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

It might be argued that since only a fraction (',~30%) of PRO-GLO was transported from the ER, that the secreted globin was transported to granules by default in these cells, or that the SRIF propeptide may have facilitated nonspecific associations with endogenous secretory proteins, thereby leading to "passive" sorting into the regulated pathway. The obvious candidate for such a protein is growth hormone which is by far the most abundant polypeptide synthesized by and secreted from GH3 cells. However, we recently demonstrated (41) that, in these cells, only 5 % of newly synthesized growth hormone is stored intracellularly in a cAMPresponsive compartment, whereas 95 % is secreted constitutively. Consequently, if PRO-GLO were "sticking" to growth hormone, for example, only ~5 % would be expected to enter the regulated pathway. In fact our data demonstrate that "~45 % of the processed PRO-GLO is stored intracellularly; i.e., significantly more efficiently than endogenous growth hormone. This figure is consistent with the observation (44) that 55 % of processed native proSRIF was sorted to the regulated pathway and secreted in response to 8-Br-cAMP. Approximately 90% of the stored intracellular material was secreted in response to cAMP; equivalent to a fourfold stimulation over the basal level of secretion. This observation is consistent with storage of globin in mature secretory granules. We conclude that the propeptide-mediated targeting to the regulated pathway and facilitated packaging of globin into a cAMP-stimulatable compartment. Morphologic studies are currently in progress to determine the precise cellular localization of these molecules.