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

Traffic between the TGN and the cell surface is ofparticular interest in regulated secretory cells since there are at least two types of carrier vesicles that leave the Golgi region carrying different classes of secretory protein . Carrier vesicles containing the two classes of protein can be resolved by density gradient centrifugation (lboze and . Using a mechanically permeabilized cell system that preserved regulated exocytosis in vitro (Martin and Walent, 1989) we found that sulfate-containing vesicles escaped from the cells. These vesicles were generated from a biosynthetic compartment that remained inside the cell ghosts, presumably the TGN . The vesicles that were released were oftwo types, both lighter in density than the membranes from which they were derived . One population of small, relatively homogeneously sized vesicles contained 60-70% of the released proteoglycans, consistent with its involvement in the constitutive secretory pathway. Their size, density, and the ratio of secretogranin II to proteoglycans (Fig. 8) remain relatively constant late after a pulse label. They are slightly enriched in sulfated proteoglycans relative to the Golgi membranes.