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Heterodimeric Capping Protein from Arabidopsis Is Regulated by Phosphatidic Acid
Shanjin Huang *,
Lisa Gao *,
Laurent Blanchoin, and
Christopher J. Staiger *
* Department of Biological Sciences and The Bindley Bioscience Center, Purdue University, West Lafayette, IN 47907-2064 Laboratoire de Physiologie Cellulaire Végétale, Commissariat à l'Energie Atomique/Centre National de la Recherche Scientifique/Université Joseph Fourier, F38054 Grenoble, France
Received for publication September 6, 2005.
Revision received December 13, 2005.
Accepted for publication January 17, 2006.
Monitoring Editor: Thomas Pollard
The cytoskeleton is a key regulator of morphogenesis, sexualreproduction, and cellular responses to extracellular stimuli.Changes in the cellular architecture are often assumed to requireactin-binding proteins as stimulus-response modulators, becausemany of these proteins are regulated directly by binding tointracellular second messengers or signaling phospholipids.Phosphatidic acid (PA) is gaining widespread acceptance as amajor, abundant phospholipid in plants that is required forpollen tube tip growth and mediates responses to osmotic stress,wounding, and phytohormones; however, the number of identifiedeffectors of PA is rather limited. Here we demonstrate thatexogenous PA application leads to significant increases in filamentousactin levels in Arabidopsis suspension cells and poppy pollengrains. To investigate further these lipid-induced changes inpolymer levels, we analyzed the properties of a key regulatorof actin filament polymerization, the heterodimeric cappingprotein from Arabidopsis thaliana (AtCP). AtCP binds to PA witha Kd value of 17 µM and stoichiometry of 1:2. It alsobinds well to PtdIns(4,5)P2, but not to several other phosphoinositideor acidic phospholipids. The interaction with PA inhibited theactin-binding activity of CP. In the presence of PA, CP is unableto block the barbed or rapidly growing and shrinking end ofactin filaments. Precapped filament barbed ends can also beuncapped by addition of PA, allowing rapid filament assemblyfrom an actin monomer pool that is buffered with profilin. Thefindings support a model in which the inhibition of CP activityin cells by elevated PA results in the stimulation of actinpolymerization from a large pool of profilin-actin. Such regulationmay be important for the response of plant cells to extracellularstimuli as well as for the normal process of pollen tube tipgrowth.