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J. Biol. Chem., Vol. 282, Issue 24, 17632-17639, June 15, 2007

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Mammalian Sec16/p250 Plays a Role in Membrane Traffic from the Endoplasmic Reticulum^*

Takayuki Iinuma, Akiko Shiga, Koji Nakamoto, Matthew B. O'Brien, Meir Aridor, Nagisa Arimitsu, Mitsuo Tagaya, and Katsuko Tani¹

From the School of Life Sciences, Tokyo University of Pharmacy and Life Sciences, Hachioji, Tokyo 192-0392, Japan and the Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261

Coat protein complex II (COPII)-coated vesicles/carriers, which mediate export of proteins from the endoplasmic reticulum (ER), are formed at special ER subdomains in mammals, termed ER exit sites or transitional ER. The COPII coat consists of a small GTPase, Sar1, and two protein complexes, Sec23-Sec24 and Sec13-Sec31. Sec23-Sec24 and Sec13-Sec31 appear to constitute the inner and the outermost layers of the COPII coat, respectively. We previously isolated two mammalian proteins (p125 and p250) that bind to Sec23. p125 was found to be a mammalian-specific, phospholipase A₁-like protein that participates in the organization of ER exit sites. Here we show that p250 is encoded by the KIAA0310 clone and has sequence similarity to yeast Sec16 protein. Although KIAA0310p was found to be localized at ER exit sites, subcellular fractionation revealed its predominant presence in the cytosol. Cytosolic KIAA0310p was recruited to ER membranes in a manner dependent on Sar1. Depletion of KIAA0310p mildly caused disorganization of ER exit sites and delayed protein transport from the ER, suggesting its implication in membrane traffic out of the ER. Overexpression of KIAA0310p affected ER exit sites in a manner different from that of p125. Binding experiments suggested that KIAA0310p interacts with both the inner and the outermost layer coat complexes, whereas p125 binds principally to the inner layer complex. Our results suggest that KIAA0310p, a mammalian homologue of yeast Sec16, builds up ER exit sites in cooperation with p125 and plays a role in membrane traffic from the ER.

Received for publication, December 7, 2006 , and in revised form, March 27, 2007.

^* This work was supported by Grants-in-Aid for Scientific Research 18570186 and 18370081 from the Ministry of Education, Science, Sports and Culture of Japan (to K. T. and M. T.) and by National Institute of Health Grant DK062318 (M. A.). 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.

¹ To whom correspondence should be addressed. Tel.: 81-426-76-7110; Fax: 81-426-76-5468; E-mail: tani{at}ls.toyaku.ac.jp.

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