The propeptide of preprosomatostatin mediates intracellular transport and secretion of alpha-globin from mammalian cells

In endocrine cells proteolytic processing and packaging of the mature hormone into secretory granules requires acidification of the TGN compartment (1, 7, 31) . To demonstrate that PRO-GLO processing occurred in an acidic intracellular compartment and to exclude the possibility of extracellular proteolysis, GH3PRO-GLO cells were incubated in the presence of increasing concentrations of chloroquine. At 25/zM chloroquine PRO-GLO processing was inhibited by '~95 % and at higher concentrations proteolysis was quantitatively inhibited resulting in secretion of only unprocessed precursor (Fig. 7) . In the absence of proteolytic processing at 25 and 50 #M chloroquine, enhanced secretion of unprocessed PRO-GLO was seen (Fig. 7, lanes 7and 8) . At higher chloroquine concentrations secretion was diminished (Fig. 7 , lane 9) most likely due to cytotoxic effects of the drug. These results contrast with those of SIG-GLO and further demonstrate that the two precursors were transported to distinct cellular compartments.