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

Concerning these questions, it has been proposed ) that (a) the aggregation of regulated secretory proteins in the TGN results from an increase in their concentration as they travel from the RER to this compartment where they reach a critical level, and (b) constitutive secretory proteins do not reach this concentration and/or have an cluded glycosaminoglycan chains which served as constitutively secreted bulk flow markers . The low pH, high calcium milieu was sufficient to induce granin aggregation in the RER. In the TGN of pituitary GH4C cells, the proportion of granins conserved as aggregates was higher upon hormonal treatment known to increase secretory granule formation . Our data suggest that a decrease in pH and an increase in calcium are sufficient to trigger the selective aggregation of the granins in the TGN, segregating them from constitutive secretory proteins .