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.