The formation mechanisms of secretory granules and synaptic-like micro-vesicles by the lead of cholesterol in endocrine cells.

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The secretory granules (SGs) of neuroendocrine cells contain granin proteins in addition to peptide hormones and prohormone convertases. Granin-family proteins, including chromogranin A (CgA), chromogranin B, secretogranin II, secretogranin III (SgIII), and 7B2, are thought to condense peptide hormones to SGs. To investigate the sorting mechanisms of peptide hormones to the SGs, we selected CgA as a model protein. Then we addressed following four findings. 1) We have screened a rat brain cDNA library with a yeast two-hybrid system using CgA as a bait. Although membrane-bound proteins were initially expected, a soluble secretory protein, SgIII, was identified. Following analyses demonstrated that CgA requires the SgIII-binding domain for its targeting to the SGs, whereas SgIII does not need the CgA-binding domain to target to the SGs. 2) SgIII was localized to the secretory granule membrane (SGM) despite lacking the transmembrane region. SgIII directly binds to cholesterol components of the SGM and targets CgA to SGs. 3) SgIII and carboxypeptidase E (CPE; a different molecule of a sorting receptor for prohormones) core the separate functional sorting complex by anchoring to cholesterol-rich secretory granule membranes, and POMC-derived peptides are transferred from CPE to SgIII, and further to CgA. With this grant, I have studied the formation mechanisms of secretory granules and synaptic-like micro-vesicles by the lead of cholesterol in endocrine cells. Moreover, we have shown that SgIII can bind the aggregated form of CgA, suggesting that a small quantity of SgIII is enough to target a large quantity of CgA to the secretory granules.