title: Malectin forms a complex with ribophorin I for enhanced association with misfolded glycoproteins.
abstract: 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.
authors.affiliation: Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Chiba, Japan.
authors.forename: Sheng-Ying
authors.initials: SY
authors.lastname: Qin
authors.forename: Dan
authors.initials: D
authors.lastname: Hu
authors.forename: Kana
authors.initials: K
authors.lastname: Matsumoto
authors.forename: Koh
authors.initials: K
authors.lastname: Takeda
authors.forename: Naoki
authors.initials: N
authors.lastname: Matsumoto
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Yamaguchi
authors.forename: Kazuo
authors.initials: K
authors.lastname: Yamamoto
mesh_terms: D000595:Amino Acid Sequence; D015153:Blotting, Western; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006023:Glycoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D047468:Immunoprecipitation; D037102:Lectins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D011134:Polysaccharides / Q000378:metabolism; D011485:Protein Binding; D017510:Protein Folding; D034622:RNA Interference; D013050:Spectrometry, Fluorescence; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D000515:alpha 1-Antitrypsin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D006023:Glycoproteins; D037102:Lectins; C554484:MLEC protein, human; D008565:Membrane Proteins; D011134:Polysaccharides; D000515:alpha 1-Antitrypsin; C034241:ribophorin