Ribophorin - Function

Function

Ribophorins I and II, transmembrane glycoprotein of the rough endoplasmic reticulum, intervene in the union of the ribosomes (they fix the large subunit, 60S, of the ribosome) to the RE membrane, and they play an important role in the co-translational translocation process which depends on this union, as the com insertion of the nascent polypeptide to the membrane or their transference to the lumen of the cistern; this is the translocation of proteins generated by polyribosomes.

Ribophorin I usually interacts with those proteins that have a wrong folding; otherwise, this protein does not interact with native state proteins. This suggests that ribophorin I may work as a chaperone that recognizes the proteins with a wrong folding. Moreover, this ribophorin can regulate the delivery of precursor proteins to the oligosaccharyltransferase (main enzyme of the N-glycosylation for proteins), through the capture of substrates and taking them to the catalytic center. So that, ribophorin I can keep those possible substrates in the proximity of the catalytic subunit of the enzyme; this way, the efficiency of the N-glycosylation reaction will improve during their biogenesis in the ER. But ribophorin I only changes drastically the N-glycosylation of determined substrates, as it is apparently dispensable in the same process with other substrates. When ribophorin is not essential, these precursors have shortcut to the catalytic center of the oligosaccharyltransferase or their presence depends on the rest of the non-catalytic subunits in the complex. It shows the specificity of ribophorin I for determined substrates; so that, this protein will regulate selectively the delivery of substrates to the catalytic center of the oligosaccharyltransferase complex.

Although ribophorin II is still a protein quite unknown, it has been discovered that this ribophorin is part of an N-oligosaccharyltransferase complex that links high mannose oligosaccharides to asparagine residues found in the Asn-X-Ser/Thr consensus motif of nascent polypeptide chains. Moreover, this protein takes part in the identification of retention signals of other proteins.

There is not much information about ribophorin II because this subunit of the complex hasn’t been as investigated as ribophorin I.