The organization of endoplasmic reticulum export complexes

VTCs are dynamic structures with varied morphology in different cell types. In the past, VTCs have been suggested to be the site of O-glycosylation (Tooze et al., 1988) , acylation (Rizzolo et al., 1985) , and generation of the mannose-6-phosphate signal for lysosomal protein targeting (Pelham, 1988) . Several endogenous proteins serve as useful markers for VTCs. These include the small GTPase Rab2 (Chavrier et al., 1990) , the transmembrane protein p58 in rat cells or its human homologue p53 (Schweizer et al., 1988; Saraste and Svensson, 1991) , which actively cycle between the ER and VTCs, and the COPI subunit 13-COP (Oprins et al., 1993; Pepperkok et al., 1993; Pind et al., 1994a; Aridor et al., 1995) . VTCs play a pivotal role in the segregation of anterograde and retrograde transported proteins (Aridor et al., 1995; Tang et al., 1995) . Segregation is believed to involve the COPI coat complex whose assembly is driven through activation of the ARF1 GTPase (Aridor et al., 1995; Letourneur et al., 1995) .