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

Peptide hormone-producing cells characteristically concentrate and store their secretory product in electron-dense secretory granules (32) . Upon stimulation by an extracellular signal, these granules fuse, through a calcium-dependent process, with the plasma membrane releasing their contents into the external milieu. This type of secretion is designated "regulated" or "stimulated" (see references 4, 14, and 33 for recent reviews). Hormone-secreting cells also undergo basal or "constitutive" secretion whereby nonhormone secretory proteins and plasma membrane proteins are neither concentrated nor stored and are transported in vesicles which continuously fuse with the plasma membrane in a calciumindependent manner (4, 14) . Since hormone-secreting cells undergo basal secretion, a mechanism must exist that discriminates between molecules destined for the regulated or constitutive pathways.