SORTING OF PC2 TO THE REGULATED SECRETORY PATHWAY IN ATT20 CELLS

PC2 and PC3 are neuroendocrine specific members of the eukaryotic subt ilisin-like proprotein convertase (PC) family. Both are sorted via the regulated secretory pathway into secretory granules. In order to iden tify sequences in PC2 which are involved in targeting to the regulated secretory pathway we expressed a series of PC2 cDNAs containing mutat ions in the C terminal or propeptide domains in the mouse corticotroph ic AtT20 cell line. Sorting of endogenous PC3 was used as a control. P C2 and PC3 were secreted with similar kinetics and sorted to secretory granules with similar efficiencies. Deletions of up to 50 amino acids from the C-terminus of proPC2 had no effect on secretion or sorting, but larger deletions completely prevented maturation or secretion. Two large deletions within the propeptide also prevented secretion. Small er deletions between the primary and secondary cleavage sites, or of t he primary cleavage site, reduced the amount of protein secreted but d id not affect sorting to secretory granules. Replacement of the propep tide of PC2 with that of the endogenous PC3 also had no effect on secr etion or sorting. The results indicate that targeting of proPC2 to the regulated secretory pathway is dependent on more than one region with in the proPC2 molecule.