A transporter's doom or destiny: SLC6A1 in health and disease, novel molecular targets and emerging therapeutic prospects

As the first member of the solute carrier 6 (SLC6) protein family, the γ-aminobutyric acid (GABA) transporter 1 (GAT1, SLC6A1), plays a pivotal role in the uptake of GABA from the synaptic cleft into neurons and astrocytes. This process facilitates the subsequent storage of GABA in presynaptic vesicles. The human SLC6A1 gene is highly susceptible to missense mutations, leading to severe clinical outcomes, such as epilepsy, in the afflicted patients. The molecular mechanisms of SLC6A1-associated disorders are discerned to some degree; many SLC6A1 mutations are now known to impair protein folding, and consequently fail to reach the plasma membrane. Inherently, once inside the endoplasmic reticulum (ER), GAT1 abides by a complex cascade of events that enable efficient intracellular trafficking. This involves association with specialized molecular chaperones responsible for steering the protein folding process, oligomerization, sorting through the Golgi apparatus, and ultimately delivery to the cell surface. The entire process is subject to stringent quality control mechanisms at multiple checkpoints. While the majority of the existing loss-of-function SLC6A1 variants interfere with folding and membrane targeting, certain mutants retain abundant surface expression. In either scenario, suppressed GAT1 activity disrupts GABAergic neurotransmission, preceding the disease manifestation in individuals harboring these mutations. The nervous system is enthralling and calls for systematic, groundbreaking research efforts to dissect the precise molecular factors associated with the onset of complex neurological disorders, and uncover additional non-canonical therapeutic targets. Recent research has given hope for some of the misfolded SLC6A1 variants, which can be salvaged by small molecules, i.e., chemical and pharmacological chaperones, acting on multiple upstream targets in the secretory pathway. We here highlight the significance of pharmacochaperoning as a therapeutic strategy for the treatment of SLC6A1-related disorders.

FERONIA: A Receptor Kinase at the Core of a Global Signaling Network

Initially identified as a key regulator of female fertility in Arabidopsis, the FERONIA (FER) receptor kinase is now recognized as crucial for almost all aspects of plant growth and survival. FER partners with a glycosylphosphatidylinositol-anchored protein of the LLG family to act as coreceptors on the cell surface. The FER-LLG coreceptor interacts with different RAPID ALKALINIZATION FACTOR (RALF) peptide ligands to function in various growth and developmental processes and to respond to challenges from the environment. The RALF-FER-LLG signaling modules interact with molecules in the cell wall, cell membrane, cytoplasm, and nucleus and mediate an interwoven signaling network. Multiple FER-LLG modules, each anchored by FER or a FER-related receptor kinase, have been studied, illustrating the functional diversity and the mechanistic complexity of the FER family signaling modules. The challenges going forward are to distill from this complexity the unifying schemes where possible and attain precision and refinement in the knowledge of critical details upon which future investigations can be built. By focusing on the extensively characterized FER, this review provides foundational information to guide the next phase of research on FER in model as well as crop species and potential applications for improving plant growth and resilience.

Potential Involvement of the South American Lungfish Intelectin-2 in Innate-Associated Immune Modulation

Intelectins belong to a family of lectins with specific and transitory carbohydrate interaction capabilities. These interactions are related to the activity of agglutinating pathogens, as intelectins play a significant role in immunity. Despite the prominent immune defense function of intelectins, limited information about its structural characteristics and carbohydrate interaction properties is available. This study investigated an intelectin transcript identified in RNA-seq data obtained from the South American lungfish (Lepidosiren paradoxa), namely LpITLN2-B. The structural analyses predicted LpITLN2-B to be a homo-trimeric globular protein with the fibrinogen-like functional domain (FReD), exhibiting a molecular mass of 57 kDa. The quaternary structure is subdivided into three monomers, A, B, and C, and each domain comprises 11 β-sheets: an anti-parallel β-sheet, a β-hairpin, and a disordered β-sheet structure. Molecular docking demonstrates a significant interaction with disaccharides rather than monosaccharides. The preferential interaction with disaccharides highlights the potential interaction with pathogen molecules, such as LPS and Poly(I:C). The hemagglutination assay inhibited lectins activity, especially maltose and sucrose, highlighting lectin activity in L. paradoxa samples. Overall, our results show the potential relevance of LpITLN2-B in L. paradoxa immune defense against pathogens.

Differential expansion and retention patterns of LRR-RLK genes across plant evolution

To maximize overall fitness, plants must accurately respond to a host of growth, developmental, and environmental signals throughout their life. Many of these internal and external signals are perceived by the leucine-rich repeat receptor-like kinases, which play roles in regulating growth, development, and immunity. This largest family of receptor kinases in plants can be divided into subfamilies based on the conservation of the kinase domain, which demonstrates that shared evolutionary history often indicates shared molecular function. Here we investigate the evolutionary history of this family across the evolution of 112 plant species. We identify lineage-specific expansions of the malectin-domain containing subfamily LRR subfamily I primarily in the Brassicales and bryophytes. Most other plant lineages instead show a large expansion in LRR subfamily XII, which in Arabidopsis is known to contain key receptors in pathogen perception. This striking asymmetric expansion may reveal a dichotomy in the evolutionary history and adaptation strategies employed by plants. A greater understanding of the evolutionary pressures and adaptation strategies acting on members of this receptor family offers a way to improve functional predictions for orphan receptors and simplify the identification of novel stress-related receptors.

Signals and Their Perception for Remodelling, Adjustment and Repair of the Plant Cell Wall

The integrity of the cell wall is important for plant cells. Mechanical or chemical distortions, tension, pH changes in the apoplast, disturbance of the ion homeostasis, leakage of cell compounds into the apoplastic space or breakdown of cell wall polysaccharides activate cellular responses which often occur via plasma membrane-localized receptors. Breakdown products of the cell wall polysaccharides function as damage-associated molecular patterns and derive from cellulose (cello-oligomers), hemicelluloses (mainly xyloglucans and mixed-linkage glucans as well as glucuronoarabinoglucans in Poaceae) and pectins (oligogalacturonides). In addition, several types of channels participate in mechanosensing and convert physical into chemical signals. To establish a proper response, the cell has to integrate information about apoplastic alterations and disturbance of its wall with cell-internal programs which require modifications in the wall architecture due to growth, differentiation or cell division. We summarize recent progress in pattern recognition receptors for plant-derived oligosaccharides, with a focus on malectin domain-containing receptor kinases and their crosstalk with other perception systems and intracellular signaling events.

Arabidopsis immune responses triggered by cellulose- and mixed-linked glucan-derived oligosaccharides require a group of leucine-rich repeat malectin receptor kinases

The plant immune system perceives a diversity of carbohydrate ligands from plant and microbial cell walls through the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs), which activate pattern-triggered immunity (PTI). Among these ligands are oligosaccharides derived from mixed-linked β-1,3/β-1,4-glucans (MLGs; e.g. β-1,4-D-(Glc)₂ -β-1,3-D-Glc, MLG43) and cellulose (e.g. β-1,4-D-(Glc)₃ , CEL3). The mechanisms behind carbohydrate perception in plants are poorly characterized except for fungal chitin oligosaccharides (e.g. β-1,4-d-(GlcNAc)₆ , CHI6), which involve several receptor kinase proteins (RKs) with LysM-ECDs. Here, we describe the isolation and characterization of Arabidopsis thaliana mutants impaired in glycan perception (igp) that are defective in PTI activation mediated by MLG43 and CEL3, but not by CHI6. igp1-igp4 are altered in three RKs - AT1G56145 (IGP1), AT1G56130 (IGP2/IGP3) and AT1G56140 (IGP4) - with leucine-rich-repeat (LRR) and malectin (MAL) domains in their ECDs. igp1 harbors point mutation E906K and igp2 and igp3 harbor point mutation G773E in their kinase domains, whereas igp4 is a T-DNA insertional loss-of-function mutant. Notably, isothermal titration calorimetry (ITC) assays with purified ECD-RKs of IGP1 and IGP3 showed that IGP1 binds with high affinity to CEL3 (with dissociation constant K_D  = 1.19 ± 0.03 μm) and cellopentaose (K_D  = 1.40 ± 0.01 μM), but not to MLG43, supporting its function as a plant PRR for cellulose-derived oligosaccharides. Our data suggest that these LRR-MAL RKs are components of a recognition mechanism for both cellulose- and MLG-derived oligosaccharide perception and downstream PTI activation in Arabidopsis.

Genomic reconstruction and features of glycosylation pathways in the apicomplexan Cryptosporidium parasites

Cryptosporidium is a genus of apicomplexan parasites infecting humans or other vertebrates. The majority of the Cryptosporidium species live in host intestines (e.g., C. parvum, C. hominis and C. ubiquitum), but there are a few gastric species (e.g., C. muris and C. andersoni). Among them, C. parvum is the most important zoonotic species, for which a number of glycoproteins have been reported for being involved in the interacting with host cells. However, little is known on the cryptosporidium glycobiology. Information on the glycosylation pathways in Cryptosporidium parasites remains sketchy and only a few studies have truly determined the glycoforms in the parasites. Here we reanalyzed the Cryptosporidium genomes and reconstructed the glycosylation pathways, including the synthesis of N- and O-linked glycans and GPI-anchors. In N-glycosylation, intestinal Cryptosporidium possesses enzymes to make a simple precursor with two terminal glucoses on the long arm (i.e., Glc2Man5GlcNAc2 vs. Glc3Man9GlcNAc2 in humans), but gastric species only makes a simpler precursor containing only the "core" structure (i.e., Man3GlcNAc2). There is an ortholog of glucosidase II (GANAB) in all Cryptosporidium species, for which the authenticity is questioned because it contains no signal peptide and exist in gastric species lacking terminal glucoses for the enzyme to act on. In O-linked glycosylation, all Cryptosporidium species may attach one-unit HexNAc (GalNAc and GlcNAc) and two-unit Fuc-type (Man-Fuc) glycans to the target proteins. Cryptosporidium lacks enzymes to further process N- and O-glycans in the Golgi. The glycosylphosphatidylinositol (GPI)-anchor in Cryptosporidium is predicted to be unbranched and unprocessed further in the Golgi. Cryptosporidium can synthesize limited nucleotide sugars, but possesses at least 12 transporters to scavenge nucleotide sugars or transport them across the ER/Golgi membranes. Overall, Cryptosporidium makes much simpler glycans than the hosts, and the N-glycoforms further differ between intestinal and gastric species. The Cryptosporidium N- and O-glycans are neutrally charged and have limited capacity to absorb water molecules in comparison to the host intestinal mucins that are negatively charged and highly expandable in waters.

CORK1, A LRR-Malectin Receptor Kinase, Is Required for Cellooligomer-Induced Responses in Arabidopsis thaliana

Cell wall integrity (CWI) maintenance is central for plant cells. Mechanical and chemical distortions, pH changes, and breakdown products of cell wall polysaccharides activate plasma membrane-localized receptors and induce appropriate downstream responses. Microbial interactions alter or destroy the structure of the plant cell wall, connecting CWI maintenance to immune responses. Cellulose is the major polysaccharide in the primary and secondary cell wall. Its breakdown generates short-chain cellooligomers that induce Ca²⁺-dependent CWI responses. We show that these responses require the malectin domain-containing CELLOOLIGOMER-RECEPTOR KINASE 1 (CORK1) in Arabidopsis and are preferentially activated by cellotriose (CT). CORK1 is required for cellooligomer-induced cytoplasmic Ca²⁺ elevation, reactive oxygen species (ROS) production, mitogen-associated protein kinase (MAPK) activation, cellulose synthase phosphorylation, and the regulation of CWI-related genes, including those involved in biosynthesis of cell wall material, secondary metabolites and tryptophan. Phosphoproteome analyses identified early targets involved in signaling, cellulose synthesis, the endoplasmic reticulum/Golgi secretory pathway, cell wall repair and immune responses. Two conserved phenylalanine residues in the malectin domain are crucial for CORK1 function. We propose that CORK1 is required for CWI and immune responses activated by cellulose breakdown products.

Cell Wall Signaling in Plant Development and Defense

Plant architecture fundamentally differs from that of other multicellular organisms in that individual cells serve as osmotic bricks, defined by the equilibrium between the internal turgor pressure and the mechanical resistance of the surrounding cell wall, which constitutes the interface between plant cells and their environment. The state and integrity of the cell wall are constantly monitored by cell wall surveillance pathways, which relay information to the cell interior. A recent surge of discoveries has led to significant advances in both mechanistic and conceptual insights into a multitude of cell wall response pathways that play diverse roles in the development, defense, stress response, and maintenance of structural integrity of the cell. However, these advances have also revealed the complexity of cell wall sensing, and many more questions remain to be answered, for example, regarding the mechanisms of cell wall perception, the molecular players in this process, and how cell wall-related signals are transduced and integrated into cellular behavior. This review provides an overview of the mechanistic and conceptual insights obtained so far and highlights areas for future discoveries in this exciting area of plant biology.

Malectin/Malectin-like domain-containing proteins: A repertoire of cell surface molecules with broad functional potential

Cell walls are at the front line of interactions between walled-organisms and their environment. They support cell expansion, ensure cell integrity and, for multicellular organisms such as plants, they provide cell adherence, support cell shape morphogenesis and mediate cell-cell communication. Wall-sensing, detecting perturbations in the wall and signaling the cell to respond accordingly, is crucial for growth and survival. In recent years, plant signaling research has suggested that a large family of receptor-like kinases (RLKs) could function as wall sensors partly because their extracellular domains show homology with malectin, a diglucose binding protein from the endoplasmic reticulum of animal cells. Studies of several malectin/malectin-like (M/ML) domain-containing RLKs (M/MLD-RLKs) from the model plant Arabidopsis thaliana have revealed an impressive array of biological roles, controlling growth, reproduction and stress responses, processes that in various ways rely on or affect the cell wall. Malectin homologous sequences are widespread across biological kingdoms, but plants have uniquely evolved a highly expanded family of proteins with ML domains embedded within various protein contexts. Here, we present an overview on proteins with malectin homologous sequences in different kingdoms, discuss the chromosomal organization of Arabidopsis M/MLD-RLKs and the phylogenetic relationship between these proteins from several model and crop species. We also discuss briefly the molecular networks that enable the diverse biological roles served by M/MLD-RLKs studied thus far.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

N-Glycan Biosynthesis: Basic Principles and Factors Affecting Its Outcome

Carbohydrate chains are the most abundant and diverse of nature's biopolymers and represent one of the four fundamental macromolecular building blocks of life together with proteins, nucleic acids, and lipids. Indicative of their essential roles in cells and in multicellular organisms, genes encoding proteins associated with glycosylation account for approximately 2% of the human genome. It has been estimated that 50-80% of all human proteins carry carbohydrate chains-glycans-as part of their structure. Despite cells utilize only nine different monosaccharides for making their glycans, their order and conformational variation in glycan chains together with chain branching differences and frequent post-synthetic modifications can give rise to an enormous repertoire of different glycan structures of which few thousand is estimated to carry important structural or functional information for a cell. Thus, glycans are immensely versatile encoders of multicellular life. Yet, glycans do not represent a random collection of unpredictable structures but rather, a collection of predetermined but still dynamic entities that are present at defined quantities in each glycosylation site of a given protein in a cell, tissue, or organism.In this chapter, we will give an overview of what is currently known about N-glycan synthesis in higher eukaryotes, focusing not only on the processes themselves but also on factors that will affect or can affect the final outcome-the dynamicity and heterogeneity of the N-glycome. We hope that this review will help understand the molecular details underneath this diversity, and in addition, be helpful for those who plan to produce optimally glycosylated antibody-based therapeutics.

Genome-Wide Identification of Populus Malectin/Malectin-Like Domain-Containing Proteins and Expression Analyses Reveal Novel Candidates for Signaling and Regulation of Wood Development

Malectin domain (MD) is a ligand-binding protein motif of pro- and eukaryotes. It is particularly abundant in Viridiplantae, where it occurs as either a single (MD, PF11721) or tandemly duplicated domain (PF12819) called malectin-like domain (MLD). In herbaceous plants, MD- or MLD-containing proteins (MD proteins) are known to regulate development, reproduction, and resistance to various stresses. However, their functions in woody plants have not yet been studied. To unravel their potential role in wood development, we carried out genome-wide identification of MD proteins in the model tree species black cottonwood (Populus trichocarpa), and analyzed their expression and co-expression networks. P. trichocarpa had 146 MD genes assigned to 14 different clades, two of which were specific to the genus Populus. 87% of these genes were located on chromosomes, the rest being associated with scaffolds. Based on their protein domain organization, and in agreement with the exon-intron structures, the MD genes identified here could be classified into five superclades having the following domains: leucine-rich repeat (LRR)-MD-protein kinase (PK), MLD-LRR-PK, MLD-PK (CrRLK1L), MLD-LRR, and MD-Kinesin. Whereas the majority of MD genes were highly expressed in leaves, particularly under stress conditions, eighteen showed a peak of expression during secondary wall formation in the xylem and their co-expression networks suggested signaling functions in cell wall integrity, pathogen-associated molecular patterns, calcium, ROS, and hormone pathways. Thus, P. trichocarpa MD genes having different domain organizations comprise many genes with putative foliar defense functions, some of which could be specific to Populus and related species, as well as genes with potential involvement in signaling pathways in other tissues including developing wood.

Mechanisms of productive folding and endoplasmic reticulum-associated degradation of glycoproteins and non-glycoproteins

BACKGROUND

The quality of proteins destined for the secretory pathway is ensured by two distinct mechanisms in the endoplasmic reticulum (ER): productive folding of newly synthesized proteins, which is assisted by ER-localized molecular chaperones and in most cases also by disulfide bond formation and transfer of an oligosaccharide unit; and ER-associated degradation (ERAD), in which proteins unfolded or misfolded in the ER are recognized and processed for delivery to the ER membrane complex, retrotranslocated through the complex with simultaneous ubiquitination, extracted by AAA-ATPase to the cytosol, and finally degraded by the proteasome.

SCOPE OF REVIEW

We describe the mechanisms of productive folding and ERAD, with particular attention to glycoproteins versus non-glycoproteins, and to yeast versus mammalian systems.

MAJOR CONCLUSION

Molecular mechanisms of the productive folding of glycoproteins and non-glycoproteins mediated by molecular chaperones and protein disulfide isomerases are well conserved from yeast to mammals. Additionally, mammals have gained an oligosaccharide structure-dependent folding cycle for glycoproteins. The molecular mechanisms of ERAD are also well conserved from yeast to mammals, but redundant expression of yeast orthologues in mammals has been encountered, particularly for components involved in recognition and processing of glycoproteins and components of the ER membrane complex involved in retrotranslocation and simultaneous ubiquitination of glycoproteins and non-glycoproteins. This may reflect an evolutionary consequence of increasing quantity or quality needs toward mammals.

GENERAL SIGNIFICANCE

The introduction of innovative genome editing technology into analysis of the mechanisms of mammalian ERAD, as exemplified here, will provide new insights into the pathogenesis of various diseases.

Blood coagulation factor VIII D1241E polymorphism leads to a weak malectin interaction and reduction of factor VIII posttranslational modification and secretion

Blood coagulation factor VIII (FVIII) is a key cofactor in regulation of blood coagulation. This study investigated the mechanism by which FVIII is translated and transported into the endoplasmic reticulum (ER) and processed in the Golgi apparatus before secretion using an in vitro cell model. HEK-293T cells were transfected with vectors carrying wild-type (WT) FVIII or polymorphic FVIII D1241E for coexpression with ER lectins and treatment with tunicamycin (an N-linked glycosylation inhibitor), 1-deoxynojirimycin (an alpha-glucosidase inhibitor), endoglycosidase H, or MG132 (Cbz-Leu-Leu-leucinal; a proteasome inhibitor). The data showed that the minor allele of FVIII D1241E was able to reduce FVIII secretion into the conditioned medium but maintain a normal level of procoagulation ability, although both FVIII WT and the minor allele of FVIII D1241E showed similar levels of transcription and translation capacities. Functionally, the D1241E polymorphism led to a reduced level of FVIII in the Golgi apparatus because of its reduced association with malectin, which interacts with newly synthesized glycoproteins in the ER for FVIII folding and trafficking, leading to degradation of the minor allele of FVIII D1241E in the cytosol. This study demonstrated that malectin is important for regulation of the FVIII posttranslational process and that the minor allele of FVIII D1241E had a reduced association with malectin but an increased capacity for proteasomal FVIII degradation. These data imply the role of the ER quality control in future recombinant FVIII development.

Oligosaccharyltransferase structures provide novel insight into the mechanism of asparagine-linked glycosylation in prokaryotic and eukaryotic cells

Asparagine-linked (N-linked) glycosylation is one of the most common protein modification reactions in eukaryotic cells, occurring upon the majority of proteins that enter the secretory pathway. X-ray crystal structures of the single subunit OSTs from eubacterial and archaebacterial organisms revealed the location of donor and acceptor substrate binding sites and provided the basis for a catalytic mechanism. Cryoelectron microscopy structures of the octameric yeast OST provided substantial insight into the organization and assembly of the multisubunit oligosaccharyltransferases. Furthermore, the cryoelectron microscopy structure of a complex consisting of a mammalian OST complex, the protein translocation channel and a translating ribosome revealed new insight into the mechanism of cotranslational glycosylation.

Crystal structures of two tandem malectin-like receptor kinases involved in plant reproduction

Complex cell-to-cell communication between the male pollen tube and the female reproductive organs is required for plant fertilization. A family of Catharanthus roseus receptor kinase 1-like (CrRLK1L) membrane receptors has been genetically implicated in this process. Here, crystal structures of the CrRLK1Ls ANXUR1 and ANXUR2 are reported at 1.48 and 1.1 Å resolution, respectively. The structures reveal a novel arrangement of two malectin-like domains connected by a short β-hairpin linker and stabilized by calcium ions. The canonical carbohydrate-interaction surfaces of related animal and bacterial carbohydrate-binding modules are not conserved in plant CrRLK1Ls. In line with this, the binding of chemically diverse oligosaccharides to ANXUR1 and HERCULES1 could not be detected. Instead, CrRLK1Ls have evolved a protein-protein interface between their malectin domains which forms a deep cleft lined by highly conserved aromatic and polar residues. Analysis of the glycosylation patterns of different CrRLK1Ls and their oligomeric states suggests that this cleft could resemble a binding site for a ligand required for receptor activation of CrRLK1Ls.

Crystal structures of the extracellular domains of the CrRLK1L receptor-like kinases ANXUR1 and ANXUR2

Catharanthus roseus Receptor-Like Kinase 1-like (CrRLK1L) proteins contain two tandem malectin-like modules in their extracellular domains (ECDs) and function in diverse signaling pathways in plants. Malectin is a carbohydrate-binding protein in animals and recognizes a number of diglucosides; however, it remains unclear how the two malectin-like domains in the CrRLK1L proteins sense the ligand molecule. In this study, we reveal the crystal structures of the ECDs of ANXUR1 and ANXUR2, two CrRLK1L members in Arabidopsis thaliana that have critical functions in controlling pollen tube rupture during the fertilization process. We show that the two malectin-like domains in these proteins pack together to form a rigid architecture. Unlike animal malectin, these malectin-like domains lack residues involved in binding to the diglucosides, suggesting that they have a distinct ligand-binding mechanism. A cleft is observed between the two malectin-like domains, which might function as a potential ligand-binding pocket.

Compatible topologies and parameters for NMR structure determination of carbohydrates by simulated annealing

The use of NMR methods to determine the three-dimensional structures of carbohydrates and glycoproteins is still challenging, in part because of the lack of standard protocols. In order to increase the convenience of structure determination, the topology and parameter files for carbohydrates in the program Crystallography & NMR System (CNS) were investigated and new files were developed to be compatible with the standard simulated annealing protocols for proteins and nucleic acids. Recalculating the published structures of protein-carbohydrate complexes and glycosylated proteins demonstrates that the results are comparable to the published structures which employed more complex procedures for structure calculation. Integrating the new carbohydrate parameters into the standard structure calculation protocol will facilitate three-dimensional structural study of carbohydrates and glycosylated proteins by NMR spectroscopy.

DC2 and KCP2 mediate the interaction between the oligosaccharyltransferase and the ER translocon

The STT3A isoform of the oligosaccharyltransferase is adjacent to the protein translocation channel to catalyze co-translational N-glycosylation of proteins in the endoplasmic reticulum. Shrimal et al. show that the DC2 and KCP2 subunits of the STT3A isoform of the oligosaccharyltransferase are responsible for mediating the interaction between the STT3A complex and the protein translocation channel to allow co-translational N-glycosylation of proteins.

In metazoan organisms, the STT3A isoform of the oligosaccharyltransferase is localized adjacent to the protein translocation channel to catalyze co-translational N-linked glycosylation of proteins in the endoplasmic reticulum. The mechanism responsible for the interaction between the STT3A complex and the translocation channel has not been addressed. Using genetically modified human cells that are deficient in DC2 or KCP2 proteins, we show that loss of DC2 causes a defect in co-translational N-glycosylation of proteins that mimics an STT3A^−/− phenotype. Biochemical analysis showed that DC2 and KCP2 are responsible for mediating the interaction between the protein translocation channel and the STT3A complex. Importantly, DC2- and KCP2-deficient STT3A complexes are stable and enzymatically active. Deletion mutagenesis revealed that a conserved motif in the C-terminal tail of DC2 is critical for assembly into the STT3A complex, whereas the lumenal loop and the N-terminal cytoplasmic segment are necessary for the functional interaction between the STT3A and Sec61 complexes.

Two FERONIA-Like Receptor Kinases Regulate Apple Fruit Ripening by Modulating Ethylene Production

Ethylene has long been known to be a critical signal controlling the ripening of climacteric fruits; however, the signaling mechanism underlying ethylene production during fruit development is unknown. Here, we report that two FERONIA-like receptor kinases (FERLs) regulate fruit ripening by modulating ethylene production in the climacteric fruit, apple (Malus×domestica). Bioinformatic analysis indicated that the apple genome contains 14 members of the FER family (MdFERL1-17), of these 17 FERLs, MdFERL6 was expressed at the highest level in fruit. Heterologous expression of MdFERL6 or MdFERL1, the apple homolog of Arabidopsis FER, in another climacteric fruit, tomato (Solanum lycopersicum) fruit delayed ripening and suppressed ethylene production. Overexpression and antisense expression of MdFERL6 in apple fruit calli inhibited and promoted ethylene production, respectively. Additionally, virus-induced gene silencing (VIGS) of SlFERL1, the tomato homolog of FER, promoted tomato fruit ripening and ethylene production. Both MdFERL6 and MdFERL1 physically interacted with MdSAMS (S-adenosylmethionine synthase), a key enzyme in the ethylene biosynthesis pathway. MdFERL6 was expressed at high levels during early fruit development, but dramatically declined when fruit ripening commenced, implying that MdFERL6 might limit ethylene production prior to fruit development and the ethylene production burst during fruit ripening. These results indicate that FERLs regulate apple and tomato fruit ripening, shedding light on the molecular mechanisms underlying ripening in climacteric fruit.

A FERONIA-Like Receptor Kinase Regulates Strawberry (Fragaria × ananassa) Fruit Ripening and Quality Formation

Ripening of fleshy fruits is controlled by a series of intricate signaling processes. Here, we report a FERONIA/FER-like receptor kinase, FaMRLK47, that regulates both strawberry (Fragaria × ananassa) fruit ripening and quality formation. Overexpression and RNAi-mediated downregulation of FaMRLK47 delayed and accelerated fruit ripening, respectively. We showed that FaMRLK47 physically interacts with FaABI1, a negative regulator of abscisic acid (ABA) signaling, and demonstrated that FaMRLK47 regulates fruit ripening by modulating ABA signaling, a major pathway governing strawberry fruit ripening. In accordance with these findings, overexpression and RNAi-mediated downregulation of FaMRLK47 caused a decrease and increase, respectively, in the ABA-induced expression of a series of ripening-related genes. Additionally, overexpression and RNAi-mediated downregulation of FaMRLK47 resulted in an increase and decrease in sucrose content, respectively, as compared with control fruits, and respectively promoted and inhibited the expression of genes in the sucrose biosynthesis pathway (FaSS and FaSPS). Collectively, this study demonstrates that FaMRLK47 is an important regulator of strawberry fruit ripening and quality formation, and sheds light on the signaling mechanisms underlying strawberry fruit development and ripening.

Plant cell wall signalling and receptor-like kinases

Communication between the extracellular matrix and the cell interior is essential for all organisms as intrinsic and extrinsic cues have to be integrated to co-ordinate development, growth, and behaviour. This applies in particular to plants, the growth and shape of which is governed by deposition and remodelling of the cell wall, a rigid, yet dynamic, extracellular network. It is thus generally assumed that cell wall surveillance pathways exist to monitor the state of the wall and, if needed, elicit compensatory responses such as altered expression of cell wall remodelling and biosynthesis genes. Here, I highlight recent advances in the field of cell wall signalling in plants, with emphasis on the role of plasma membrane receptor-like kinase complexes. In addition, possible roles for cell wall-mediated signalling beyond the maintenance of cell wall integrity are discussed.

Plant Lectins and Lectin Receptor-Like Kinases: How Do They Sense the Outside?

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.

Insights into Carbohydrate Recognition by 3D Structure Determination of Protein–Carbohydrate Complexes Using NMR

This chapter provides an overview of protein–carbohydrate complex structures determined with NMR spectroscopy and deposited in the Protein Data Bank (PDB). These 14 structures include protein–carbohydrate interactions ranging from nanomolar to millimolar affinities. Two complexes are discussed in detail, one representing a tightly bound complex and one a weak but specific interaction. This review illustrates that NMR spectroscopy is a competitive method for three-dimensional structure determination of protein–carbohydrate complexes, especially in the case of weak interactions. The number of biological functions in which protein–carbohydrate interactions are involved is steadily growing. Essential functions of the immune system such as the distinction between self and non-self, or the resolution of inflammation, involve critical protein–carbohydrate recognition events. It is therefore expected that by providing atomic details, NMR spectroscopy can make a significant contribution in the near future to unexplored pathways of the immune system and of many other biological processes.

Ligand Receptor-Mediated Regulation of Growth in Plants

Growth and development of multicellular organisms are coordinately regulated by various signaling pathways involving the communication of inter- and intracellular components. To form the appropriate body patterns, cellular growth and development are modulated by either stimulating or inhibiting these pathways. Hormones and second messengers help to mediate the initiation and/or interaction of the various signaling pathways in all complex multicellular eukaryotes. In plants, hormones include small organic molecules, as well as larger peptides and small proteins, which, as in animals, act as ligands and interact with receptor proteins to trigger rapid biochemical changes and induce the intracellular transcriptional and long-term physiological responses. During the past two decades, the availability of genetic and genomic resources in the model plant species, Arabidopsis thaliana, has greatly helped in the discovery of plant hormone receptors and the components of signal transduction pathways and mechanisms used by these immobile but highly complex organisms. Recently, it has been shown that two of the most important plant hormones, auxin and abscisic acid (ABA), act through signaling pathways that have not yet been recognized in animals. For example, auxins stimulate cell elongation by bringing negatively acting transcriptional repressor proteins to the proteasome to be degraded, thus unleashing the gene expression program required for increasing cell size. The "dormancy" inducing hormone, ABA, binds to soluble receptor proteins and inhibits a specific class of protein phosphatases (PP2C), which activates phosphorylation signaling leading to transcriptional changes needed for the desiccation of the seeds prior to entering dormancy. While these two hormone receptors have no known animal counterparts, there are also many similarities between animal and plant signaling pathways. For example, in plants, the largest single gene family in the genome is the protein kinase family (approximately 5% of the protein coding genes), although the specific function for only a few dozen of these kinases is clearly established. Recent comparative genomics studies have revealed that parasitic nematodes and pathogenic microbes produce plant peptide hormone mimics that target specific plant plasma membrane receptor-like protein kinases, thus usurping endogenous signaling pathways for their own pathogenic purposes. With biochemical, genetic, and physiological analyses of the regulation of hormone receptor signal pathways, we are thus just now beginning to understand how plants optimize the development of their body shape and cope with constantly changing environmental conditions.

Genome-Wide Identification and Expression Analysis of MRLK Family Genes Associated with Strawberry (Fragaria vesca) Fruit Ripening and Abiotic Stress Responses

Malectin-like domain-containing receptor-like kinases (MRLK) constitute a large and divergent family of proteins in plants; however, little is known about the role of MRLKs in fruit growth and development. In this study, we characterized MRLK family genes in diploid strawberry, Fragaria vesca. Based on an analysis of malectin-like domain and a search in the strawberry genome and NCBI database, we identified 62 FvMRLKs in the strawberry genome, and classified these genes into six subfamilies with distinct malectin domains in the extracellular regions of the encoded proteins. Gene expression analysis indicated that more than 80% of the FvMRLKs were expressed in various tissues, with higher levels in roots than in other organs. Thirty-three FvMRLKs were found to be expressed in fruits during the early stages of development, and over 60% of these exhibited dramatic decreases in expression during fruit growth and development. Moreover, the expression of some FvMRLKs was sensitive to both environmental and internal cues that play critical roles in regulating strawberry fruit development and ripening. Collectively, this study provides valuable insight into the FvMRLKs gene family and its role in regulating strawberry fruit development and ripening.

Conserved Roles of CrRLK1L Receptor-Like Kinases in Cell Expansion and Reproduction from Algae to Angiosperms

Receptor-like kinases (RLKs) are regulators of plant development through allowing cells to sense their extracellular environment. They facilitate detection of local endogenous signals, in addition to external biotic and abiotic stimuli. The Catharanthus roseus RLK1-like (CrRLK1L) protein kinase subfamily, which contains FERONIA, plays a central role in regulating fertilization and in cell expansion mechanisms such as cell elongation and tip growth, as well as having indirect links to plant-pathogen interactions. Several components of CrRLK1L signaling pathways have been identified, including an extracellular ligand, coreceptors, and downstream signaling elements. The presence and abundance of the CrRLK1L proteins in the plant kingdom suggest an origin within the Streptophyta lineage, with a notable increase in prevalence in the seeded land plants. Given the function of the sole CrRLK1L protein in a charophycean alga, the possibility of a conserved role in detection and/or regulation of cell wall integrity throughout the Strephtophytes is discussed. Orthologs of signaling pathway components are also present in extant representatives of non-vascular land plants and early vascular land plants including the liverwort Marchantia polymorpha, the moss Physcomitrella patens and the lycophyte Selaginella moellendorffii. Deciphering the roles in development of the CrRLK1L protein kinases in early diverging land plants will provide insights into their ancestral function, furthering our understanding of this diversified subfamily of receptors in higher plants.

Co- and Post-Translational Protein Folding in the ER

The biophysical rules that govern folding of small, single-domain proteins in dilute solutions are now quite well understood. The mechanisms underlying co-translational folding of multidomain and membrane-spanning proteins in complex cellular environments are often less clear. The endoplasmic reticulum (ER) produces a plethora of membrane and secretory proteins, which must fold and assemble correctly before ER exit - if these processes fail, misfolded species accumulate in the ER or are degraded. The ER differs from other cellular organelles in terms of the physicochemical environment and the variety of ER-specific protein modifications. Here, we review chaperone-assisted co- and post-translational folding and assembly in the ER and underline the influence of protein modifications on these processes. We emphasize how method development has helped advance the field by allowing researchers to monitor the progression of folding as it occurs inside living cells, while at the same time probing the intricate relationship between protein modifications during folding.

N-linked glycosylation and homeostasis of the endoplasmic reticulum

As a major site of protein biosynthesis, homeostasis of the endoplasmic reticulum is critical for cell viability. Asparagine linked glycosylation of newly synthesized proteins by the oligosaccharyltransferase plays a central role in ER homeostasis due to the use of protein-linked oligosaccharides as recognition and timing markers for glycoprotein quality control pathways that discriminate between correctly folded proteins and terminally malfolded proteins destined for ER associated degradation. Recent findings indicate how the oligosaccharyltransferase achieves efficient and accurate glycosylation of the diverse proteins that enter the endoplasmic reticulum. In metazoan organisms two distinct OST complexes cooperate to maximize the glycosylation of nascent proteins. The STT3B complex glycosylates acceptor sites that have been skipped by the translocation channel associated STT3A complex.

Glycosylphosphatidylinositol-anchored proteins as chaperones and co-receptors for FERONIA receptor kinase signaling in Arabidopsis

The Arabidopsis receptor kinase FERONIA (FER) is a multifunctional regulator for plant growth and reproduction. Here we report that the female gametophyte-expressed glycosylphosphatidylinositol-anchored protein (GPI-AP) LORELEI and the seedling-expressed LRE-like GPI-AP1 (LLG1) bind to the extracellular juxtamembrane region of FER and show that this interaction is pivotal for FER function. LLG1 interacts with FER in the endoplasmic reticulum and on the cell surface, and loss of LLG1 function induces cytoplasmic retention of FER, consistent with transport of FER from the endoplasmic reticulum to the plasma membrane in a complex with LLG1. We further demonstrate that LLG1 is a component of the FER-regulated RHO GTPase signaling complex and that fer and llg1 mutants display indistinguishable growth, developmental and signaling phenotypes, analogous to how lre and fer share similar reproductive defects. Together our results support LLG1/LRE acting as a chaperone and co-receptor for FER and elucidate a mechanism by which GPI-APs enable the signaling capacity of a cell surface receptor.

DOI:http://dx.doi.org/10.7554/eLife.06587.001

Plants respond to changes in their environment by altering how they grow and when they reproduce. A protein called FERONIA is found in most types of cells and regulates many of the processes that drive these responses, such as cell growth and communication between male and female cells. FERONIA sits in the membrane that surrounds the cell, where it can detect molecules in the cell wall and from outside the cell, and send signals to locations within the cell. However, it is not clear how FERONIA is able to specifically regulate different processes to produce the right response in a particular cell at a particular time.

A family of proteins called glycosylphosphatidylinositol-anchored proteins (GPI-APs for short) play important roles in plants, animals, and other eukaryotic organisms. Li et al. studied FERONIA and two closely related GPI-APs called LLG1—which is produced in seedlings, and LORELEI, which is only found in female sex cells. The experiments show that plants missing either LLG1 or FERONIA had similar defects in growth and in how they respond to plant hormones. Plants missing LORELEI had similar defects in their ability to reproduce as the plants missing FERONIA. This suggests that FERONIA works with either LLG1 or LORELEI to regulate similar processes in different situations.

Li et al. found that FERONIA binds to LLG1 in a compartment within the cell called the endoplasmic reticulum—where proteins are assembled—before both proteins are moved together to the cell membrane. In the absence of LLG1, FERONIA fails to reach the cell membrane, and a large amount of FERONIA remains trapped in the endoplasmic reticulum. Therefore, LLG1 acts as a ‘chaperone’ that delivers FERONIA to the membrane where it is required to regulate plant growth. Li et al. found that LORELEI also interacts with FERONIA. Both LLG1 and LORELEI bind to the same region of FERONIA, which is on the outer surface of the cell membrane.

These findings show that FERONIA is able to perform different roles in cells by teaming up with different members of the GPI-AP family of proteins. The next challenges will be to find out if, and how, LLG1 and LORELEI affect the ability of FERONIA to respond to signals from the cell wall and outside the cell.

DOI:http://dx.doi.org/10.7554/eLife.06587.002

N-linked sugar-regulated protein folding and quality control in the ER

Asparagine-linked glycans (N-glycans) are displayed on the majority of proteins synthesized in the endoplasmic reticulum (ER). Removal of the outermost glucose residue recruits the lectin chaperone malectin possibly involved in a first triage of defective polypeptides. Removal of a second glucose promotes engagement of folding and quality control machineries built around the ER lectin chaperones calnexin (CNX) and calreticulin (CRT) and including oxidoreductases and peptidyl-prolyl isomerases. Deprivation of the last glucose residue dictates the release of N-glycosylated polypeptides from the lectin chaperones. Correctly folded proteins are authorized to leave the ER. Non-native polypeptides are recognized by the ER quality control key player UDP-glucose glycoprotein glucosyltransferase 1 (UGT1), re-glucosylated and re-addressed to the CNX/CRT chaperone binding cycle to provide additional opportunity for the protein to fold in the ER. Failure to attain the native structure determines the selection of the misfolded polypeptides for proteasome-mediated degradation.

Functional analysis of duplicated Symbiosis Receptor Kinase (SymRK) genes during nodulation and mycorrhizal infection in soybean (Glycine max)

Association between legumes and rhizobia results in the formation of root nodules, where symbiotic nitrogen fixation occurs. The early stages of this association involve a complex of signalling events between the host and microsymbiont. Several genes dealing with early signal transduction have been cloned, and one of them encodes the leucine-rich repeat (LRR) receptor kinase (SymRK; also termed NORK). The Symbiosis Receptor Kinase gene is required by legumes to establish a root endosymbiosis with Rhizobium bacteria as well as mycorrhizal fungi. Using degenerate primer and BAC sequencing, we cloned duplicated SymRK homeologues in soybean called GmSymRKα and GmSymRKβ. These duplicated genes have high similarity of nucleotide (96%) and amino acid sequence (95%). Sequence analysis predicted a malectin-like domain within the extracellular domain of both genes. Several putative cis-acting elements were found in promoter regions of GmSymRKα and GmSymRKβ, suggesting a participation in lateral root development, cell division and peribacteroid membrane formation. The mutant of SymRK genes is not available in soybean; therefore, to know the functions of these genes, RNA interference (RNAi) of these duplicated genes was performed. For this purpose, RNAi construct of each gene was generated and introduced into the soybean genome by Agrobacterium rhizogenes-mediated hairy root transformation. RNAi of GmSymRKβ gene resulted in an increased reduction of nodulation and mycorrhizal infection than RNAi of GmSymRKα, suggesting it has the major activity of the duplicated gene pair. The results from the important crop legume soybean confirm the joint phenotypic action of GmSymRK genes in both mycorrhizal and rhizobial infection seen in model legumes.

Cotranslational and posttranslocational N-glycosylation of proteins in the endoplasmic reticulum

Asparagine linked glycosylation of proteins is an essential protein modification reaction in most eukaryotic organisms. N-linked oligosaccharides are important for protein folding and stability, biosynthetic quality control, intracellular traffic and the physiological function of many N-glycosylated proteins. In metazoan organisms, the oligosaccharyltransferase is composed of a catalytic subunit (STT3A or STT3B) and a set of accessory subunits. Duplication of the catalytic subunit gene allowed cells to evolve OST complexes that act sequentially to maximize the glycosylation efficiency of the large number of proteins that are glycosylated in metazoan organisms. We will summarize recent progress in understanding the mechanism of (a) cotranslational glycosylation by the translocation channel associated STT3A complex, (b) the role of the STT3B complex in mediating cotranslational or posttranslocational glycosylation of acceptor sites that have been skipped by the STT3A complex, and (c) the role of the oxidoreductase MagT1 in STT3B-dependent glycosylation of cysteine-proximal acceptor sites.

Oxidoreductase activity is necessary for N-glycosylation of cysteine-proximal acceptor sites in glycoproteins

Stabilization of protein tertiary structure by disulfides can interfere with glycosylation of acceptor sites (NXT/S) in nascent polypeptides. Here, we show that MagT1, an ER-localized thioredoxin homologue, is a subunit of the STT3B isoform of the oligosaccharyltransferase (OST). The lumenally oriented active site CVVC motif in MagT1 is required for glycosylation of STT3B-dependent acceptor sites including those that are closely bracketed by disulfides or contain cysteine as the internal residue (NCT/S). The MagT1- and STT3B-dependent glycosylation of cysteine-proximal acceptor sites can be reduced by eliminating cysteine residues. The predominant form of MagT1 in vivo is oxidized, which is consistent with transient formation of mixed disulfides between MagT1 and a glycoprotein substrate to facilitate access of STT3B to unmodified acceptor sites. Cotranslational N-glycosylation by the STT3A isoform of the OST, which lacks MagT1, allows efficient modification of acceptor sites in cysteine-rich protein domains before disulfide bond formation. Thus, mammalian cells use two mechanisms to achieve N-glycosylation of cysteine proximal acceptor sites.

Lectin structures: classification based on the 3-D structures

Recent progress in structural biology has elucidated the three-dimensional structures and carbohydrate-binding mechanisms of most lectin families. Lectins are classified into 48 families based on their three-dimensional structures. A ribbon drawing gallery of the crystal and solution structures of representative lectins or lectin-like proteins is appended and may help to convey the diversity of lectin families, the similarity and differences between lectin families, as well as the carbohydrate-binding architectures of lectins.

Malectin forms a complex with ribophorin I for enhanced association with misfolded glycoproteins

Malectin is an endoplasmic reticulum-resident lectin, which recognizes di-glucosylated Glc(2)Man(9)GlcNAc(2) (G2M9) N-glycans on newly synthesized glycoproteins. We previously demonstrated that malectin preferentially associates with misfolded glycoproteins and inhibits their secretion (Chen, Y., Hu, D., Yabe, R., Tateno, H., Qin, S. Y., Matsumoto, N., Hirabayashi, J., and Yamamoto, K. (2011) Mol. Biol. Cell 22, 3559-3570). The sugar binding activity of malectin is required for this process. However, because G2M9 N-glycans are generated at the very early stage of processing and are typically found on both misfolded glycoproteins and glycoproteins undergoing folding, other mechanisms must underlie the preference of malectin for misfolded glycoproteins. Here, we searched for proteins that were co-immunoprecipitated with malectin, and we found that malectin formed a stable complex with an endoplasmic reticulum-resident transmembrane protein, ribophorin I. Co-expression of malectin and ribophorin I significantly enhanced the association between malectin and a folding-defective α1-antitrypsin variant (null Hong Kong) and reduced its secretion; however, secretion of wild-type α1-antitrypsin was not affected. The enhanced association and reduced secretion were counteracted by siRNA-mediated down-regulation of ribophorin I. Also, a reporter assay revealed that ribophorin I preferentially interacted with misfolded ribonuclease A but not with the native form, suggesting that ribophorin I may function as a chaperone that recognizes misfolded proteins inside cells. These results provide the first evidence of the mechanism by which malectin preferentially associates with misfolded glycoproteins.

Distant plant homologues: don't throw out the baby

Plants and metazoans share many similarities in terms of conserved proteins. Antibodies have been used extensively to detect remote homologues, many of which are yet to be identified conclusively. Genome sequencing and the creation of novel sequence or structure comparison programs have assisted greatly in the identification of distant protein homologues. The continuing development of new software algorithms and the combining of bioinformatics with proteomics offer hope that remaining homologues will be soon identified.

Proteomic profiling of thyroid papillary carcinoma

Papillary thyroid carcinoma (PTC) is the most common endocrine malignancy. We performed shotgun liquid chromatography (LC)/tandem mass spectrometry (MS/MS) analysis on pooled protein extracts from patients with PTC and compared the results with those from normal thyroid tissue validated by real-time (RT) PCR and immunohistochemistry (IHC). We detected 524 types of protein in PTC and 432 in normal thyroid gland. Among these proteins, 145 were specific to PTC and 53 were specific to normal thyroid gland. We have also identified two important new markers, nephronectin (NPNT) and malectin (MLEC). Reproducibility was confirmed with several known markers, but the one of two new candidate markers such as MLEC did not show large variations in expression levels. Furthermore, IHC confirmed the overexpression of both those markers in PTCs compared with normal surrounding tissues. Our protein data suggest that NPNT and MLEC could be a characteristic marker for PTC.

THESEUS 1, FERONIA and relatives: a family of cell wall-sensing receptor kinases?

The plant cell wall provides form and integrity to the cell as well as a dynamic interface between a cell and its environment. Therefore mechanisms capable of policing changes in the cell wall, signaling cellular responses including those that would feedback regulate cell wall properties are expected to play important roles in facilitating growth and ensuring survival. Discoveries in the last few years that the Arabidopsis THESEUS 1 receptor-like kinase (RLK) may function as a sensor for cell wall defects to regulate growth and that its relatives FERONIA and ANXURs regulate pollen tube integrity imply strongly that they play key roles in cell wall-related processes. Furthermore, FERONIA acts as a cell surface regulator for RAC/ROP GTPases and activates production of reactive oxygen species which are, respectively, important molecular switches and mediators for diverse processes. These findings position the THESEUS 1/FERONIA family RLKs as surface regulators and potential cell wall sensors capable of broadly and profoundly impacting cellular pathways in response to diverse signals.

Role of malectin in Glc(2)Man(9)GlcNAc(2)-dependent quality control of α1-antitrypsin

In cells, human malectin stably interacted with newly synthesized AT^NHK, but not AT, via G2M9 glycans. The interaction of AT^NHK with malectin resulted in enhanced ERAD of AT^NHK and prevented the secretion of the misfolded glycoprotein. These findings provide evidence of a role of malectin in glycoprotein quality control via recognition of G2M9.

Malectin was first discovered as a novel endoplasmic reticulum (ER)–resident lectin from Xenopus laevis that exhibits structural similarity to bacterial glycosylhydrolases. Like other intracellular lectins involved in glycoprotein quality control, malectin is highly conserved in animals. Here results from in vitro membrane-based binding assays and frontal affinity chromatography confirm that human malectin binds specifically to Glc₂Man₉GlcNAc₂ (G2M9) N-glycan, with a K_a of 1.97 × 10⁵ M⁻¹, whereas binding to Glc₁Man₉GlcNAc₂ (G1M9), Glc₃Man₉GlcNAc₂ (G3M9), and other N-glycans is barely detectable. Metabolic labeling and immunoprecipitation experiments demonstrate that before entering the calnexin cycle, the folding-defective human α1-antitrypsin variant null Hong Kong (AT^NHK) stably associates with malectin, whereas wild-type α1-antitrypsin (AT) or N-glycan–truncated variant of AT^NHK (AT^NHK-Q3) dose not. Moreover, malectin overexpression dramatically inhibits the secretion of AT^NHK through a mechanism that involves enhanced ER-associated protein degradation; by comparison, the secretion of AT and AT^NHK-Q3 is only slightly affected by malectin overexpression. ER-stress induced by tunicamycin results in significantly elevated mRNA transcription of malectin. These observations suggest a possible role of malectin in regulating newly synthesized glycoproteins via G2M9 recognition.

New insights into the functional roles of CrRLKs in the control of plant cell growth and development

Receptor-like kinases (RLKs) are a family of transmembrane proteins with a variable ligand-binding extracellular domain and a cytoplasmic kinase domain. In Arabidopsis, there are ~600 RLKs believed to have diverse functions during plant growth, development and interactions with the environment. Based on the variable extracellular domain, RLKs can be classified into different subfamilies. The CrRLK subfamily contains 17 members in Arabidopsis and characterization of some of its members suggests a role for these proteins in the regulation of growth and reproduction. The present review focuses on the roles of CrRLKs in the regulation of polarized growth with emphasis on the newly identified signal transduction pathways activated downstream of CrRLKs. A picture is emerging where CrRLKs are part of a conserved signal transduction cascade important for growth maintenance in different cell types.

Malectin participates in a backup glycoprotein quality control pathway in the mammalian ER

Malectin is a conserved, endoplasmic reticulum (ER)-resident lectin that recognizes high mannose oligosaccharides displaying terminal glucose residues. Here we show that Malectin is an ER stress-induced protein that selectively associates with glycopolypeptides without affecting their entry and their retention in the Calnexin chaperone system. Analysis of the obligate Calnexin client influenza virus hemagglutinin (HA) revealed that Calnexin and Malectin associated with different timing to different HA conformers and that Malectin associated with misfolded HA. Analysis of the facultative Calnexin clients NHK and α1-antitrypsin (α1AT) revealed that induction of Malectin expression to simulate conditions of ER stress resulted in persistent association between the ER lectin and the model cargo glycoproteins, interfered with processing of cargo-linked oligosaccharides and reduced cargo secretion. We propose that Malectin intervention is activated upon ER stress to inhibit secretion of defective gene products that might be generated under conditions of aberrant functioning of the ER quality control machinery.