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

A characteristic morphological feature of many endocrine cells is the possession of dense core secretory granules in which the mature hormone is stored in a semicrystalline form. Upon stimulation, these granules fuse with the plasma membrane releasing their contents into the circulation. In general, there is little sequence homology among a diverse array of proteins targeted to secretory granules, thus, it is unlikely that a primary consensus sequence is involved in targeting. Recent evidence, using chimeric proteins, implies targeting to the regulated secretory pathway may be an "active" sorting process that contrasts with constitutive secretion which probably occurs by bulk flow (4, 33, 47) . For exam- pie, a fusion between growth hormone and a truncated form of a constitutively secreted viral glycoprotein (vesicular stomatitis virus glycoprotein [VSV-G]) resulted in transport to the regulated pathway (24) . Although this result suggests that a positive signal in growth hormone confers a "dominant" sorting phenotype on the truncated VSV-G protein, the sorting sequence was not identified.