Location
Ribophorins are a kind of protein located only in eukaryotic cells and, furthermore, they are only found in mammal cells. In the cell, ribophorins are positioned on the membrane of the rough endoplasmic reticulum, as they interact with the ribosomes for protein translocation.
Both ribophorin I and II possess a type I membrane topology with the bulk of their polypeptide chains directed towards the ER-lumen and they are part of the mammalian protein complex OST; this complex effects the cotranslational N-glycosylation of newly synthesized polypeptides, and is composed by four RER specific membrane proteins, which are the ribophorins (I and II), the OST48 and the Dadl. In order to form the OST complex, there are specific interactions between the proteins; because of that, the lumen domains of ribophorin I and II interact with the lumen domain of OST48. Nevertheless, there is not a direct interaction between both ribophorins.
As they are transmembrane proteins, ribophorins cross the ER membrane and, so that, the protein has a cytoplasmic, a transmembrane and a lumen domain. In the case of the ribophorin II, the transmembrane and the cytoplasmic domains are the ones that have the retention function on the ER; but on the other hand, the lumen domain is the one with the retention function for ribophorin I.
Ribophorin I resides in the ER membrane with a single spanning sequence from amino acids 416 to 434, having a cytoplasmic C terminus of 150 amino acids and a luminal N-terminal domain consisting of 415 residues. Ribophotin II is disposed on a similar way in the ER membrane, but now the membrane-spanning domain is located at residues 517-539 and asparagine residues 544 and 547 would be disposed at the cytoplasm leaving only Asn84 as the putative site for oligosaccharide addition; the cytoplasmic domain will have a maximum length of 70 residues. In order to be disposed on the ER membrane, ribophorins have a signal sequence which will determine their location.
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