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

The reduction in pH and the increase in calcium ion concentration exerted a synergistic effect on the aggregation of the granins . This raises the possibility that the change in one ofthese parameters is sufficient to trigger granin aggregation in the TGN in vivo, provided that the other parameter is at an "aggregative" value. We found that at a free calcium ion concentration above 1 mM, a change in pH from neutrality to pH 6.4 was sufficient to recovery SgII in an aggregated state. Translated to the in vivo situation this may mean that calcium, which is thought to enter the secretory pathway at least to some extent at the level of the ER (for review see Meldolesi et al., 1990) , binds to the granins already in this compartment but does not promote their aggregation until they reach the slightly acidic environment of the TGN . If this is the case, the increase in the level of calcium in the trans-Golgi area as compared to the ER (Stoeckel et al., 1975; Ravazzola, 1976; Mata et al., 1987; Roos, 1988) would be the consequence of the aggregation of secretory calcium binding proteins. Alternatively, the higher level of calcium in the trans-Golgi than in the ER may reflect the existence, at least in regulated secretory cells, ofa calcium uptake system in the TGN which causes the calcium ion concentration to raise above 1 mM, and thereby facilitates granin aggregation in this compartment . The presence ofa Cal+ATPase in the Golgi complex of neurons (Mata and Fink, 1989 ) is consistent with this possibility. At any rate, if the aggregation is a crucial step in the sorting of the granins, the requirement of a slightly acidic pH for this process to occur would, alone, be sufficient to explain the missorting of these proteins in the presence of weak bases in vivo (Gerdes et al ., 1989) .