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

The calcium concentration of the aggregative milieu corresponded to that believed to exist in the lumen of the TGN of regulated secretory cells (Stoeckel et al., 1975 ; Ravazzola, 1976; Mata et al., 1987; Roos, 1988) . It is not known which proportion of the total calcium in the TGN of these cell types is "free:' However, it should be borne in mind that not only the "free, but also the "bound" calcium participates in the aggregation process, because most of the "bound" calcium in the TGN probably reflects the low affinity-, high capacity-binding to proteins passing through this compartment, the bulk ofwhich is constituted by regulated secretory proteins.