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

Granin Aggregates in the TGNafter Aermeabilization in a Low pH-, High Calcium Milieu When ["S]sulfate-labeled TGN vesicles were permeabilized with saponin in aggregative milieu, the majority of SgII was recovered in the pellet (Fig. 2 A, top) . Although not ap- parent from the fluorogram shown in Fig . 2 A, due to the presence of the hsPG, the same was observed for CgB (see legend to Fig. 2 A, top) . Thus, this low pH-, high calcium milieu was sufficient to maintain the granins in an aggregated state . Quantitation of the radioactivity contained in the SgII band from five different experiments (Table I) showed that in the nonaggregative milieu -23% of the [33S]sulfatelabeled SgII was recovered in the pellet, whereas -82% was recovered in the pellet in the aggregative milieu . Protein staining ofthe gel ( Fig. 2 A, bottom) revealed that, apart from CgB and SgII, the protein pattern ofthe pellet obtained in nonaggregative and aggregative milieu was very similar, as was the protein pattern of the supernatant in the two conditions, showing that the retention of the granins in the TGN after saponin permeabilization in aggregative milieu was a selective event .