The Arabidopsis homolog of the mammalian OS-9 protein plays a key role in the endoplasmic reticulum-associated degradation of misfolded receptor-like kinases

Analysis of Protein Glycosylation in the ER

Protein N-glycosylation is an essential posttranslational modification which is initiated in the endoplasmic reticulum (ER). In plants, the N-glycans play a pivotal role in protein folding and quality control. Through the interaction of glycan processing and binding reactions mediated by ER-resident glycosidases and specific carbohydrate-binding proteins, the N-glycans contribute to the adoption of a native protein conformation. Properly folded glycoproteins are released from these processes and allowed to continue their transit to the Golgi where further processing and maturation of N-glycans leads to the formation of more complex structures with different functions. Incompletely folded glycoproteins are removed from the ER by a highly conserved degradation process to prevent the accumulation or secretion of misfolded proteins and maintain ER homeostasis. Here, we describe methods to analyze the N-glycosylation status and the glycan-dependent ER-associated degradation process in plants.

Transcription factor NAC1 activates expression of peptidase-encoding AtCEPs in roots to limit root hair growth

Plant genomes encode a unique group of papain-type Cysteine EndoPeptidases (CysEPs) containing a KDEL endoplasmic reticulum (ER) retention signal (KDEL-CysEPs or CEPs). CEPs process the cell-wall scaffolding EXTENSIN (EXT) proteins that regulate de novo cell-wall formation and cell expansion. Since CEPs cleave EXTs and EXT-related proteins, acting as cell-wall-weakening agents, they may play a role in cell elongation. The Arabidopsis (Arabidopsis thaliana) genome encodes 3 CEPs (AtCPE1-AtCEP3). Here, we report that the genes encoding these 3 Arabidopsis CEPs are highly expressed in root-hair (RH) cell files. Single mutants have no evident abnormal RH phenotype, but atcep1-3 atcep3-2 and atcep1-3 atcep2-2 double mutants have longer RHs than wild-type (Wt) plants, suggesting that expression of AtCEPs in root trichoblasts restrains polar elongation of the RH. We provide evidence that the transcription factor NAC1 (petunia NAM and Arabidopsis ATAF1, ATAF2, and CUC2) activates AtCEPs expression in roots to limit RH growth. Chromatin immunoprecipitation indicates that NAC1 binds to the promoter of AtCEP1, AtCEP2, and, to a lower extent, AtCEP3 and may directly regulate their expression. Inducible NAC1 overexpression increases AtCEP1 and AtCEP2 transcript levels in roots and leads to reduced RH growth while the loss of function nac1-2 mutation reduces AtCEP1-AtCEP3 gene expression and enhances RH growth. Likewise, expression of a dominant chimeric NAC1-SRDX repressor construct leads to increased RH length. Finally, we show that RH cell walls in the atcep1-3 atcep3-2 double mutant have reduced levels of EXT deposition, suggesting that the defects in RH elongation are linked to alterations in EXT processing and accumulation. Our results support the involvement of AtCEPs in controlling RH polar growth through EXT processing and insolubilization at the cell wall.

Mechanisms of Endoplasmic Reticulum Protein Homeostasis in Plants

Maintenance of proteome integrity is essential for cell function and survival in changing cellular and environmental conditions. The endoplasmic reticulum (ER) is the major site for the synthesis of secretory and membrane proteins. However, the accumulation of unfolded or misfolded proteins can perturb ER protein homeostasis, leading to ER stress and compromising cellular function. Eukaryotic organisms have evolved sophisticated and conserved protein quality control systems to ensure protein folding fidelity via the unfolded protein response (UPR) and to eliminate potentially harmful proteins via ER-associated degradation (ERAD) and ER-phagy. In this review, we summarize recent advances in our understanding of the mechanisms of ER protein homeostasis in plants and discuss the crosstalk between different quality control systems. Finally, we will address unanswered questions in this field.

Orosomucoid proteins limit endoplasmic reticulum stress in plants

Sphingolipids are cell membrane components and signaling molecules that induce endoplasmic reticulum (ER) stress responses, but the underlying mechanism is unknown. Orosomucoid proteins (ORMs) negatively regulate serine palmitoyltransferase activity, thus helping maintain proper sphingolipid levels in humans, yeast, and plants. In this report, we explored the roles of ORMs in regulating ER stress in Arabidopsis thaliana. Loss of ORM1 and ORM2 function caused constitutive activation of the unfolded protein response (UPR), as did treatment with the ceramide synthase inhibitor Fumonisin B1 (FB1) or ceramides. FB1 treatment induced the transcription factor bZIP28 to relocate from the ER membrane to the nucleus. The transcription factor WRKY75 positively regulates the UPR and physically interacted with bZIP28. We also found that the orm mutants showed impaired ER-associated degradation (ERAD), blocking the degradation of misfolded MILDEW RESISTANCE LOCUS-O 12 (MLO-12). ORM1 and ORM2 bind to EMS-MUTAGENIZED BRI1 SUPPRESSOR 7 (EBS7), a plant-specific component of the Arabidopsis ERAD complex, and regulate its stability. These data strongly suggest that ORMs in the ER membrane play vital roles in the UPR and ERAD pathways to prevent ER stress in Arabidopsis. Our results reveal that ORMs coordinate sphingolipid homeostasis with ER quality control and play a role in stress responses.

TMT-Based Proteomic Analysis of Hannaella sinensis-Induced Apple Resistance-Related Proteins

Studies on the molecular mechanism of antagonistic yeasts to control apple postharvest diseases are not comprehensive enough. Our preliminary investigations screened the biocontrol effect of Hannaella sinensis, an antagonistic yeast, and discovered its control efficacy on apple blue mold decay. However, the molecular mechanism of H. sinensis-induced resistance in apple has not been studied. In this study, proteins from apple treated with H. sinensis and sterile saline were analyzed using TMT proteomics technology. It was found that H. sinensis treatment induced the expressions of apple resistance-related proteins. Among the proteins in H. sinensis-induced apple, proteins related to plant defense mechanisms, such as reactive oxygen species scavenging, improvement of plant resistance and synthesis of resistant substances, improvement of plant disease resistance, the degradation of the pathogen cell wall, cell signaling, antibacterial activity, transport of defense-related substances, and protein processing, were differentially regulated. The results of this study revealed the underlying molecular mechanisms of H. sinensis-induced apple resistance at the protein level; the results also provided a theoretical basis for the commercial application of H. sinensis.

An endoplasmic reticulum-associated degradation-related E2-E3 enzyme pair controls grain size and weight through the brassinosteroid signaling pathway in rice

Grain size is an important agronomic trait, but our knowledge about grain size determination in crops is still limited. Endoplasmic reticulum (ER)-associated degradation (ERAD) is a special ubiquitin proteasome system that is involved in degrading misfolded or incompletely folded proteins in the ER. Here, we report that SMALL GRAIN 3 (SMG3) and DECREASED GRAIN SIZE 1 (DGS1), an ERAD-related E2-E3 enzyme pair, regulate grain size and weight through the brassinosteroid (BR) signaling pathway in rice (Oryza sativa). SMG3 encodes a homolog of Arabidopsis (Arabidopsis thaliana) UBIQUITIN CONJUGATING ENZYME 32, which is a conserved ERAD-associated E2 ubiquitin conjugating enzyme. SMG3 interacts with another grain size regulator, DGS1. Loss of function of SMG3 or DGS1 results in small grains, while overexpression of SMG3 or DGS1 leads to long grains. Further analyses showed that DGS1 is an active E3 ubiquitin ligase and colocates with SMG3 in the ER. SMG3 and DGS1 are involved in BR signaling. DGS1 ubiquitinates the BR receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) and affects its accumulation. Genetic analysis suggests that SMG3, DGS1, and BRI1 act together to regulate grain size and weight. In summary, our findings identify an ERAD-related E2-E3 pair that regulates grain size and weight, which gives insight into the function of ERAD in grain size control and BR signaling.

A Predominant Role of AtEDEM1 in Catalyzing a Rate-Limiting Demannosylation Step of an Arabidopsis Endoplasmic Reticulum-Associated Degradation Process

Endoplasmic reticulum-associated degradation (ERAD) is a key cellular process for degrading misfolded proteins. It was well known that an asparagine (N)-linked glycan containing a free α1,6-mannose residue is a critical ERAD signal created by Homologous to α-mannosidase 1 (Htm1) in yeast and ER-Degradation Enhancing α-Mannosidase-like proteins (EDEMs) in mammals. An earlier study suggested that two Arabidopsis homologs of Htm1/EDEMs function redundantly in generating such a conserved N-glycan signal. Here we report that the Arabidopsis irb1 (reversal of bri1) mutants accumulate brassinosteroid-insensitive 1-5 (bri1-5), an ER-retained mutant variant of the brassinosteroid receptor BRI1 and are defective in one of the Arabidopsis Htm1/EDEM homologs, AtEDEM1. We show that the wild-type AtEDEM1, but not its catalytically inactive mutant, rescues irb1-1. Importantly, an insertional mutation of the Arabidopsis Asparagine-Linked Glycosylation 3 (ALG3), which causes N-linked glycosylation with truncated glycans carrying a different free α1,6-mannose residue, completely nullifies the inhibitory effect of irb1-1 on bri1-5 ERAD. Interestingly, an insertional mutation in AtEDEM2, the other Htm1/EDEM homolog, has no detectable effect on bri1-5 ERAD; however, it enhances the inhibitory effect of irb1-1 on bri1-5 degradation. Moreover, AtEDEM2 transgenes rescued the irb1-1 mutation with lower efficacy than AtEDEM1. Simultaneous elimination of AtEDEM1 and AtEDEM2 completely blocks generation of α1,6-mannose-exposed N-glycans on bri1-5, while overexpression of either AtEDEM1 or AtEDEM2 stimulates bri1-5 ERAD and enhances the bri1-5 dwarfism. We concluded that, despite its functional redundancy with AtEDEM2, AtEDEM1 plays a predominant role in promoting bri1-5 degradation.

Molecular Mechanisms of Plant Responses to Salt Stress

Saline-alkali soils pose an increasingly serious global threat to plant growth and productivity. Much progress has been made in elucidating how plants adapt to salt stress by modulating ion homeostasis. Understanding the molecular mechanisms that affect salt tolerance and devising strategies to develop/breed salt-resilient crops have been the primary goals of plant salt stress signaling research over the past few decades. In this review, we reflect on recent major advances in our understanding of the cellular and physiological mechanisms underlying plant responses to salt stress, especially those involving temporally and spatially defined changes in signal perception, decoding, and transduction in specific organelles or cells.

The Role of SBI2/ALG12/EBS4 in the Regulation of Endoplasmic Reticulum-Associated Degradation (ERAD) Studied by a Null Allele

Redundancy and lethality is a long-standing problem in genetics but generating minimal and lethal phenotypes in the knockouts of the same gene by different approaches drives this problem to a new high. In Asn (N)-linked glycosylation, a complex and ubiquitous cotranslational and post-translational protein modification required for the transfer of correctly folded proteins and endoplasmic reticulum-associated degradation (ERAD) of misfolded proteins, ALG12 (EBS4) is an α 1, 6-mannosyltransferase catalyzing a mannose into Glc₃Man₉GlcNAc₂. In Arabidopsis, T-DNA knockout alg12-T is lethal while likely ebs4 null mutants isolated by forward genetics are most healthy as weak alleles, perplexing researchers and demanding further investigations. Here, we isolated a true null allele, sbi2, with the W258Stop mutation in ALG12/EBS4. sbi2 restored the sensitivity of brassinosteroid receptor mutants bri1-5, bri1-9, and bri1-235 with ER-trapped BRI1 to brassinosteroids. Furthermore, sbi2 maturated earlier than the wild-type. Moreover, concomitant with impaired and misfolded proteins accumulated in the ER, sbi2 had higher sensitivity to tunicamycin and salt than the wild-type. Our findings thus clarify the role of SBI2/ALG12/EBS4 in the regulation of the ERAD of misfolded glycoproteins, and plant growth and stress response. Further, our study advocates the necessity and importance of using multiple approaches to validate genetics study.

A lipid viewpoint on the plant endoplasmic reticulum stress response

Organisms, including humans, seem to be constantly exposed to various changes, which often have undesirable effects, referred to as stress. To keep up with these changes, eukaryotic cells may have evolved a number of relevant cellular processes, such as the endoplasmic reticulum (ER) stress response. Owing to presumably intimate links between human diseases and the ER function, the ER stress response has been extensively investigated in various organisms for a few decades. Based on these studies, we now have a picture of the molecular mechanisms of the ER stress response, one of which, the unfolded protein response (UPR), is highly conserved among yeasts, mammals, higher plants, and green algae. In this review, we attempt to highlight the plant UPR from the perspective of lipids, especially membrane phospholipids. Phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) are the most abundant membrane phospholipids in eukaryotic cells. The ratio of PtdCho to PtdEtn and the unsaturation of fatty acyl tails in both phospholipids may be critical factors for the UPR, but the pathways responsible for PtdCho and PtdEtn biosynthesis are distinct in animals and plants. We discuss the plant UPR in comparison with the system in yeasts and animals in the context of membrane phospholipids.

Endoplasmic Reticulum Stress and Unfolded Protein Response Signaling in Plants

Plants are sensitive to a variety of stresses that cause various diseases throughout their life cycle. However, they have the ability to cope with these stresses using different defense mechanisms. The endoplasmic reticulum (ER) is an important subcellular organelle, primarily recognized as a checkpoint for protein folding. It plays an essential role in ensuring the proper folding and maturation of newly secreted and transmembrane proteins. Different processes are activated when around one-third of newly synthesized proteins enter the ER in the eukaryote cells, such as glycosylation, folding, and/or the assembling of these proteins into protein complexes. However, protein folding in the ER is an error-prone process whereby various stresses easily interfere, leading to the accumulation of unfolded/misfolded proteins and causing ER stress. The unfolded protein response (UPR) is a process that involves sensing ER stress. Many strategies have been developed to reduce ER stress, such as UPR, ER-associated degradation (ERAD), and autophagy. Here, we discuss the ER, ER stress, UPR signaling and various strategies for reducing ER stress in plants. In addition, the UPR signaling in plant development and different stresses have been discussed.

A Non-redundant Function of MNS5: A Class I α-1, 2 Mannosidase, in the Regulation of Endoplasmic Reticulum-Associated Degradation of Misfolded Glycoproteins

Endoplasmic Reticulum-Associated Degradation (ERAD) is one of the major processes in maintaining protein homeostasis. Class I α-mannosidases MNS4 and MNS5 are involved in the degradation of misfolded variants of the heavily glycosylated proteins, playing an important role for glycan-dependent ERAD in planta. MNS4 and MNS5 reportedly have functional redundancy, meaning that only the loss of both MNS4 and MNS5 shows phenotypes. However, MNS4 is a membrane-associated protein while MNS5 is a soluble protein, and both can localize to the endoplasmic reticulum (ER). Furthermore, MNS4 and MNS5 differentially demannosylate the glycoprotein substrates. Importantly, we found that their gene expression patterns are complemented rather than overlapped. This raises the question of whether they indeed work redundantly, warranting a further investigation. Here, we conducted an exhaustive genetic screen for a suppressor of the bri1-5, a brassinosteroid (BR) receptor mutant with its receptor downregulated by ERAD, and isolated sbi3, a suppressor of bri1-5 mutant named after sbi1 (suppressor of bri1). After genetic mapping together with whole-genome re-sequencing, we identified a point mutation G343E in AT1G27520 (MNS5) in sbi3. Genetic complementation experiments confirmed that sbi3 was a loss-of-function allele of MNS5. In addition, sbi3 suppressed the dwarf phenotype of bri1-235 in the proteasome-independent ERAD pathway and bri1-9 in the proteasome-dependent ERAD pathway. Importantly, sbi3 could only affect BRI1/bri1 with kinase activities such that it restored BR-sensitivities of bri1-5, bri1-9, and bri1-235 but not null bri1. Furthermore, sbi3 was less tolerant to tunicamycin and salt than the wild-type plants. Thus, our study uncovers a non-redundant function of MNS5 in the regulation of ERAD as well as plant growth and ER stress response, highlighting a need of the traditional forward genetic approach to complement the T-DNA or CRISPR-Cas9 systems on gene functional study.

Two ubiquitin-associated ER proteins interact with COPT copper transporters and modulate their accumulation

The endoplasmic reticulum (ER) contains an elaborate protein quality control network that promotes protein folding and prevents accumulation of misfolded proteins. Evolutionarily conserved UBIQUITIN-ASSOCIATED DOMAIN-CONTAINING PROTEIN 2 (UBAC2) is involved in ER-associated protein degradation in metazoans. We have previously reported that two close UBAC2 homologs from Arabidopsis (Arabidopsis thaliana) not only participate in selective autophagy of ER components but also interact with plant-specific PATHOGEN-ASSOCIATED MOLECULAR PATTERN (PAMP)-INDUCED COILED COIL (PICC) protein to increase the accumulation of POWDERY MILDEW-RESISTANT 4 callose synthase. Here, we report that UBAC2s also interacted with COPPER (Cu) TRANSPORTER 1 (COPT1) and plasma membrane-targeted members of the Cu transporter family. The ubac2 mutants were significantly reduced in both the accumulation of COPT proteins and Cu content, and also displayed increased sensitivity to a Cu chelator. Therefore, UBAC2s positively regulate the accumulation of COPT transporters, thereby increasing Cu uptake by plant cells. Unlike with POWDERY MILDEW RESISTANCE 4, however, the positive role of UBAC2s in the accumulation of COPT1 is not dependent on PICC or the UBA domain of UBAC2s. When COPT1 was overexpressed under the CaMV 35S promoter, the increased accumulation of COPT1 was strongly UBAC2-dependent, particularly when a signal peptide was added to the N-terminus of COPT1. Further analysis using inhibitors of protein synthesis and degradation strongly suggested that UBAC2s stabilize newly synthesized COPT proteins against degradation by the proteasome system. These results indicate that plant UBAC2s are multifunctional proteins that regulate the degradation and accumulation of specific ER-synthesized proteins.

SIMP1 modulates salt tolerance by elevating ERAD efficiency through UMP1A-mediated proteasome maturation in plants

Salt stress significantly induces accumulation of misfolded or unfolded proteins in plants. Endoplasmic reticulum (ER)-associated protein degradation (ERAD) and other degradative machineries function in the degradation of these abnormal proteins, leading to enhanced salt tolerance in plants. Here we characterise that a novel receptor-like kinase, Salt-Induced Malectin-like domain-containing Protein1 (SIMP1), elevates ERAD efficiency during salt stress through UMP1A, a putative proteasome maturation factor in Arabidopsis. SIMP1 loss-of-function caused a salt-hypersensitive phenotype. SIMP1 interacts and phosphorylates UMP1A, and the protein stability of UMP1A is positively regulated by SIMP1. SIMP1 modulates the 26S proteasome maturation possibly through enhancing the recruitment of specific β subunits of the core catalytic particle to UMP1A. Functionally, the SIMP1-UMP1A module plays a positive role in ERAD efficiency in Arabidopsis. The degradation of misfolded/unfolded proteins was impaired in both simp1 and ump1a mutants during salt stress. Consistently, both simp1 and ump1a plants exhibited reduced ER stress tolerance. Phenotypic analysis revealed that SIMP1 regulates salt tolerance through UMP1A at least in part. Taken together, our work demonstrated that SIMP1 modulates plant salt tolerance by promoting proteasome maturation via UMP1A, therefore mitigating ER stress through enhanced ERAD efficiency under saline conditions.

Endoplasmic reticulum-related E3 ubiquitin ligases: Key regulators of plant growth and stress responses

Accumulating evidence has revealed that the ubiquitin proteasome system plays fundamental roles in the regulation of diverse cellular activities in eukaryotes. The ubiquitin protein ligases (E3s) are central to the proteasome system because of their ability to determine its substrate specificity. Several studies have demonstrated the essential role of a group of ER (endoplasmic reticulum)-localized E3s in the positive or negative regulation of cell homeostasis. Most ER-related E3s are conserved between plants and mammals, and a few plant-specific components have been reported. In this review, we summarize the functions of ER-related E3s in plant growth, ER-associated protein degradation and ER-phagy, abiotic and biotic stress responses, and hormone signaling. Furthermore, we highlight several questions that remain to be addressed and suggest directions for further research on ER-related E3 ubiquitin ligases.

An Argon-Ion-Induced Pale Green Mutant of Arabidopsis Exhibiting Rapid Disassembly of Mesophyll Chloroplast Grana

Argon-ion beam is an effective mutagen capable of inducing a variety of mutation types. In this study, an argon ion-induced pale green mutant of Arabidopsis thaliana was isolated and characterized. The mutant, designated Ar50-33-pg1, exhibited moderate defects of growth and greening and exhibited rapid chlorosis in photosynthetic tissues. Fluorescence microscopy confirmed that mesophyll chloroplasts underwent substantial shrinkage during the chlorotic process. Genetic and whole-genome resequencing analyses revealed that Ar50-33-pg1 contained a large 940 kb deletion in chromosome V that encompassed more than 100 annotated genes, including 41 protein-coding genes such as TYRAAt1/TyrA1, EGY1, and MBD12. One of the deleted genes, EGY1, for a thylakoid membrane-localized metalloprotease, was the major contributory gene responsible for the pale mutant phenotype. Both an egy1 mutant and F1 progeny of an Ar50-33-pg1 × egy1 cross-exhibited chlorotic phenotypes similar to those of Ar50-33-pg1. Furthermore, ultrastructural analysis of mesophyll cells revealed that Ar50-33-pg1 and egy1 initially developed wild type-like chloroplasts, but these were rapidly disassembled, resulting in thylakoid disorganization and fragmentation, as well as plastoglobule accumulation, as terminal phenotypes. Together, these data support the utility of heavy-ion mutagenesis for plant genetic analysis and highlight the importance of EGY1 in the structural maintenance of grana in mesophyll chloroplasts.

Protein Quality Control in Plant Organelles: Current Progress and Future Perspectives

The endoplasmic reticulum, chloroplasts, and mitochondria are major plant organelles for protein synthesis, photosynthesis, metabolism, and energy production. Protein homeostasis in these organelles, maintained by a balance between protein synthesis and degradation, is essential for cell functions during plant growth, development, and stress resistance. Nucleus-encoded chloroplast- and mitochondrion-targeted proteins and ER-resident proteins are imported from the cytosol and undergo modification and maturation within their respective organelles. Protein folding is an error-prone process that is influenced by both developmental signals and environmental cues; a number of mechanisms have evolved to ensure efficient import and proper folding and maturation of proteins in plant organelles. Misfolded or damaged proteins with nonnative conformations are subject to degradation via complementary or competing pathways: intraorganelle proteases, the organelle-associated ubiquitin-proteasome system, and the selective autophagy of partial or entire organelles. When proteins in nonnative conformations accumulate, the organelle-specific unfolded protein response operates to restore protein homeostasis by reducing protein folding demand, increasing protein folding capacity, and enhancing components involved in proteasome-associated protein degradation and autophagy. This review summarizes recent progress on the understanding of protein quality control in the ER, chloroplasts, and mitochondria in plants, with a focus on common mechanisms shared by these organelles during protein homeostasis.

Quantitative Proteomic Analysis of ER Stress Response Reveals both Common and Specific Features in Two Contrasting Ecotypes of Arabidopsis thaliana

Accumulation of unfolded and misfolded proteins in endoplasmic reticulum (ER) elicits a well-conserved response called the unfolded protein response (UPR), which triggers the upregulation of downstream genes involved in protein folding, vesicle trafficking, and ER-associated degradation (ERAD). Although dynamic transcriptomic responses and the underlying major transcriptional regulators in ER stress response in Arabidopsis have been well established, the proteome changes induced by ER stress have not been reported in Arabidopsis. In the current study, we found that the Arabidopsis Landsberg erecta (Ler) ecotype was more sensitive to ER stress than the Columbia (Col) ecotype. Quantitative mass spectrometry analysis with Tandem Mass Tag (TMT) isobaric labeling showed that, in total, 7439 and 7035 proteins were identified from Col and Ler seedlings, with 88 and 113 differentially regulated (FC > 1.3 or <0.7, p < 0.05) proteins by ER stress in Col and Ler, respectively. Among them, 40 proteins were commonly upregulated in Col and Ler, among which 10 were not upregulated in bzip28 bzip60 double mutant (Col background) plants. Of the 19 specifically upregulated proteins in Col, as compared with that in Ler, components in ERAD, N-glycosylation, vesicle trafficking, and molecular chaperones were represented. Quantitative RT-PCR showed that transcripts of eight out of 19 proteins were not upregulated (FC > 1.3 or <0.7, p < 0.05) by ER stress in Col ecotype, while transcripts of 11 out of 19 proteins were upregulated by ER stress in both ecotypes with no obvious differences in fold change between Col and Ler. Our results experimentally demonstrated the robust ER stress response at the proteome level in plants and revealed differentially regulated proteins that may contribute to the differed ER stress sensitivity between Col and Ler ecotypes in Arabidopsis.

Regulation of Three Key Kinases of Brassinosteroid Signaling Pathway

Brassinosteroids (BRs) are important plant growth hormones that regulate a wide range of plant growth and developmental processes. The BR signals are perceived by two cell surface-localized receptor kinases, Brassinosteroid-Insensitive1 (BRI1) and BRI1-Associated receptor Kinase (BAK1), and reach the nucleus through two master transcription factors, bri1-EMS suppressor1 (BES1) and Brassinazole-resistant1 (BZR1). The intracellular transmission of the BR signals from BRI1/BAK1 to BES1/BZR1 is inhibited by a constitutively active kinase Brassinosteroid-Insensitive2 (BIN2) that phosphorylates and negatively regulates BES1/BZR1. Since their initial discoveries, further studies have revealed a plethora of biochemical and cellular mechanisms that regulate their protein abundance, subcellular localizations, and signaling activities. In this review, we provide a critical analysis of the current literature concerning activation, inactivation, and other regulatory mechanisms of three key kinases of the BR signaling cascade, BRI1, BAK1, and BIN2, and discuss some unresolved controversies and outstanding questions that require further investigation.

Insights into endoplasmic reticulum-associated degradation in plants

Secretory and transmembrane protein synthesis and initial modification are essential processes in protein maturation, and these processes are important for maintaining protein homeostasis in the endoplasmic reticulum (ER). ER homeostasis can be disrupted by the accumulation of misfolded proteins, resulting in ER stress, due to specific intra- or extracellular stresses. Processes including the unfolded protein response (UPR), ER-associated degradation (ERAD) and autophagy are thought to play important roles in restoring ER homeostasis. Here, we focus on summarizing and analysing recent advances in our understanding of the role of ERAD in plant physiological processes, especially in plant adaption to biotic and abiotic stresses, and also identify several issues that still need to be resolved in this field.

The Crucial Role of Demannosylating Asparagine-Linked Glycans in ERADicating Misfolded Glycoproteins in the Endoplasmic Reticulum

Most membrane and secreted proteins are glycosylated on certain asparagine (N) residues in the endoplasmic reticulum (ER), which is crucial for their correct folding and function. Protein folding is a fundamentally inefficient and error-prone process that can be easily interfered by genetic mutations, stochastic cellular events, and environmental stresses. Because misfolded proteins not only lead to functional deficiency but also produce gain-of-function cellular toxicity, eukaryotic organisms have evolved highly conserved ER-mediated protein quality control (ERQC) mechanisms to monitor protein folding, retain and repair incompletely folded or misfolded proteins, or remove terminally misfolded proteins via a unique ER-associated degradation (ERAD) mechanism. A crucial event that terminates futile refolding attempts of a misfolded glycoprotein and diverts it into the ERAD pathway is executed by removal of certain terminal α1,2-mannose (Man) residues of their N-glycans. Earlier studies were centered around an ER-type α1,2-mannosidase that specifically cleaves the terminal α1,2Man residue from the B-branch of the three-branched N-linked Man₉GlcNAc₂ (GlcNAc for N-acetylglucosamine) glycan, but recent investigations revealed that the signal that marks a terminally misfolded glycoprotein for ERAD is an N-glycan with an exposed α1,6Man residue generated by members of a unique folding-sensitive α1,2-mannosidase family known as ER-degradation enhancing α-mannosidase-like proteins (EDEMs). This review provides a historical recount of major discoveries that led to our current understanding on the role of demannosylating N-glycans in sentencing irreparable misfolded glycoproteins into ERAD. It also discusses conserved and distinct features of the demannosylation processes of the ERAD systems of yeast, mammals, and plants.

Arabidopsis OTU1, a linkage-specific deubiquitinase, is required for endoplasmic reticulum-associated protein degradation

Endoplasmic reticulum (ER)-associated degradation (ERAD) is part of the ER protein quality-control system (ERQC), which is critical for the conformation fidelity of most secretory and membrane proteins in eukaryotic organisms. ERAD is thought to operate in plants with core machineries highly conserved to those in human and yeast; however, little is known about the plant ERAD system. Here we report the characterization of a close homolog of human OTUB1 in Arabidopsis thaliana, designated as AtOTU1. AtOTU1 selectively hydrolyzes several types of ubiquitin chains and these activities depend on its conserved protease domain and/or the unique N-terminus. The otu1 null mutant is sensitive to high salinity stress, and particularly agents that cause protein misfolding. It turns out that AtOTU1 is required for the processing of known plant ERAD substrates such as barley powdery mildew O (MLO) alleles by virtue of its association with the CDC48 complex through its N-terminal region. These observations collectively define AtOTU1 as an OTU domain-containing deubiquitinase involved in Arabidopsis ERAD.

PAWH1 and PAWH2 are plant-specific components of an Arabidopsis endoplasmic reticulum-associated degradation complex

Endoplasmic reticulum-associated degradation (ERAD) is a unique mechanism to degrade misfolded proteins via complexes containing several highly-conserved ER-anchored ubiquitin ligases such as HMG-CoA reductase degradation1 (Hrd1). Arabidopsis has a similar Hrd1-containing ERAD machinery; however, our knowledge of this complex is limited. Here we report two closely-related Arabidopsis proteins, Protein Associated With Hrd1-1 (PAWH1) and PAWH2, which share a conserved domain with yeast Altered Inheritance of Mitochondria24. PAWH1 and PAWH2 localize to the ER membrane and associate with Hrd1 via EMS-mutagenized Bri1 Suppressor7 (EBS7), a plant-specific component of the Hrd1 complex. Simultaneously elimination of two PAWHs constitutively activates the unfolded protein response and compromises stress tolerance. Importantly, the pawh1 pawh2 double mutation reduces the protein abundance of EBS7 and Hrd1 and inhibits degradation of several ERAD substrates. Our study not only discovers additional plant-specific components of the Arabidopsis Hrd1 complex but also reveals a distinct mechanism for regulating the Hrd1 stability.

HSFA2 Functions in the Physiological Adaptation of Undifferentiated Plant Cells to Spaceflight

Heat Shock Factor A2 (HsfA2) is part of the Heat Shock Factor (HSF) network, and plays an essential role beyond heat shock in environmental stress responses and cellular homeostatic control. Arabidopsis thaliana cell cultures derived from wild type (WT) ecotype Col-0 and a knockout line deficient in the gene encoding HSFA2 (HSFA2 KO) were grown aboard the International Space Station (ISS) to ascertain whether the HSF network functions in the adaptation to the novel environment of spaceflight. Microarray gene expression data were analyzed using a two-part comparative approach. First, genes differentially expressed between the two environments (spaceflight to ground) were identified within the same genotype, which represented physiological adaptation to spaceflight. Second, gene expression profiles were compared between the two genotypes (HSFA2 KO to WT) within the same environment, which defined genes uniquely required by each genotype on the ground and in spaceflight-adapted states. Results showed that the endoplasmic reticulum (ER) stress and unfolded protein response (UPR) define the HSFA2 KO cells' physiological state irrespective of the environment, and likely resulted from a deficiency in the chaperone-mediated protein folding machinery in the mutant. Results further suggested that additional to its universal stress response role, HsfA2 also has specific roles in the physiological adaptation to spaceflight through cell wall remodeling, signal perception and transduction, and starch biosynthesis. Disabling HsfA2 altered the physiological state of the cells, and impacted the mechanisms induced to adapt to spaceflight, and identified HsfA2-dependent genes that are important to the adaption of wild type cells to spaceflight. Collectively these data indicate a non-thermal role for the HSF network in spaceflight adaptation.

Arabidopsis Endoplasmic Reticulum-Localized UBAC2 Proteins Interact with PAMP-INDUCED COILED-COIL to Regulate Pathogen-Induced Callose Deposition and Plant Immunity

Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) is initiated upon PAMP recognition by pattern recognition receptors (PRR). PTI signals are transmitted through activation of mitogen-activated protein kinases (MAPKs), inducing signaling and defense processes such as reactive oxygen species (ROS) production and callose deposition. Here, we examine mutants for two Arabidopsis thaliana genes encoding homologs of UBIQUITIN-ASSOCIATED DOMAIN-CONTAINING PROTEIN 2 (UBAC2), a conserved endoplasmic reticulum (ER) protein implicated in ER protein quality control. The ubac2 mutants were hypersusceptible to a type III secretion-deficient strain of the bacterial pathogen Pseudomonas syringae, indicating a PTI defect. The ubac2 mutants showed normal PRR biogenesis, MAPK activation, ROS burst, and PTI-associated gene expression. Pathogen- and PAMP-induced callose deposition, however, was compromised in ubac2 mutants. UBAC2 proteins interact with the plant-specific long coiled-coil protein PAMP-INDUCED COILED COIL (PICC), and picc mutants were compromised in callose deposition and PTI. Compromised callose deposition in the ubac2 and picc mutants was associated with reduced accumulation of the POWDERY MILDEW RESISTANT 4 (PMR4) callose synthase, which is responsible for pathogen-induced callose synthesis. Constitutive overexpression of PMR4 restored pathogen-induced callose synthesis and PTI in the ubac2 and picc mutants. These results uncover an ER pathway involving the conserved UBAC2 and plant-specific PICC proteins that specifically regulate pathogen-induced callose deposition in plant innate immunity.

A Temperature-Sensitive Misfolded bri1-301 Receptor Requires Its Kinase Activity to Promote Growth

BRASSINOSTEROID-INSENSITIVE1 (BRI1) is a leucine-rich-repeat receptor-like kinase that functions as the cell surface receptor for brassinosteroids (BRs). Previous studies showed that BRI1 requires its kinase activity to transduce the extracellular BR signal into the nucleus. Among the many reported mutant bri1 alleles, bri1-301 is unique, as its glycine-989-to-isoleucine mutation completely inhibits its kinase activity in vitro but only gives rise to a weak dwarf phenotype compared with strong or null bri1 alleles, raising the question of whether kinase activity is essential for the biological function of BRI1. Here, we show that the Arabidopsis (Arabidopsis thaliana) bri1-301 mutant receptor exhibits weak BR-triggered phosphorylation in vivo and absolutely requires its kinase activity for the limited growth that occurs in the bri1-301 mutant. We also show that bri1-301 is a temperature-sensitive misfolded protein that is rapidly degraded in the endoplasmic reticulum and at the plasma membrane by yet unknown mechanisms. A temperature increase from 22°C to 29°C reduced the protein stability and biochemical activity of bri1-301, likely due to temperature-enhanced protein misfolding. The bri1-301 protein could be used as a model to study the degradation machinery for misfolded membrane proteins with cytosolic structural lesions and the plasma membrane-associated protein quality-control mechanism.

Function of N-glycosylation in plants

Protein N-glycosylation is one of the major post-translational modifications in eukaryotic cells. In lower unicellular eukaryotes, the known functions of N-glycans are predominantly in protein folding and quality control within the lumen of the endoplasmic reticulum (ER). In multicellular organisms, complex N-glycans are important for developmental programs and immune responses. However, little is known about the functions of complex N-glycans in plants. Formed in the Golgi apparatus, plant complex N-glycans have structures distinct from their animal counterparts due to a set of glycosyltransferases unique to plants. Severe basal underglycosylation in the ER lumen induces misfolding of newly synthesized proteins, which elicits the unfolded protein response (UPR) and ER protein quality control (ERQC) pathways. The former promotes higher capacity of proper protein folding and the latter degradation of misfolded proteins to clear the ER. Although our knowledge on plant complex N-glycan functions is limited, genetic studies revealed the importance of complex N-glycans in cellulose biosynthesis and growth under stress.

Trimming of N-Glycans by the Golgi-Localized α-1,2-Mannosidases, MNS1 and MNS2, Is Crucial for Maintaining RSW2 Protein Abundance during Salt Stress in Arabidopsis

Asparagine (Asn/N)-linked glycans are important for protein folding, trafficking, and endoplasmic reticulum-associated degradation in eukaryotes. The maturation of glycoproteins involves the trimming of mannosyl residues by mannosidases and addition of other sugar molecules to three-branched N-glycans in the Golgi. However, the biological importance of Golgi-mediated mannose trimming is not fully understood. Here, we show that abolishment of two functionally redundant mannosidases, MNS1 and MNS2, responsible for α-1,2-mannose trimming on the A and C branches of plant N-glycans lead to severe root growth inhibition under salt stress conditions in Arabidopsis. In contrast, mutants with defects in the biosynthesis of the oligosaccharide precursor displayed enhanced salt tolerance in the absence of mannose trimming. However, mutation in EBS3, which is required for the formation of the branched N-glycan precursor, suppressed the salt-sensitive phenotype of mns1 mns2 double mutant. Interestingly, we observed that cellulose biosynthesis was compromised in mns1 mns2 roots under high salinity. Consistently, abundance of a membrane anchored endo-β-1,4-endoglucanase (RSW2/KOR) that plays a key role in cellulose biosynthesis and its mutant variant rsw2-1 were modulated by α-1,2-mannose trimming under salt stress. Overexpression of RSW2 could partially rescue the salt-sensitive phenotype of mns1 mns2. Taken together, these results suggest that MNS1/2-mediated mannose trimming of N-glycans is crucial in modulating glycoprotein abundance to withstand salt stress in plants.

Protein Quality Control in the Endoplasmic Reticulum of Plants

The endoplasmic reticulum (ER) is the site of maturation for roughly one-third of all cellular proteins. ER-resident molecular chaperones and folding catalysts promote folding and assembly in a diverse set of newly synthesized proteins. Because these processes are error-prone, all eukaryotic cells have a quality-control system in place that constantly monitors the proteins and decides their fate. Proteins with potentially harmful nonnative conformations are subjected to assisted folding or degraded. Persistent folding-defective proteins are distinguished from folding intermediates and targeted for degradation by a specific process involving clearance from the ER. Although the basic principles of these processes appear conserved from yeast to animals and plants, there are distinct differences in the ER-associated degradation of misfolded glycoproteins. The general importance of ER quality-control events is underscored by their involvement in the biogenesis of diverse cell surface receptors and their crucial maintenance of protein homeostasis under diverse stress conditions.

Protein Quality Control in the Endoplasmic Reticulum of Plants

The endoplasmic reticulum (ER) is the site of maturation for roughly one-third of all cellular proteins. ER-resident molecular chaperones and folding catalysts promote folding and assembly in a diverse set of newly synthesized proteins. Because these processes are error-prone, all eukaryotic cells have a quality-control system in place that constantly monitors the proteins and decides their fate. Proteins with potentially harmful nonnative conformations are subjected to assisted folding or degraded. Persistent folding-defective proteins are distinguished from folding intermediates and targeted for degradation by a specific process involving clearance from the ER. Although the basic principles of these processes appear conserved from yeast to animals and plants, there are distinct differences in the ER-associated degradation of misfolded glycoproteins. The general importance of ER quality-control events is underscored by their involvement in the biogenesis of diverse cell surface receptors and their crucial maintenance of protein homeostasis under diverse stress conditions.

The glycan-dependent ERAD machinery degrades topologically diverse misfolded proteins

Many soluble and integral membrane proteins fold in the endoplasmic reticulum (ER) with the help of chaperones and folding factors. Despite these efforts, protein folding is intrinsically error prone and amino acid changes, alterations in post-translational modifications or cellular stress can cause protein misfolding. Folding-defective non-native proteins are cleared from the ER and typically undergo ER-associated degradation (ERAD). Here, we investigated whether different misfolded glycoproteins require the same set of ERAD factors and are directed to HRD1 complex-mediated degradation in plants. We generated a series of glycoprotein ERAD substrates harboring a misfolded domain from Arabidopsis STRUBBELIG or the BRASSINOSTEROID INSENSITVE 1 receptor fused to different membrane anchoring regions. We show that single pass and multispanning ERAD substrates are subjected to glycan-dependent degradation by the HRD1 complex. However, the presence of a powerful ER exit signal in the multispanning ERAD substrates causes competition with ER quality control and targeting of misfolded glycoproteins to the vacuole. Our results demonstrate that the same machinery is used for degradation of topologically different misfolded glycoproteins in the ER of plants.

The Extracellular Domain of Pollen Receptor Kinase 3 is structurally similar to the SERK family of co-receptors

During reproduction in flowering plants, the male gametophyte delivers an immotile male gamete to the female gametophyte in the pistil by formation of pollen tubes. In Arabidopsis thaliana, two synergid cells situated on either side of the egg cell produce cysteine-rich chemoattractant peptide LURE that guides the pollen tube to the female gametophyte for sexual reproduction. Recently, in Arabidopsis thaliana, Pollen Receptor Kinase 3 (PRK3), along with PRK1, PRK6, and PRK8, have been predicted to be the receptors responsible for sensing LURE. These receptors belong to the Leucine Rich Repeat Receptor Like Kinases (LRR-RLKs), the largest family of receptor kinases found in Arabidopsis thaliana. How PRKs regulate the growth and development of the pollen tube remains elusive. In order to better understand the PRK-mediated signaling mechanism in pollen tube growth and guidance, we have determined the crystal structure of the extracellular domain (ecd) of PRK3 at 2.5 Å, which resembles the SERK family of plant co-receptors. The structure of ecdPRK3 is composed of a conserved surface that coincides with the conserved receptor-binding surface of the SERK family of co-receptors. Our structural analyses of PRK3 have provided a template for future functional studies of the PRK family of LRR-RLK receptors in the regulation of pollen tube development.

Analysis of Protein Glycosylation in the ER

Protein N-glycosylation is an essential posttranslational modification which is initiated in the endoplasmic reticulum. In plants, the N-glycans play a pivotal role for protein folding and quality control. Through the interaction of glycan processing and binding reactions mediated by ER-resident glycosidases and specific carbohydrate binding proteins, the N-glycans contribute to the adoption of a native protein conformation. Properly folded glycoproteins are released from these processes and allowed to continue their transit to the Golgi where further processing and maturation of N-glycans leads to the formation of more complex structures with different functions. Incompletely folded glycoproteins are removed from the ER by a highly conserved degradation process to prevent the accumulation or secretion of misfolded proteins and maintain ER homeostasis. Here, we describe methods to analyze the N-glycosylation status and the glycan-dependent ER-associated degradation process in plants.

Plant glyco-biotechnology

Glycosylation is an important protein modification in all eukaryotes. Whereas the early asparagine-linked glycosylation (N-glycosylation) and N-glycan processing steps in the endoplasmic reticulum are conserved between mammals and plants, the maturation of complex N-glycans in the Golgi apparatus differs considerably. Due to a restricted number of Golgi-resident N-glycan processing enzymes and the absence of nucleotide sugars such as CMP-N-acetylneuraminic acid, plants produce only a limited repertoire of different N-glycan structures. Moreover, mammalian mucin-type O-glycosylation of serine or threonine residues has not been described in plants and the required machinery is not encoded in their genome which enables de novo build-up of the pathway. As a consequence, plants are very well-suited for the production of homogenous N- and O-glycans and are increasingly used for the production of recombinant glycoproteins with custom-made glycans that may result in the generation of biopharmaceuticals with improved therapeutic potential.

Scanning for New BRI1 Mutations via TILLING Analysis

The identification and characterization of a mutational spectrum for a specific protein can help to elucidate its detailed cellular functions. BRASSINOSTEROID INSENSITIVE1 (BRI1), a multidomain transmembrane receptor-like kinase, is a major receptor of brassinosteroids in Arabidopsis (Arabidopsis thaliana). Within the last two decades, over 20 different bri1 mutant alleles have been identified, which helped to determine the significance of each domain within BRI1. To further understand the molecular mechanisms of BRI1, we tried to identify additional alleles via targeted induced local lesions in genomes. Here, we report our identification of 83 new point mutations in BRI1, including nine mutations that exhibit an allelic series of typical bri1 phenotypes, from subtle to severe morphological alterations. We carried out biochemical analyses to investigate possible mechanisms of these mutations in affecting brassinosteroid signaling. A number of interesting mutations have been isolated via this study. For example, bri1-702, the only weak allele identified so far with a mutation in the activation loop, showed reduced autophosphorylation activity. bri1-705, a subtle allele with a mutation in the extracellular portion, disrupts the interaction of BRI1 with its ligand brassinolide and coreceptor BRI1-ASSOCIATED RECEPTOR KINASE1. bri1-706, with a mutation in the extracellular portion, is a subtle defective mutant. Surprisingly, root inhibition analysis indicated that it is largely insensitive to exogenous brassinolide treatment. In this study, we found that bri1-301 possesses kinase activity in vivo, clarifying a previous report arguing that kinase activity may not be necessary for the function of BRI1. These data provide additional insights into our understanding of the early events in the brassinosteroid signaling pathway.

New Insights on Leucine-Rich Repeats Receptor-Like Kinase Orthologous Relationships in Angiosperms

Leucine-Rich Repeats Receptor-Like Kinase (LRR-RLK) genes represent a large and complex gene family in plants, mainly involved in development and stress responses. These receptors are composed of an LRR-containing extracellular domain (ECD), a transmembrane domain (TM) and an intracellular kinase domain (KD). To provide new perspectives on functional analyses of these genes in model and non-model plant species, we performed a phylogenetic analysis on 8,360 LRR-RLK receptors in 31 angiosperm genomes (8 monocots and 23 dicots). We identified 101 orthologous groups (OGs) of genes being conserved among almost all monocot and dicot species analyzed. We observed that more than 10% of these OGs are absent in the Brassicaceae species studied. We show that the ECD structural features are not always conserved among orthologs, suggesting that functions may have diverged in some OG sets. Moreover, we looked at targets of positive selection footprints in 12 pairs of OGs and noticed that depending on the subgroups, positive selection occurred more frequently either in the ECDs or in the KDs.

N-glycan containing a core α1,3-fucose residue is required for basipetal auxin transport and gravitropic response in rice (Oryza sativa)

In plants, α1,3-fucosyltransferase (FucT) catalyzes the transfer of fucose from GDP-fucose to asparagine-linked GlcNAc of the N-glycan core in the medial Golgi. To explore the physiological significance of this processing, we isolated two Oryza sativa (rice) mutants (fuct-1 and fuct-2) with loss of FucT function. Biochemical analyses of the N-glycan structure confirmed that α1,3-fucose is missing from the N-glycans of allelic fuct-1 and fuct-2. Compared with the wild-type cv Kitaake, fuct-1 displayed a larger tiller angle, shorter internode and panicle lengths, and decreased grain filling as well as an increase in chalky grains with abnormal shape. The mutant allele fuct-2 gave rise to similar developmental abnormalities, although they were milder than those of fuct-1. Restoration of a normal tiller angle in fuct-1 by complementation demonstrated that the phenotype is caused by the loss of FucT function. Both fuct-1 and fuct-2 plants exhibited reduced gravitropic responses. Expression of the genes involved in tiller and leaf angle control was also affected in the mutants. We demonstrate that reduced basipetal auxin transport and low auxin accumulation at the base of the shoot in fuct-1 account for both the reduced gravitropic response and the increased tiller angle.

Managing the protein folding demands in the endoplasmic reticulum of plants

Endoplasmic reticulum (ER) stress occurs in plants during certain developmental stages or under adverse environmental conditions, as a result of the accumulation of unfolded or misfolded proteins in the ER. To minimize the accumulation of misfolded proteins in the ER, a protein quality control (PQC) system monitors protein folding and eliminates misfolded proteins through either ER-associated protein degradation (ERAD) or autophagy. ER stress elicits the unfolded protein response (UPR), which enhances the operation in plant cells of the ER protein folding machinery and the PQC system. The UPR also reduces protein folding demands in the ER by degrading mRNAs encoding secretory proteins. In plants subjected to severe or chronic stress, UPR promotes programmed cell death (PCD). Progress in the field in recent years has provided insights into the regulatory networks and signaling mechanisms of the ER stress responses in plants. In addition, novel physiological functions of the ER stress responses in plants for coordinating plant growth and development with changing environment have been recently revealed.

HRD1-mediated ERAD tuning of ER-bound E2 is conserved between plants and mammals

When membrane proteins and secretory proteins are misfolded or incompletely folded, they are retained in the endoplasmic reticulum (ER) for further folding or degradation. The HMG-COA reductase degradation 1 (HRD1) and degradation of alpha2 10 (DOA10) complexes are two major components involved in the ER-associated protein degradation (ERAD) system in eukaryotic organisms(1-4). However, the relationship between these two complexes is largely unknown, especially in higher eukaryotes. Here, we report that the plant ubiquitin-conjugating enzyme 32 (UBC32), an ER-bound E2 working in the DOA10 complex, is maintained at low levels under standard conditions by proteasome-dependent degradation mediated by the HRD1 complex, the other E3 complex involved in ERAD. Loss of this negative regulation under ER stress increases capacity for degradation of misfolded proteins retained in the ER. Consistently, UBE2J1, the homologue of UBC32 in mammals, was also identified to be targeted by HRD1 for degradation. Taken together, these results suggest that the regulation of UBC32 (or UBE2J1) by the HRD1 complex is conserved between plants and mammals.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

Plant protein glycosylation

Protein glycosylation is an essential co- and post-translational modification of secretory and membrane proteins in all eukaryotes. The initial steps of N-glycosylation and N-glycan processing are highly conserved between plants, mammals and yeast. In contrast, late N-glycan maturation steps in the Golgi differ significantly in plants giving rise to complex N-glycans with β1,2-linked xylose, core α1,3-linked fucose and Lewis A-type structures. While the essential role of N-glycan modifications on distinct mammalian glycoproteins is already well documented, we have only begun to decipher the biological function of this ubiquitous protein modification in different plant species. In this review, I focus on the biosynthesis and function of different protein N-linked glycans in plants. Special emphasis is given on glycan-mediated quality control processes in the ER and on the biological role of characteristic complex N-glycan structures.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

Comprehensive analysis of the endoplasmic reticulum stress response in the soybean genome: conserved and plant-specific features

BACKGROUND

Despite the relevance of the eukaryotic endoplasmic reticulum (ER)-stress response as an integrator of multiple stress signals into an adaptive response, knowledge about these ER-mediated cytoprotective pathways in soybean (Glycine max) is lacking. Here, we searched for genes involved in the highly conserved unfolded protein response (UPR) and ER stress-induced plant-specific cell death signaling pathways in the soybean genome.

METHODS

Previously characterized Arabidopsis UPR genes were used as prototypes for the identification of the soybean orthologs and the in silico assembly of the UPR in soybean, using eggNOG v4.0 software. Functional studies were also conducted by analyzing the transcriptional activity of soybean UPR transducers.

RESULTS

As a result of this search, we have provided a complete profile of soybean UPR genes with significant predicted protein similarities to A. thaliana UPR-associated proteins. Both arms of the plant UPR were further examined functionally, and evidence is presented that the soybean counterparts are true orthologs of previously characterized UPR transducers in Arabidopsis. The bZIP17/bZI28 orthologs (GmbZIP37 and GmbZIP38) and ZIP60 ortholog (GmbZIP68) from soybean have similar structural organizations as their Arabidopsis counterparts, were induced by ER stress and activated an ERSE- and UPRE-containing BiP promoter. Furthermore, the transcript of the putative substrate of GmIREs, GmbZIP68, harbors a canonical site for IRE1 endonuclease activity and was efficiently spliced under ER stress conditions. In a reverse approach, we also examined the Arabidopsis genome for components of a previously characterized ER stress-induced cell death signaling response in soybean. With the exception of GmERD15, which apparently does not possess an Arabidopsis ortholog, the Arabidopsis genome harbors conserved GmNRP, GmNAC81, GmNAC30 and GmVPE sequences that share significant structural and sequence similarities with their soybean counterparts. These results suggest that the NRP/GmNAC81 + GmNAC30/VPE regulatory circuit may transduce cell death signals in plant species other than soybean.

CONCLUSIONS

Our in silico analyses, along with current and previous functional data, permitted generation of a comprehensive overview of the ER stress response in soybean as a framework for functional prediction of ER stress signaling components and their possible connections with multiple stress responses.

EBS7 is a plant-specific component of a highly conserved endoplasmic reticulum-associated degradation system in Arabidopsis

Endoplasmic reticulum (ER)-associated degradation (ERAD) is an essential part of an ER-localized protein quality-control system for eliminating terminally misfolded proteins. Recent studies have demonstrated that the ERAD machinery is conserved among yeast, animals, and plants; however, it remains unknown if the plant ERAD system involves plant-specific components. Here we report that the Arabidopsis ethyl methanesulfonate-mutagenized brassinosteroid-insensitive 1 suppressor 7 (EBS7) gene encodes an ER membrane-localized ERAD component that is highly conserved in land plants. Loss-of-function ebs7 mutations prevent ERAD of brassinosteroid insensitive 1-9 (bri1-9) and bri1-5, two ER-retained mutant variants of the cell-surface receptor for brassinosteroids (BRs). As a result, the two mutant receptors accumulate in the ER and consequently leak to the plasma membrane, resulting in the restoration of BR sensitivity and phenotypic suppression of the bri1-9 and bri1-5 mutants. EBS7 accumulates under ER stress, and its mutations lead to hypersensitivity to ER and salt stresses. EBS7 interacts with the ER membrane-anchored ubiquitin ligase Arabidopsis thaliana HMG-CoA reductase degradation 1a (AtHrd1a), one of the central components of the Arabidopsis ERAD machinery, and an ebs7 mutation destabilizes AtHrd1a to reduce polyubiquitination of bri1-9. Taken together, our results uncover a plant-specific component of a plant ERAD pathway and also suggest its likely biochemical function.

The effects of induced production of reactive oxygen species in organelles on endoplasmic reticulum stress and on the unfolded protein response in arabidopsis

BACKGROUND AND AIMS

Accumulation of unfolded proteins caused by inefficient chaperone activity in the endoplasmic reticulum (ER) is termed 'ER stress', and it is perceived by a complex gene network. Induction of these genes triggers a response termed the 'unfolded protein response' (UPR). If a cell cannot overcome the accumulation of unfolded proteins, the ER-associated degradation (ERAD) system is induced to degrade those proteins. In addition to other factors, reactive oxygen species (ROS) are also produced during oxidative protein-folding in the ER. It has been shown in animal systems that there is a tight association between mitochondrial ROS and ER stress. However, in plants there are no reports concerning how induced ROS production in mitochondria and chloroplasts affects ER stress and if there is a possible role of organelle-originated ROS as a messenger molecule in the unfolded protein response. To address this issue, electron transport in chloroplasts and mitochondria and carnitine acetyl transferase (CAT) activity in peroxisomes were inhibited in wild-type Arabidopsis thaliana to induce ROS production. Expression of UPR genes was then investigated.

METHODS

Plants of A. thaliana ecotype Col-0 were treated with various H2O2- and ROS-producing agents specific to different organelles, including the mitochondria, chloroplasts and peroxisomes. The expression of ER stress sensor/transducer genes (bZIP28, bZIP17, IRE1A, IRE1B, BiP1, BiP3), genes related to protein folding (CNX, ERO1) and ERAD genes (HRD1, SEL1, DER1, UBC32) were evaluated by qRT-PCR analysis.

KEY RESULTS

Relatively low concentrations of ROS were more effective for induction of the ER stress response. Mitochondrial and chloroplastic ROS production had different induction mechanisms for the UPR and ER stress responses.

CONCLUSIONS

Chloroplast- and mitochondria-originated ROS have distinct roles in triggering the ER stress response. In general, low concentrations of ROS induced the transcription of ER stress-related genes, which can be attributed to the roles of ROS as secondary messengers. This is the first time that ROS production in organelles has been shown to affect the ER stress response in a plant system.

The molecular circuitry of brassinosteroid signaling

Because they are tethered in space, plants have to make the most of their local growth environment. In order to grow in an ever-changing environment, plants constantly remodel their shapes. This adaptive attribute requires the orchestration of complex environmental signals at the cellular and organismal levels. A battery of small molecules, classically known as phytohormones, allows plants to change their body plan by using highly integrated signaling networks and transcriptional cascades. Amongst these hormones, brassinosteroids (BRs), the polyhydroxylated steroid of plants, influence plant responsiveness to the local environment and exquisitely promote, or interfere with, many aspects of plant development. The molecular circuits that wire steroid signals at the cell surface to the promoters of thousands of genes in the nucleus have been defined in the past decade. This review recapitulates how the transduction of BR signals impacts the temporally unfolding programs of plant growth. First, we summarize the paradigmatic BR signaling pathway acting primarily in cellular expansion. Secondly, we describe the current wiring diagram and the temporal dynamics of the BR signal transduction network. And finally we provide an overview of how key players in BR signaling act as molecular gates to transduce BR signals onto other signaling pathways.

A context-independent N-glycan signal targets the misfolded extracellular domain of Arabidopsis STRUBBELIG to endoplasmic-reticulum-associated degradation

N-glycosylation of proteins plays an important role in the determination of the fate of newly synthesized glycoproteins in the endoplasmic reticulum (ER). Specific oligosaccharide structures recruit molecular chaperones that promote folding or mannose-binding lectins that assist in the clearance of improperly-folded glycoproteins by delivery to ER-associated degradation (ERAD). In plants, the mechanisms and factors that recognize non-native proteins and sort them to ERAD are poorly understood. In the present study, we provide evidence that a misfolded variant of the STRUBBELIG (SUB) extracellular domain (SUBEX-C57Y) is degraded in a glycan-dependent manner in plants. SUBEX-C57Y is an ER-retained glycoprotein with three N-glycans that is stabilized in the presence of kifunensine, a potent inhibitor of α-mannosidases. Stable expression in Arabidopsis thaliana knockout mutants revealed that SUBEX-C57Y degradation is dependent on the ER lectin OS9 and its associated ERAD factor SEL1L. SUBEX-C57Y was also stabilized in plants lacking the α-mannosidases MNS4 and MNS5 that generate a terminal α1,6-linked mannose on the C-branch of N-glycans. Notably, the glycan signal for degradation is not constrained to a specific position within SUBEX-C57Y. Structural analysis revealed that SUBEX-C57Y harbours considerable amounts of Glc1Man7GlcNAc2 N-glycans suggesting that the ER-quality control processes involving calnexin/calreticulin (CNX/CRT) and ERAD are tightly interconnected to promote protein folding or disposal by termination of futile folding attempts.

FLS2-BAK1 extracellular domain interaction sites required for defense signaling activation

Signaling initiation by receptor-like kinases (RLKs) at the plasma membrane of plant cells often requires regulatory leucine-rich repeat (LRR) RLK proteins such as SERK or BIR proteins. The present work examined how the microbe-associated molecular pattern (MAMP) receptor FLS2 builds signaling complexes with BAK1 (SERK3). We first, using in vivo methods that validate separate findings by others, demonstrated that flg22 (flagellin epitope) ligand-initiated FLS2-BAK1 extracellular domain interactions can proceed independent of intracellular domain interactions. We then explored a candidate SERK protein interaction site in the extracellular domains (ectodomains; ECDs) of the significantly different receptors FLS2, EFR (MAMP receptors), PEPR1 (damage-associated molecular pattern (DAMP) receptor), and BRI1 (hormone receptor). Repeat conservation mapping revealed a cluster of conserved solvent-exposed residues near the C-terminus of models of the folded LRR domains. However, site-directed mutagenesis of this conserved site in FLS2 did not impair FLS2-BAK1 ECD interactions, and mutations in the analogous site of EFR caused receptor maturation defects. Hence this conserved LRR C-terminal region apparently has functions other than mediating interactions with BAK1. In vivo tests of the subsequently published FLS2-flg22-BAK1 ECD co-crystal structure were then performed to functionally evaluate some of the unexpected configurations predicted by that crystal structure. In support of the crystal structure data, FLS2-BAK1 ECD interactions were no longer detected in in vivo co-immunoprecipitation experiments after site-directed mutagenesis of the FLS2 BAK1-interaction residues S554, Q530, Q627 or N674. In contrast, in vivo FLS2-mediated signaling persisted and was only minimally reduced, suggesting residual FLS2-BAK1 interaction and the limited sensitivity of co-immunoprecipitation data relative to in vivo assays for signaling outputs. However, Arabidopsis plants expressing FLS2 with the Q530A+Q627A double mutation were impaired both in detectable interaction with BAK1 and in FLS2-mediated responses, lending overall support to current models of FLS2 structure and function.