Control of glycosylation of MHC class II‐associated invariant chain by translocon‐associated RAMP4

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.