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

The complexity of regulated secretory proteins makes it likely that certain specific features of the sorting mechanism may vary depending onthe regulated secretory protein under study. We have been focusing on the granins (chromogranins/secretogranins) as model proteins to study sorting (Huttner et al., 1991) because they are found in secretory granules ofmost endocrine cells and neurons (Ross et al., 1985 ; Wiedenmann and Hutmer, 1989) . Hence results regarding their sorting in one cell type can be generalized to most endocrine and neuronal cells . The granins are a family ofat least three proteins (chromogranin A, chromogranin B [CgB], and secretogranin II [SgH]) . The deduced primary structure of the granins (Benedum et al., 1986; Iacangelo et al., 1986; Benedum et al., 1987; Gerdes et al., 1989) predicts an abundance of acidic residues and a secondary structure alternating between helix and turns. These features are consistent with their aggregation at low pH in the presence of calcium ions. In vitro studies have shown that granins bind calcium (Reiffen and Gratzl, 1986; Cozzi and Zanini, 1988) andthat these proteins, but not various constitutive secretory proteins, aggregate in the presence of millimolar calcium ions at acidic pH (Gerdes et al., 1989; Gorr et al., 1989; Yoo and Albanesi, 1990) . However, these studies were performed with granins isolated from secretory granules and were carried out at a pH corresponding to that of neuroendocrine secretory granules rather than that of the TGN . Hence, the relevance of these studies for the aggregation of the granins occurring in the TGN remains to be established. We therefore investigated in the present study whether a low pH-, high calcium milieu corresponding to that believed to exist in the lumen of the TGN is sufficient to (a) induce and maintain the aggregation of the granins in the secretory pathway, and (b) segregate these proteins from bulk flow markers and resident proteins.