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

PRO-GLO processing efficiency and the kinetics of a-globin secretion were determined by excising globin-specific polypeptides from gels similar to that shown in Fig. 2 and determining the radioactivity (Fig. 5) . With increasing chase times, PRO-GLO decreased with a concomitant increase in the level of intracellular mature c~-globin (Fig. 5 A) which remained constant from 90 to 180 min of chase. Processing of PRO-GLO to mature globin was detected between 15-30 min of chase and some c~-globin was secreted into the medium following a lag of "~30 min (Fig. 5 B) . It is noteworthy that in these cells, the kinetics of PRO-GLO cleavage to c~-globin were identical to those of native proSRIF processing to the mature hormone (41) , suggesting that the chimera and native precursors were transported through the same intracellular compartments at similar rates. About 30 % of the total pulse-labeled globin-immunoreactive material could be recovered intracellularly and in the medium at 120 min of chase. We presume that the initially synthesized PRO-GLO which was not recovered after 15 min of chase was degraded intracellularly in the same compartment as SIG-GLO. Since PRO-GLO contains three methionine residues whereas mature ct-globin has two methionines, we estimate that "~60-65 % of the transported PRO-GLO, i.e., that present after the 15-min chase, was processed to mature ~-globin at 2 h of chase. Of this processed globin, ,'~45 % was stored intracellularly with a half-life >4 h.