Secretory granules are found in specialized cell types, including endocrine cells, suggesting that a coordinated programme of gene expression is involved in their biogenesis. Indeed, it has been proposed that chromogranin A (CgA) acts as an on/off switch for secretory granule biogenesis. However, this proposed function is difficult to reconcile with the large body of evidence suggesting that secretory granules exist in the absence of CgA and that cells can synthesize CgA in the absence of secretory granules. Indeed, recent evidence suggests that, rather than a master gene or universal on/off switch, a series of on/off switches combines to induce expression of subsets of secretory granule-associated genes. The assembly of newly synthesized proteins and the inclusion of existing granule proteins would produce functional secretory granules. CgA and related proteins might act as assembly factors in this process.
Bibliographical noteFunding Information:
Work in our laboratories was supported by PHS grants R01 DK53367, DE12205, Jewish Hospital Foundation, Louisville, KY, USA (S-U.G.), the Juvenile Diabetes Research Foundation and the Canadian Institutes of Health Research (R.D.). R.D. is a scholar of the Fonds de la Recherche en Sante du Quebec.