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

Approximately 60 % of the transported PRO-GLO was processed to mature ~-globin. Partial NH~-terminal sequencing was performed to determine if proteolytic cleavage had occurred at the predicted processing sites: on the carboxyl side of lysine in the hexapeptide domain in PRO-GLO and the junction of the signal peptide and t~-globin in SIG-GLO (Fig. 6) . Chimpanzee c~-globin has methionine residues at positions 1 and 33. Consistent with correct signal peptide cleavage, methionine residues were detected at positions 1 and 33 in the ot-globin polypeptide from GH3SIG-GLO cells. PRO-GLO contains four additional NH2-terminal amino acids (AlaAspProArg; Fig. 1 ). Therefore, if the prohormone processing enzyme(s) recognized the endoproteolytic processing site of PRO-GLO (ArgLys), a methionine residue should be present at position 5. The sequence data demonstrated that this was the case (Fig. 6) . Thus, despite the pres- Fig. 8 ). The chase medium was treated with antihemoglobin antiserum, the immunoprecipitates separated on a SEP-PAK and applied directly to the sequencer, and subjected to 21 cycles of automated Edman degradation. The radioactivity in each cycle was determined by liquid scintillation counting.