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

We recently demonstrated (41) that growth hormone-producing cells (GH3) efficiently cleave proSRIF and sort the mature hormone into the regulated secretory pathway. We previously postulated that the SRIF propeptide might function in mediating intracellular transport (46) . To test this hypothesis directly, we used a retroviral vector, pLJ (15) , to express a fusion protein (PRO-GLO) consisting of the SRIF signal peptide and proregion and chimpanzee at-globin in pituitary GH3 cells. The o~-globin polypeptide was chosen as a reporter group for these studies since (a) it is a cytoplasmic protein and is unlikely to possess intrinsic sorting information; (b) previous work demonstrated that attachment of a bacterial signal peptide to the NH2 terminus of tx-globin resulted in its translocation into the ER in vitro and in vivo (17, 40) ; and (c) expression of an ovalbumin signal peptidect-globin fusion protein in Xenopus oocytes led to efficient segregation of c~-globin into the ER but not to its secretion (42) . Here we show that a signal peptide-globin fusion protein (SIG-GLO) was translocated into the ER in vivo, but in the absence of the SRIF propeptide it was rapidly degraded with a half-life of 4-5 min. In contrast, PRO-GLO was transported from the ER to the TGN where it underwent proteolytic cleavage yielding mature u-globin which was secreted into the medium.