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J. Biol. Chem., Vol. 282, Issue 13, 9392-9400, March 30, 2007

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The Stardust Family Protein MPP7 Forms a Tripartite Complex with LIN7 and DLG1 That Regulates the Stability and Localization of DLG1 to Cell Junctions^*

From the Department of Pathology, University of Virginia School of Medicine, Charlottesville, Virginia 22908

MPP7, a previously uncharacterized member of the p55 Stardust family of membrane-associated guanylate kinase (MAGUK) proteins, was found in a tripartite complex with DLG1 and LIN7A or LIN7C. MPP7 dimerizes with all three LIN7 family members (LIN7A, -B, and -C) through interaction of the single L27 domain of LIN7 with the carboxyl-terminal L27 domain of MPP7, thereby stabilizing both proteins. The dimer of MPP7 with LIN7A or LIN7C associates with DLG1 through an interaction requiring the amino-terminal L27 domain of MPP7. The amino-terminal L27 domain of MPP7 is not sufficient for interaction with DLG1 but interacts efficiently only if MPP7 is in a complex with LIN7A or -C. Thus the specificity of interaction of DLG1 with the LIN7-MPP7 complex is determined by L27 interactions with both MPP7 and LIN7. The tripartite complex forms in a ratio of 1:1:1 and localizes to epithelial adherens junctions in a manner dependent upon MPP7. Expression of MPP7 stabilizes DLG1 in an insoluble compartment. Expression of MPP7 deleted of the PDZ or Src homology 3 domain redistributes MPP7, DLG1, and LIN7 out of adherens junctions and into the soluble cytoplasmic fraction without changing the localization of E-cadherin. Thus, the stability and localization of DLG1 to cell-cell junctions are complex functions determined by the expression and association of particular Stardust family members together with particular LIN7 family members.

Received for publication, October 25, 2006 , and in revised form, December 26, 2006.

^* This work was supported by National Institutes of Health Grants CA80931 and CA69292 (to S. V.) and by the University of Virginia Department of Pathology Collaborative Mass Spectrometry Facility. 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S4.

¹ To whom correspondence should be addressed: Dept. of Pathology, University of Virginia. P. O. Box 800904, Charlottesville, VA 22908. Tel.: 434-924-1603; Fax: 434-924-2151; E-mail: vandepol{at}virginia.edu.

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