SECRETOGRANIN-III IS A SULFATED PROTEIN UNDERGOING PROTEOLYTIC PROCESSING IN THE REGULATED SECRETORY PATHWAY

Secretogranin III (SgIII) is an acidic protein of unknown function tha t is present in the storage vesicles of many neuroendocrine cells, It is coexpressed with the prohormone proopiomelanocortin in the intermed iate pituitary of Xenopus laevis, We developed an antiserum to investi gate the biosynthesis of SgIII in pulse-chase incubated Xenopus neuroi ntermediate lobes, SgIII was synthesized as a 61- or 63-kDa (N-glycosy lated) protein and processed to a 48-kDa form which, in turn, was part ially cleaved to fragments of 28 and 20 kDa, The 48-, 28-, and 20-kDa cleavage products, but not their precursors, were secreted, This secre tion is regulated and can be blocked in parallel with that of proopiom elanocortin-derived peptides by the hypothalamic factors dopamine, gam ma-aminobutyric acid, and neuropeptide Y. Coexpression of Xenopus SgII I with prohormone convertase (PC)1 or PC2 in transfected fibroblasts w as sufficient to reconstitute the processing events observed in the ne urointermediate lobes, Site-directed mutagenesis revealed that Xenopus SgIII is cleaved at two dibasic sites, namely Lys(68)-Arg(69) and Arg (237)-Arg(238), Pulse-chase incubations of lobes with Na-2[S-35]SO4 sh owed that SgIII is sulfated in the trans-Golgi network before it is pr ocessed, Finally, SgIII processing was found in several neuroendocrine cell types from various species, We conclude that SgIII is a precurso r protein and that the intact molecule can only have an intracellular function, whereas an extracellular role can only be attributed to its cleavage products.