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.