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J. Biol. Chem., Vol. 280, Issue 27, 25424-25435, July 8, 2005

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Retrieval and Reuse of Pituitary Secretory Granule Proteins^*

Francesco Ferraro, Betty A. Eipper, and Richard E. Mains

From the Neuroscience Department, University of Connecticut Health Center, Farmington, Connecticut 06030-3401

The pituitary contains professional secretory cells, devoting a large fraction of their energy to the synthesis of hormones that are stored for secretion in response to a complex mixture of inputs. Ba²⁺, a substitute for Ca²⁺, and phorbol ester, a mimic for diacylglycerol, have a synergistic effect on exocytosis. By using these secretagogues, we developed a paradigm in which phorbol ester potentiation of Ba²⁺-evoked exocytosis produces a robust secretory response in multiple pituitary cell types. Because cells subjected to this stimulatory paradigm remain healthy despite their greatly reduced hormone content, we used this paradigm to study the fate of granule membrane proteins. We examined the turnover of peptidylglycine -amidating monooxygenase (PAM), a membrane enzyme involved in the final maturation of many peptides, and VAMP2, a vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE). The stability of recently synthesized PAM was increased by sustained exocytosis. Biotinylation studies established that the appearance of integral membrane PAM at the plasma membrane was stimulated along with hormone secretion. PAM biotinylated on the cell surface undergoes cleavage to yield soluble peptidylglycine--hydroxylating monooxygenase that can then be secreted in a regulated fashion. Consistent with a kiss-and-run or cavicapture mode of secretion (Taraska, J. W., Perrais, D., Ohara-Imaizumi, M., Nagamatsu, S., and Almers, W. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2070–2075), biotinylated prolactin was also retained by the cells and later released in response to secretagogues. Thus, pituitary cells can retrieve and reuse components of the machinery involved in the final stages of exocytosis (the SNAREs) as well as soluble and membrane granule proteins.

Received for publication, December 16, 2004 , and in revised form, April 18, 2005.

^* This work was supported by National Institutes of Health Grant DK32948 (to B. A. E. and R. E. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Neuroscience Dept., University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030-3401. Tel.: 860-679-8894; Fax: 860-679-1885; E-mail: mains{at}uchc.edu.

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