Hubei 430072, China (X.Y., Y.W.); and Group of Molecular and Functional Plant Biology, Research Institute for Bioresources, Okayama University, Kurashiki, Okayama 710–0046, Japan (T.H., M.K.)
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Horie T, Hauser F, Schroeder JI. HKT transporter-mediated salinity resistance mechanisms in Arabidopsis and monocot crop plants. TRENDS IN PLANT SCIENCE 2009; 14:660-8. [PMID: 19783197 PMCID: PMC2787891 DOI: 10.1016/j.tplants.2009.08.009] [Citation(s) in RCA: 273] [Impact Index Per Article: 18.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/30/2009] [Revised: 08/04/2009] [Accepted: 08/17/2009] [Indexed: 05/17/2023]
Abstract
The salinization of irrigated lands is increasingly detrimental to plant biomass production and agricultural productivity, as most plant species are sensitive to high concentrations of sodium (Na(+)), which causes combined Na(+) toxicity and osmotic stress. Plants have multiple Na(+)-transport systems to circumvent Na(+) toxicity. Essential physiological functions of major Na(+) transporters and their mechanisms mediating salinity resistance have been identified in Arabidopsis , including the AtSOS1, AtNHX and AtHKT1;1 transporters. As we discuss here, recent studies have demonstrated that a class of xylem-parenchyma-expressed Na(+)-permeable plant HKT transporters represent a primary mechanism mediating salt tolerance and Na(+) exclusion from leaves in Arabidopsis, and that major salt-tolerance quantitative trait loci in monocot crop plants are also based on this HKT-mediated mechanism.
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Affiliation(s)
- Tomoaki Horie
- Group of Molecular and Functional Plant Biology, Research Institute for Bioresources, Okayama University, Kurashiki, Okayama 710-0046, Japan.
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50
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Characterization and function analysis of a Halo-alkaline-adaptable Trk K+ uptake system in Alkalimonas amylolytica strain N10. ACTA ACUST UNITED AC 2009; 52:949-57. [DOI: 10.1007/s11427-009-0132-2] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2009] [Accepted: 06/25/2009] [Indexed: 10/20/2022]
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