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

Abstract. Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation . The factors responsible for this aggregation are unknown . We show here that two widespread regulated secretory proteins, chromogranin B and secretogranin II (granins), remain in an aggregated state when TGN vesicles from neuroendocrine cells (PC12) are permeabilized at pH 6.4 in 1-10 mM calcium, conditions believed to exist in this compartment. Permeabilization of immature secretory granules under these conditions allowed the recovery of electron dense cores. The granin aggregates in the TGN largely ex-HE mechanism by which constitutive and regulated secretory proteins are sorted in the trans-Golgi network (TGN)' is poorly understood (for review see iB urgess and Kelly, 1987) . It has been postulated that a key step in this sorting process is the selective aggregation of regulated, but not constitutive, secretory proteins in the TGN (Kelly, 1985; Burgess and Kelly, 1987; Pfeffer and Rothman, 1987; Huttner et al ., 1988; Gerdes et al., 1989; Huttner and Tooze, 1989; Tooze et al., 1989) . This hypothesis is largely based on morphological data showing that regulated secretory proteins form electron-dense cores in the TGN (for review see Farquhar and Palade, 1981; Orci et al., 1987; Tooze et al ., 1987) . Aggregation of a protein inevitably segregates it from proteins that do not aggregate but remain in solution in the fluid phase that is excluded from the aggregate. Thus, if aggregation is a key step in the sorting of secretory proteins, the crucial questions arising are: (a) why does aggregation occur specifically in the TGN ; and (b) why is the aggregation selective for regulated secretory proteins?