Ripeptides containing the acceptor sequence for Asn-linked glycosylatlon (AsnXSerlThr) were added to CHO and HepGP cells. The trlpeptldes were glycosylated in the ER and then secreted into the medium, via the Golgi complex in which the oligosaccharide chains were processed. The half-time for secretion, 40 min, was faster than that of known proteins transported through the same pathway. Since much evidence suggests that ol (gosaccharlde chains are not signals for transport, it appears that no signal is necessary for rapid and efficient transport from the ER to the Golgi, or from the Golgi to the cell surface. Rather, it appears that proteins Mained as permanent testdents en route through the ER-Golgi transport pathway must contain specific retention signals. introduction

The endoplasmic reticulum (ER) retains one set of proteins but releases another for transport to plasma membranes, lysosomes, and secretory vesicles (for review see Farquhar, 1985; Pfeffer and Rothman, 1987). What determines whether a given protein is exported or retained? Export from the ER could be selective with an unselective retention. If so, the exported proteins must possess signals that mark them for transport, while the retained proteins would be unmarked. Alternatively, retention could be selective, and export unselective. This mechanism would require that the proteins retained as permanent residents of the ER be marked with retention signals; any protein in the ER not marked for retention would automatically be transported away.