Malectin: a novel carbohydrate-binding protein of the endoplasmic reticulum and a candidate player in the early steps of protein N-glycosylation.
N-Glycosylation starts in the endoplasmic reticulum (ER) where a 14-sugar glycan composed of three glucoses, nine mannoses, and two N-acetylglucosamines (Glc(3)Man(9)GlcNAc(2)) is transferred to nascent proteins. The glucoses are sequentially trimmed by ER-resident glucosidases. The Glc(3)Man(9)GlcNAc(2) moiety is the substrate for oligosaccharyltransferase; the Glc(1)Man(9)GlcNAc(2) and Man(9)GlcNAc(2) intermediates are signals for glycoprotein folding and quality control in the calnexin/calreticulin cycle. Here, we report a novel membrane-anchored ER protein that is highly conserved in animals and that recognizes the Glc(2)-N-glycan. Structure determination by nuclear magnetic resonance showed that its luminal part is a carbohydrate binding domain that recognizes glucose oligomers. Carbohydrate microarray analyses revealed a uniquely selective binding to a Glc(2)-N-glycan probe. The localization, structure, and binding specificity of this protein, which we have named malectin, open the way to studies of its role in the genesis, processing and secretion of N-glycosylated proteins.
PubMed ID: 18524852
PMC ID: PMC2488313
Article link: Mol Biol Cell.
Grant support: Medical Research Council
Genes referenced: act3 calr canx man1a1 mlec pdia2 sult2a1 txk
Article Images: [+] show captions
|Figure 1. Broad expression of malectin in embryonic and adult X. laevis. (A) Analysis of embryos by whole mount in situ hybridization showing malectin expression in the anterior neuroectoderm (ne) and neural crest (nc) at stages 18 and 20 (A1, A2), and at later stages, e.g., 32, in the hatching gland (hg), retina (re), otic vesicle (ot), epibranchial placodes (eb), pronephros (pn), and the tail tip (tp); anterior (a); posterior (p). At stage 41 (A4), transcripts are detected in the liver (li), dorsal and ventral pancreas (dp and vp), branchial arches (ba), and the proctodeum (pd). (B) Analysis of embryonic expression of malectin by RT-PCR showing expression in the oocyte (stage VI) and continued expression throughout development, and the contrasting expression of protein disulfide isomerase, xPDIp, only from late tadpole stage 39 onward. Abbreviations: stage VI oocyte (VI); unfertilized egg (0) and fertilized egg (1). (C) The expression analysis in adult tissue by semiquantitative RT-PCR showing a broad distribution of malectin in comparison with xPDIp, which is detected in pancreas and stomach only. Additional abbreviations not defined in A: gall bladder (gb); heart (he); intestine (in); kidney (ki); lung (lu); muscle (mu); pancreas (pa); stomach (st).|
|Figure 5. One dimensional STD spectra for glucose and glucose disaccharides: glucose (A), cellobiose (B), maltose (C) nigerose (D), kojibiose (E), and isomaltose (F). Structures of the tested carbohydrates are depicted on the left of the corresponding spectra. and denote the stereoisomers of the anomeric center of the reducing-end glucose ring. The ring protons of the nonreducing residues are labeled 1–6, and those of the reducing residues 1 –6 . Assignment tables of the carbohydrate ligands are in Supplemental Table S4.|
|Figure S1: Sequence alignment of animal malectin proteins vs. Arabidopsis thaliana and Oryza sativa RLK proteins. (A) Alignment of representative arabidopsis and rice RLK-malectin-like domains with the malectin core domain of X. laevis, humans, rat and mouse. Sequences are labelled with UniProtKB/TrEMBL accession numbers. (XENLA = Xenopus laevis, DROME = Drosophila melanogaster, SCHJA = Schistosoma japonicum, CAEEL = Caenorhabditis elegans, CRYPV = Cryptospridium parvum, ARATH = Arabidopsis thaliana, ORYSA = Oryza sativa). The aromatic residues and the aspartate that in X. laevis mediate interactions with the glucose residues (Fig.4D) are marked by red crosses, and are not conserved in plants. (B) Domain topologies of plant and animal proteins that contain the malectin core domain. The two topologies among plant RLKs are shown. Labels: SP - signal peptide; TM- transmembrane helix; LRR - leucine rich repeat: STKinase – serine/threonine receptor-like kinase.|
|Figure S2: Malectin binding to glucose disaccharides studied by isothermal titration calorimetry. Kojibiose (A), nigerose (B) and maltose (C). The raw data are shown in the upper panel, and the integrated heat data, corrected for dilution, are shown in the lower panel. ITC measurements were carried out using a VP-ITC Mircocal clorimeter (Mircocal, Northhampton, MA, USA) in 20mM phosphate buffer (pH 6.8), 150mM KCl and 1mM TCEP. A typical titration consisted of injecting 10μl of the sugar into the malectin sample, at time intervals of 5min, to ensure that the titration peak returned to the baseline.|
|Figure S3: NOEs between malectin and nigerose. (A) Part of a 13C-edited half-filtered-NOESY experiment (mixing time 150 ms) showing intermolecular NOEs between malectin and nigerose. (B) Structure of α-nigerose.|
|Figure S4: Microarray analyses of the interactions of malectin with Glc1-, Glc2- and Glc3-high mannose N-glycans and gluco-oligosaccharide probes. The oligosaccharide probes were printed as duplicate spots and binding was assayed with malectin at 20, 5, 1 and 0.5 μg/ml (panels A to D, respectively). Numerical scores are shown for the binding signals [means of duplicate values at 2 and 7 fmol/spot, (blue and red bars, respectively) with error bars]. At a malectin concentration of 20 μg/ml, the binding signals for the Glc2-high mannose N-glycan probe, both at 2 and 7 fmol, were too high to be accurately quantified (asterisk in A) and were annotated as >> 50000 in Table 1. Other oligosaccharide probes tested included the glucose disaccharides kojibiose (Glcα1-2Glc) nigerose (Glcα1-3Glc), maltodextrins (Glcα1-4Glc, dp 2-7); and oligosaccharides from dextran (isomalto) (Glcα1-6Glc, dp 2-7); laminarin (Glcβ1-3Glc, dp 2-7); cellulose (Glcβ1-4Glc, dp 2-6); and pustulan (Glcβ1-6Glc, dp 2-7). Abbreviations G3N, G2N and G1N designate Glc3Man7(D1)GlcNAc, Glc2Man7(D1)GlcNAc and Glc1Man9GlcNAc2 N-glycan probes, respectively; dp, degree of polymerization of the gluco-oligomers.|
|mlec (malectin) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 20, dorsal view, anterior left, dorsal up.|
|mlec (malectin) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 32, lateral view, anterior left, dorsal up.|
|mlec (malectin) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 41, lateral view, anterior left, dorsal up.|