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J. Biol. Chem., Vol. 280, Issue 34, 30018-30024, August 26, 2005

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BAP31 and Its Caspase Cleavage Product Regulate Cell Surface Expression of Tetraspanins and Integrin-mediated Cell Survival^*

Marina Stojanovic, Marc Germain, Mai Nguyen, and Gordon C. Shore¶||

From the Department of Biochemistry and ^¶McGill Cancer Center, McIntyre Medical Sciences Building, McGill University, Montreal, Quebec H3G 1Y6, Canada

BAP31, a resident integral protein of the endoplasmic reticulum membrane, regulates the export of other integral membrane proteins to the downstream secretory pathway. Here we show that cell surface expression of the tetraspanins CD9 and CD81 is compromised in mouse cells from which the Bap31 gene has been deleted. CD9 and CD81 facilitate the function of multiprotein complexes at the plasma membrane, including integrins. Of note, BAP31 does not appear to influence the egress of 51 or v3 integrins to the cell surface, but in Bap31-null mouse cells, these integrins are not able to maintain cellular adhesion to the extracellular matrix in the presence of reduced serum. Consequently, Bap31-null cells are sensitive to serum starvation-induced apoptosis. Reconstitution of wild-type BAP31 into these Bap31-null cells restores integrin-mediated cell attachment and cell survival after serum stress, whereas interference with the functions of CD9, 51, or v3 by antagonizing antibodies makes BAP31 cells act similar to Bap31-null cells in these respects. Finally, in human KB epithelial cells protected from apoptosis by BCL-2, the caspase-8 cleavage product, p20 BAP31, inhibits egress of tetraspanin and integrin-mediated cell attachment. Thus, p20 BAP31 can operate upstream of BCL-2 in living cells to influence cell surface properties due to its effects on protein egress from the endoplasmic reticulum.

Received for publication, February 3, 2005 , and in revised form, June 8, 2005.

^* This work was supported by the Canadian Institutes of Health Research and the National Cancer Institute of Canada through funds provided by the Canadian Cancer Society. 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.

Recipient of the Canadian Institutes of Health Research Doctoral Research award.

^|| To whom correspondence should be addressed: Dept. of Biochemistry, McIntyre, Medical Sciences Bldg., McGill University, 3655 Promenade Sir William Osler, Montreal, Quebec H3G 1Y6, Canada. Tel.: 514-398-7282; Fax: 514-398-7384; E-mail: gordon.shore{at}mcgill.ca.

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