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Vol. 17, Issue 4, 1606-1619, April 2006

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Cell Surface Transglutaminase Promotes RhoA Activation via Integrin Clustering and Suppression of the Src–p190RhoGAP Signaling Pathway

Anna Janiak ^* , Evgeny A. Zemskov ^*, and Alexey M. Belkin ^*  

^* Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD 21201; Marlene and Stewart Greenebaum Cancer Center, University of Maryland School of Medicine, Baltimore, MD 21201; and Center for Vascular and Inflammatory Diseases, University of Maryland School of Medicine, Baltimore, MD 21201

Submitted June 21, 2005; Revised January 18, 2006; Accepted January 20, 2006
Monitoring Editor: Martin A. Schwartz

Tissue transglutaminase (tTG) is a multifunctional protein that serves as cross-linking enzyme and integrin-binding adhesion coreceptor for fibronectin on the cell surface. Previous work showed activation of small GTPase RhoA via enzymatic transamidation by cytoplasmic tTG. Here, we report an alternative nonenzymatic mechanism of RhoA activation by cell surface tTG. Direct engagement of surface tTG with specific antibody or the fibronectin fragment containing modules I₆II_1,2I_7-9 increases RhoA-GTP levels. Integrin-dependent signaling to RhoA and its downstream target Rho-associated coiled-coil containing serine/threonine protein kinase (ROCK) is amplified by surface tTG. tTG expression on the cell surface elevates RhoA-GTP levels in nonadherent and adherent cells, delays maximal RhoA activation upon cell adhesion to fibronectin and accelerates a rise in RhoA activity after binding soluble integrin ligands. These data indicate that surface tTG induces integrin clustering regardless of integrin–ligand interactions. This notion is supported by visualization of integrin clusters, increased susceptibility of integrins to chemical cross-linking, and biochemical detection of large integrin complexes in cells expressing tTG. In turn, integrin aggregation by surface tTG inhibits Src kinase activity and decreases activation of the Src substrate p190RhoGAP. Moreover, pharmacological inhibition of Src kinase reveals inactivation of Src signaling as the primary cause of elevated RhoA activity in cells expressing tTG. Together, these findings show that surface tTG amplifies integrin-mediated signaling to RhoA/ROCK via integrin clustering and down-regulation of the Src–p190RhoGAP regulatory pathway.

Abbreviations used: DTSSP, 3,3'-dithiobis(sulfosuccinimidyl propionate); LPA, lysophosphatidic acid; MBS, myosin-binding subunit of myosin phosphatase; MLC2, myosin regulatory light chain; RBD, RhoA-binding domain (amino acids 7–89) of Rhotekin; ROCK, Rho-associated coiled-coil containing serine/threonine protein kinase.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Present address: Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, MD 21205.

Address correspondence to: Alexey M. Belkin (abelk001{at}umaryland.edu).

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