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

von Castle (1987, 1989) have suggested that early phase secretion of secretory granule components is because of the budding of vesicles from immature granules. From their analysis of the types of protein secreted in the early phase they concluded that sorting involves condensation and continues after the formation of immature granules from the TGN . Both their model and Fig . 9 raise the issue of whether the late sorting compartments should be considered part of the TGN, or an immature secretory granule . We are experimentally classifying components that stay with the cell ghosts as Golgi associated and those intermediate vesicles that escape as immature or precursor secretory granules . Chung et al . (1989) have suggested that sorting of regulated proteins involves a receptor that recycles between the TGN and a low pH compartment that is analogous in some ways to the prelysosomal or late endosome compartment . The fraction we call immature secretory granules could in principle be such a sorting compartment in which ligand dissociates from receptor at low pH. The crucial difference be-tween the two models is whether condensation precedes sorting ( Fig . 9 ) or vice versa Tooze et al ., 1987) .