Complete Golgi passage of glycotripeptides generated in the endoplasmic reticulum of mammalian cells

The tripeptide, N-octanoyl-Asn-[125I]Tyr-Thr-NH2, which contains the acceptor sequence for N-glycosylation, is readily taken up by cell culture cells, glycosylated in the endoplasmic reticulum (ER), and secreted into the medium. Therefore such glycosylated tripeptides have been used as markers for the vesicular flow from the endoplasmic reticulum to the plasma membrane [(1987) Cell 50, 289-300; (1990) J. Biol. Chem. 265, 20027-20032]. We have now studied the pathway taken by the glycotripeptides in mammals in more detail. In the perfused rat liver, the glycotripeptides secreted to the medium were analyzed by digestion with exoglycosidases, and a significant fraction was found to contain the terminating sequence -Gal-Sial, which is generated by processing enzymes that reside in the late Golgi apparatus. Thus we conclude that these glycotripeptides have passed through the complete Golgi complex on their way from the ER to the cell surface.