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Published online 27 December 2000. doi:10.1083/jcb.151.7.1435
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© The Rockefeller University Press, 0021-9525/2000/12/1435/ $5.00
The Journal of Cell Biology, Volume 151, Number 7, December 25, 2000 1435-1448


Original Article

Actopaxin, a New Focal Adhesion Protein That Binds Paxillin LD Motifs and Actin and Regulates Cell Adhesion

Sotiris N. Nikolopoulosa and Christopher E. Turnera
a Department of Cell and Developmental Biology, State University of New York Upstate Medical University, Syracuse, New York 13210

Correspondence to: Christopher E. Turner, Department of Cell and Developmental Biology, SUNY Upstate Medical University, 750 East Adams St., Syracuse, NY 13210. Tel:(315) 464-8598 Fax:(315) 464-8535 E-mail:turnerc{at}mail.upstate.edu.

Paxillin is a focal adhesion adapter protein involved in the integration of growth factor– and adhesion-mediated signal transduction pathways. Paxillin LD motifs have been demonstrated to bind to several proteins associated with remodeling of the actin cytoskeleton including the focal adhesion kinase, vinculin, and a complex of proteins comprising p95PKL, PIX, and PAK (Turner, C.E., M.C. Brown, J.A. Perrotta, M.C. Riedy, S.N. Nikolopoulos, A.R. McDonald, S. Bagrodia, S. Thomas, and P.S. Leventhal. 1999. J. Cell Biol. 145:851–863). In this study, we report the cloning and initial characterization of a new paxillin LD motif–binding protein, actopaxin. Analysis of the deduced amino acid sequence of actopaxin reveals a 42-kD protein with two calponin homology domains and a paxillin-binding subdomain (PBS). Western blotting identifies actopaxin as a widely expressed protein. Actopaxin binds directly to both F-actin and paxillin LD1 and LD4 motifs. It exhibits robust focal adhesion localization in several cultured cell types but is not found along the length of the associated actin-rich stress fibers. Similar to paxillin, it is absent from actin-rich cell–cell adherens junctions. Also, actopaxin colocalizes with paxillin to rudimentary focal complexes at the leading edge of migrating cells. An actopaxin PBS mutant incapable of binding paxillin in vitro cannot target to focal adhesions when expressed in fibroblasts. In addition, ectopic expression of the PBS mutant and/or the COOH terminus of actopaxin in HeLa cells resulted in substantial reduction in adhesion to collagen. Together, these results suggest an important role for actopaxin in integrin-dependent remodeling of the actin cytoskeleton during cell motility and cell adhesion.

Key Words: actin-binding protein, CH domains, focal adhesions, paxillin, hic-5


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