Evidence for the transfer of mannose to the extension peptides of procollagen within the cisternae of the rough endoplasmic reticulum

Collagens from various sources contain galactose and glucosylgalactose in O-glycosidic linkage to hydroxylysyl residues (reviewed [l-3] ). These are the only sugars in mammalian collagens, but the biosynthetic precursor of collagen, procollagen, contains additional carbohydrate units in its amino-terminal and carboxy-terminal peptide extensions [4-9] . These propeptides do not contain the hydroxylysyllinked carbohydrate units [l-3] . The propeptides can be labelled by incubation with [2-3H]mannose [5,7,8] , most of the label being located in the carboxyterminal peptide [8]. Direct carbohydrate analyses indicate the presence in the carboxy-terminal propeptide of chick embryo tendon procollagen of 2 residues ofN-acetylglucosamine and 9-13 residues of rnannose per peptide [9]. The hydroxylysyl-linked carbohydrate units are obviously synthesized within the cisternae of the rough endoplasmic reticulum [ 1 ,l l-161 . Nothing is known, however, about the intracellular site of the glycosylation of the propeptides. This site was investigated in the present work by inhibiting the movement of procollagen from the cisternae of the rough endoplasmic reticulum into the Golgi vacuoles and by determining the effect of this inhibition on the extent of mannosylation of the intracellular procollagen. There should be no decrease in the extent of this reaction if the mannose transfer can occur within the cisternae, whereas a distinct decrease should be observed if the mannose transfer occurs within the