Milieu-induced, selective aggregation of regulated secretory proteins in the trans-Golgi network

inherently lower tendency to aggregate than regulated secretory proteins. Concerning the first point, both constitutive and regulated secretory proteins are believed to move from the RER to the TGN by bulk flow (for review see Pfeffer and Rothman, 1987) . In bulk flow, an increase in the concentration of secretory proteins in a distal compartment as compared to a proximal compartment can only occur as the result of the selective removal of molecules, e.g., the recycling of soluble ER-resident proteins or the net movement of water across the membrane into the cytoplasm . in the concentration of regulated secretory proteins in the trans-Golgi compared to the RER have been reported (Bendayan et al., 1980; Salpeter and Farquhar, 1981; Bendayan, 1984; Posthuma et al., 1988) . However, it is unclear to what extent aggregates of regulated secretory proteins were included in these quantitations, and thus it is difficult to exclude the possibility that the observed concentration increase was the consequence ofaggregation .