Localization of the Lys, Asp, Glu, Leu tetrapeptide receptor to the Golgi complex and the intermediate compartment in mammalian cells

There is now compelling evidence that COP vesicles mediate both ER to Golgi, as well as intra-Golgi traffic (Pepperkok et al., 1993; Peter et al., 1993; Rothman and Orci, 1992) . In permeabilized cells, the budding and/or fusion of these vesicles can be arrested with GTP3,S, a treatment that leads to a considerable increase in the frequency of COP vesicles (Krijnse-Locker et al., 1994) . A significant finding in the present study was the presence of easily detectable levels of the KDEL-R in these GTP-yS-arrested COP buds and/or vesicles. Using the MHV (as well as rabl: see Fig. 5 A) to identify the IC, we can conclude that at least some of these buds/vesicles originate from the IC, and that they are probably involved in anterograde traffic from the IC into the Golgi complex (presumably the Man I compartment). The simplest interpretation is that the "empty" receptor follows the normal vesicular traffic route from the ER (IC) to the Golgi. Whether any of the COP vesicles may also be involved in the retrograde trafficking pathway can only be investigated when a suitable system becomes available for following this retrograde transport step in more detail.