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

The effect of calcium concentration and pH on the state of aggregation of SgII was studied by exposing the content of [ 35S]sulfate-labeled TGN vesicles, obtained according to the standard procedure, to various buffers in the presence of Triton X-100 ( Fig . 7) . At pH 6 .4, the amount of ["S]sulfate-labeled SgII recovered in the pellet increased rapidly when millimolar calcium was added. Half-maximal and maximal amounts of aggregated SgII were observed at a calcium concentration of tit-2 and -10 mM, respectively. In contrast, at pH 6.9, much less [35S]sulfate-labeled SgII was found in the pellet with increasing calcium concentrations . At pH 7.4, no significant amount of ["S]sulfate-labeled SgII was observed in the pellet at any of the calcium concentrations studied . Thus, slightly acidic pH and millimolar calcium concentrations had a synergistic effect on the state of aggregation of TGN-derived SgII .