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First published online October 11, 2005
doi: 10.1242/10.1242/jcs.02608

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Interaction between secretogranin III and carboxypeptidase E facilitates prohormone sorting within secretory granules

Masahiro Hosaka^1,*, Tsuyoshi Watanabe^2,*, Yuko Sakai², Takeshi Kato³ and Toshiyuki Takeuchi^1,

¹ Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi 371-8512, Japan
² Department of Anatomy II, Asahikawa Medical College, Asahikawa 078-8510, Japan
³ Laboratory of Molecular Recognition, Graduate School of Integrated Science, Yokohama City University, Yokohama 236-0027, Japan

Author for correspondence (e-mail: tstake{at}showa.gunma-u.ac.jp)

Accepted 2 August 2005

Secretogranin III (SgIII) and carboxypeptidase E (CPE) bind specifically to cholesterol-rich secretory granule (SG) membranes. We previously showed that SgIII binds chromogranin A (CgA) and targets CgA to the SGs in endocrine cells. We investigated the binding of SgIII and CPE because they frequently localize close to the periphery of SGs, and they bind each other in mouse corticotrope-derived AtT-20 cells. In Cpe^fat mouse corticotropes, which have defective CPE, proopiomelanocortin (POMC)-derived adrenocorticotrophin hormone (ACTH)-containing peptides were distributed over the entire surface of the SGs, and displayed a regulated secretion by secretagogues. The Cpe^fat pituitary exhibited elevated levels of SgIII and CgA, which suggests that they compensate for a sorting function of CPE for POMC and its intermediates to ACTH. Indeed, both SgIII and CgA were able to bind POMC-derived intermediates. In a competitive pull-down assay, excessive SgIII led to a decrease in CPE-bound POMC-derived intermediate molecules, and SgIII pulled-down by anti-ACTH antibody increased proportionately. We suggest that SgIII and CPE form the separate functional sorting complex by anchoring to cholesterol-rich SG membranes, and POMC-derived peptides are transferred from CPE to SgIII, and subsequently to CgA.

Key words: Secretogranin III, Carboxypeptidase E, Chromogranin A, Proopiomelanocortin, Prohormone sorting

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