The organization of endoplasmic reticulum export complexes

As illustrated in Fig. 4 , ER-budding profles predomi-nately faced into an area filled with VTCs in vivo. VTCs, defined as a group of four or more 60-80-nm vesicular profiles with a characteristic shape and coat, were never detected in regions lacking adjacent ER buds. The membrane profiles of VTCs were distinct from the ER and found to be more variable in size compared to the rather homogeneous appearance of buds emanating from the ER surface. The average diameter of VTCs based on serialthin sections was found to be ~0.4 Cm (Table II) with a range that varied from ~0.2 ~m to >1 ~m (Fig. 5 B) . Although technical limitations do not allow us to make a complete three-dimensional reconstruction of membrane continuities between elements of VTCs, individual profiles often appeared to be continuous in several consecutive sections and consisted of short tubules that could be connected together in a more fenestrated structure (Fig. 1, asterisks) . Tubules within VTCs characteristically had a dense COPI-like coat at their tips (Figs. 1 and 3, arrows) (Melancon et al., 1987; Orci et al., 1993a) . The presence of a COPI-like coat within VTCs is consistent with the numerous morphological studies that have demonstrated that these pre-Golgi intermediates are a major site of COPI localization when examined using indirect immunofluorescence (Lippincott-Schwartz, 1993; Aridor et al., 1995) or TEM (Oprins et al., 1993; Pind et al., 1994a; Griffiths et al., 1995b) .