title: Control of glycosylation of MHC class II-associated invariant chain by translocon-associated RAMP4.
abstract: Protein translocation across the membrane of the endoplasmic reticulum (ER) proceeds through a proteinaceous translocation machinery, the translocon. To identify components that may regulate translocation by interacting with nascent polypeptides in the translocon, we used site-specific photo-crosslinking. We found that a region C-terminal of the two N-glycosylation sites of the MHC class II-associated invariant chain (Ii) interacts specifically with the ribosome-associated membrane protein 4 (RAMP4). RAMP4 is a small, tail-anchored protein of 66 amino acid residues that is homologous to the yeast YSY6 protein. YSY6 suppresses a secretion defect of a secY mutant in Escherichia coli. The interaction of RAMP4 with Ii occurred when nascent Ii chains reached a length of 170 amino acid residues and persisted until Ii chain completion, suggesting translocational pausing. Site-directed mutagenesis revealed that the region of Ii interacting with RAMP4 contains essential hydrophobic amino acid residues. Exchange of these residues for serines led to a reduced interaction with RAMP4 and inefficient N-glycosylation. We propose that RAMP4 controls modification of Ii and possibly also of other secretory and membrane proteins containing specific RAMP4-interacting sequences. Efficient or variable glycosylation of Ii may contribute to its capacity to modulate antigen presentation by MHC class II molecules.
authors.affiliation: Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), Postfach 106249, 69052 Heidelberg, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Schröder
authors.forename: B
authors.initials: B
authors.lastname: Martoglio
authors.forename: M
authors.initials: M
authors.lastname: Hofmann
authors.forename: C
authors.initials: C
authors.lastname: Hölscher
authors.forename: E
authors.initials: E
authors.lastname: Hartmann
authors.forename: S
authors.initials: S
authors.lastname: Prehn
authors.forename: T A
authors.initials: TA
authors.lastname: Rapoport
authors.forename: B
authors.initials: B
authors.lastname: Dobberstein
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000944:Antigens, Differentiation, B-Lymphocyte / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D006031:Glycosylation; D000949:Histocompatibility Antigens Class II / Q000378:metabolism*; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D016296:Mutagenesis; D011233:Precipitin Tests; D011485:Protein Binding; D014176:Protein Biosynthesis; D051381:Rats; D012270:Ribosomes / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013997:Time Factors
chemical_list: D000944:Antigens, Differentiation, B-Lymphocyte; D018076:DNA, Complementary; D000949:Histocompatibility Antigens Class II; D008565:Membrane Proteins; C121893:Serp1 protein, rat; C045084:invariant chain