ACTIN Phosphatidic Acid Inhibits Arabidopsis Actin Capping Protein

Regulation of the cytoskeleton is a key feature of many cellular responses. In plants, changes in the actin cytoskeleton and phosphatidic acid (PA) are associated with pollen germination and tip growth, as well as the response to various pathogens and stresses. Huang et al. report that Arabidopsis thaliana capping protein (AtCP) is inhibited by PA, thereby directly connecting the actin cytoskeleton to signals that alter PA abundance. Application of PA to pollen from field poppy plants or Arabidopsis suspension cells resulted in uptake of PA and increased filamentous actin, an effect that was specific to PA and not produced by phosphatidylserine or lysophosphatidic acid. In a protein-lipid blot overlay assay, AtCP exhibited preferential binding to PA, and in a tryptophan fluorescence quenching assay, AtCP exhibited moderate affinity binding to PA and phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂]. In kinetic assays for actin polymerization in vitro, addition of PA or PI(4,5)P2 increased the lag time of actin nucleation in the presence of AtCP (indicating that the actin filament assembly activity of AtCP was inhibited) and promoted actin filament disassembly in the presence of AtCP (indicating that the barbed-end capping activity of AtCP was inhibited). PA also promoted the uncapping of AtCP from preformed actin filaments, and kinetic modeling suggested that PA decreased the affinity of the AtCP for the barbed ends of actin.

S. Huang, L. Gao, L. Blanchoin, C. J. Staiger, Heterodimeric capping protein from Arabidopsis is regulated by phosphatidic acid. Mol. Biol. Cell 17, 1946-1958 (2006). [Abstract] [Full Text]

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