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J. Biol. Chem., Vol. 278, Issue 45, 44832-44842, November 7, 2003

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Arabidopsis Capping Protein (AtCP) Is a Heterodimer That Regulates Assembly at the Barbed Ends of Actin Filaments^*

Shanjin Huang, Laurent Blanchoin¶, David R. Kovar||**, and Christopher J. Staiger

From the Department of Biological Sciences and Purdue Motility Group, Purdue University, West Lafayette, Indiana 47907-2064, Laboratoire de Physiologie Cellulaire Végétale, Commissariat à l'Energie Atomique (CEA)/CNRS/Université Joseph Fourier, CEA F38054 Grenoble, France, and ^||Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, Connecticut 06511

The precise regulation of actin filament polymerization and depolymerization is essential for many cellular processes and is choreographed by a multitude of actin-binding proteins (ABPs). In higher plants the number of well characterized ABPs is quite limited, and some evidence points to significant differences in the biochemical properties of apparently conserved proteins. Here we provide the first evidence for the existence and biochemical properties of a heterodimeric capping protein from Arabidopsis thaliana (AtCP). The purified recombinant protein binds to actin filament barbed ends with K_d values of 12–24 nM, as assayed both kinetically and at steady state. AtCP prevents the addition of profilin actin to barbed ends during a seeded elongation reaction and suppresses dilution-mediated depolymerization. It does not, however, sever actin filaments and does not have a preference for the source of actin. During assembly from Mg-ATP-actin monomers, AtCP eliminates the initial lag period for actin polymerization and increases the maximum rate of polymerization. Indeed, the efficiency of actin nucleation of 0.042 pointed ends created per AtCP polypeptide compares favorably with mouse CapZ, which has a maximal nucleation of 0.17 pointed ends per CapZ polypeptide. AtCP activity is not affected by calcium but is sensitive to phosphatidylinositol 4,5-bisphosphate. We propose that AtCP is a major regulator of actin dynamics in plant cells that, together with abundant profilin, is responsible for maintaining a large pool of actin subunits and a surprisingly small population of F-actin.

Received for publication, June 24, 2003 , and in revised form, August 6, 2003.

^* This research was supported by Department of Energy, Energy Biosciences Division Grant DE-FG02-99ER20337A01 (to C. J. S.). Microscopy facilities were established with partial support from the Showalter Fund, Purdue University and National Science Foundation Grant 0217552-DBI. 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^TM in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ Supported by an Actions Thématiques et Incitives sur Programmes et Equipes from the CNRS.

^** Supported by a National Institutes of Health post-doctoral fellowship.

To whom correspondence should be addressed: Dept. of Biological Sciences, Purdue University, 333 Hansen Life Sciences Bldg., 201 S. University St., West Lafayette, IN 47907-2064. Fax: 765-496-1496; E-mail: cstaiger{at}bilbo.bio.purdue.edu.

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