Intermediates in the constitutive and regulated secretory pathways released in vitro from semi-intact cells

Constitutive vesicles released in the presence of ATP contained -23 % of the total labeled proteoglycans in the cell (Fig. 5 B) . The ability to recover constitutive vesicles so efficiently is presumably because vesicles are more likely to escape permeabilized cells rather than fuse with the cell surface. Constitutive secretory vesicles, also called exocytosic transport vesicles, have been generated in vitro from liver Golgi membranes bound to magnetic beads (Salamero et al ., 1990) , mechanically permeabilized polarized epithelia (Bennett et al ., 1988 ; Bomsel et al., 1990; Podbilewicz and Mellman, 1990 ) and cell-free TGN membranes from BHK cells (de Curtis and Simons, 1989) . They also accumulate in yeast cells defective in the GTP-binding protein, sec4 (Holcomb et al ., 1987; Walworth and Novick, 1987) . PC12 constitutive vesicles have properties in common with the vesicles described in these studies, being relatively uniform in sedimentation velocity and density, which implies a uniformity in size.