A Trk/HKT-type K+ transporter from Trypanosoma brucei

Insights into Leishmania donovani potassium channel family and their biological functions

Leishmania donovani is the causative organism for visceral leishmaniasis. Although this parasite was discovered over a century ago, nothing is known about role of potassium channels in L. donovani. Potassium channels are known for their crucial roles in cellular functions in other organisms. Recently the presence of a calcium-activated potassium channel in L. donovani was reported which prompted us to look for other proteins which could be potassium channels and to investigate their possible physiological roles. Twenty sequences were identified in L. donovani genome and subjected to estimation of physio-chemical properties, motif analysis, localization prediction and transmembrane domain analysis. Structural predictions were also done. The channels were majorly α-helical and predominantly localized in cell membrane and lysosomes. The signature selectivity filter of potassium channel was present in all the sequences. In addition to the conventional potassium channel activity, they were associated with gene ontology terms for mitotic cell cycle, cell death, modulation by virus of host process, cell motility etc. The entire study indicates the presence of potassium channel families in L. donovani which may have involvement in several cellular pathways. Further investigations on these putative potassium channels are needed to elucidate their roles in Leishmania.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03692-y.

Two Trk/Ktr/HKT-type potassium transporters, TrkG and TrkH, perform distinct functions in Escherichia coli K-12

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.

Infection by phloem-limited phytoplasma affects mineral nutrient homeostasis in tomato leaf tissues

Phytoplasmas are sieve-elements restricted wall-less, pleomorphic pathogenic microorganisms causing devastating damage to over 700 plant species worldwide. The invasion of sieve elements by phytoplasmas has several consequences on nutrient transport and metabolism, anyway studies about changes of the mineral-nutrient profile following phytoplasma infections are scarce and offer contrasting results. Here, we examined changes in macro- and micronutrient concentration in tomato plant upon 'Candidatus Phytoplasma solani' infection. To investigate possible effects of 'Ca. P. solani' infection on mineral element allocation, the mineral elements were separately analysed in leaf midrib, leaf lamina and root. Moreover, we focused our analysis on the transcriptional regulation of genes encoding trans-membrane transporters of mineral nutrients. To this aim, a manually curated inventory of differentially expressed genes encoding transporters in tomato leaf midribs was mined from the transcriptional profile of healthy and infected tomato leaf midribs. Results highlighted changes in ion homeostasis in the host plant, and significant modulations at transcriptional level of genes encoding ion transporters and channels.

Promises and Pitfalls of Parasite Patch-clamp

Protozoan parasites acquire essential ions, nutrients, and other solutes from their insect and vertebrate hosts by transmembrane uptake. For intracellular stages, these solutes must cross additional membranous barriers. At each step, ion channels and transporters mediate not only this uptake but also the removal of waste products. These transport proteins are best isolated and studied with patch-clamp, but these methods remain accessible to only a few parasitologists due to specialized instrumentation and the required training in both theory and practice. Here, we provide an overview of patch-clamp, describing the advantages and limitations of the technology and highlighting issues that may lead to incorrect conclusions. We aim to help non-experts understand and critically assess patch-clamp data in basic research studies.

TbIRK is a signature sequence free potassium channel from Trypanosoma brucei locating to acidocalcisomes

Potassium channels from prokaryotes and eukaryotes are usually recognized by a typical amino acid sequence TXTGY(F)G representing the ionic selectivity filter. Using a screening approach with ion channel family profiles but without the above motif, we identified a gene in Trypanosoma brucei that exhibits homology to inward rectifying potassium channels. We report here cloning of this ion channel named TbIRK. The protein is localized to acidocalcisomes in procyclic and in bloodstream form parasites. Functional properties of this channel were established after expression in Xenopus oocytes. Currents recorded in potassium medium show inward rectification and little time dependence. Surprisingly, this channel retains selectivity for potassium ions over sodium ions >7, in spite of the lack of the classical selectivity filter. The sequence GGYVG was predicted in silico to replace this filter motif. Point mutations of the corresponding glycine residues confirmed this at the functional level. The channel is inhibited by caesium ions but remains unaffected by barium ions up to 10 mM. TbIRK is to our knowledge the first potassium channel in T. brucei that localizes to the acidocalcisomes, organelles involved in the storage of phosphates and the response to osmotic stress that occurs during the life cycle of trypanosomes.

Cyclic di-AMP impairs potassium uptake mediated by a cyclic di-AMP binding protein in Streptococcus pneumoniae

Cyclic di-AMP (c-di-AMP) has been shown to play important roles as a second messenger in bacterial physiology and infections. However, understanding of how the signal is transduced is still limited. Previously, we have characterized a diadenylate cyclase and two c-di-AMP phosphodiesterases in Streptococcus pneumoniae, a Gram-positive pathogen. In this study, we identified a c-di-AMP binding protein (CabP) in S. pneumoniae using c-di-AMP affinity chromatography. We demonstrated that CabP specifically bound c-di-AMP and that this interaction could not be interrupted by competition with other nucleotides, including ATP, cAMP, AMP, phosphoadenylyl adenosine (pApA), and cyclic di-GMP (c-di-GMP). By using a bacterial two-hybrid system and genetic mutagenesis, we showed that CabP directly interacted with a potassium transporter (SPD_0076) and that both proteins were required for pneumococcal growth in media with low concentrations of potassium. Interestingly, the interaction between CabP and SPD_0076 and the efficiency of potassium uptake were impaired by elevated c-di-AMP in pneumococci. These results establish a direct c-di-AMP-mediated signaling pathway that regulates pneumococcal potassium uptake.

Sodium transport system in plant cells

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.

Trypanocidal activity of salinomycin is due to sodium influx followed by cell swelling

Background

The few currently available drugs for treatment of African trypanosomiasis are outdated and show problems with toxicity and resistance. Hence, there is an urgent need for the discovery and development of new anti-trypanosomal agents.

Findings

In this study, the ionophorous antibiotic salinomycin was investigated for its trypanocidal activity in vitro using culture-adapted bloodstream forms of Trypanosoma brucei. The concentrations of salinomycin to reduce the growth rate by 50% and to kill the parasites were 0.31 μM and 1 μM, respectively. The trypanocidal action of the ionophore was shown to be the result of an influx of Na⁺ resulting in an increased intracellular Na⁺ concentration followed by cell swelling. This mode of action differs from the mechanism for the anti-cancer activity of salinomycin reported to be by induction of apoptosis.

Conclusion

Here we have shown that salinomycin is an effective agent against bloodstream forms of T. brucei and might be a potential candidate for treatment of African trypanosomiasis.

The Xenopus oocyte: a single-cell model for studying Ca2+ signaling

In the four decades since the Xenopus oocyte was first demonstrated to have the capacity to translate exogenous mRNAs, this system has been exploited for many different experimental purposes. Typically, the oocyte is used either as a "biological test tube" for heterologous expression of proteins without any particular cell biological insight or, alternatively, it is used for applications where cell biology is paramount, such as investigations of the cellular adaptations that power early development. In this article, we discuss the utility of the Xenopus oocyte for studying Ca(2+) signaling in both these contexts.

Molecular and electrophysiological characterization of a novel cation channel of Trypanosoma cruzi

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.

The KtrA and KtrE subunits are required for Na+-dependent K+ uptake by KtrB across the plasma membrane in Synechocystis sp. strain PCC 6803

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.