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

TLDR

Regulated secretory proteins are sorted in the trans‑Golgi network by selective aggregation, but the factors driving this process remain unknown. Experiments demonstrate that low pH and high calcium are sufficient to induce selective aggregation of chromogranin B and secretogranin II in the trans‑Golgi network and the rough ER, excluding glycosaminoglycans and increasing aggregate proportion upon hormonal stimulation, thereby segregating granins from constitutive secretory proteins.

Abstract

Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation. The factors responsible for this aggregation are unknown. We show here that two widespread regulated secretory proteins, chromogranin B and secretogranin II (granins), remain in an aggregated state when TGN vesicles from neuroendocrine cells (PC12) are permeabilized at pH 6.4 in 1-10 mM calcium, conditions believed to exist in this compartment. Permeabilization of immature secretory granules under these conditions allowed the recovery of electron dense cores. The granin aggregates in the TGN largely excluded 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 GH4C1 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.