Abstract

An extensive survey was carried out for compounds capable of regulating actin-binding proteins in a manner similar to phosphatidylinositol 4,5 bisphosphate (PI 4,5-P₂). For this purpose we developed a sensitive assay involving release of radioactively phosphorylated actin from the fragminP–actin complex. We found that the structurally simplest lysophospholipid, lysophosphatidic acid (LPA), dissociated the complex between fragminP and actin, whereas other lysophospholipids or sphingosine-1-phosphate were inactive. Furthermore, LPA inhibited the F-actin severing activity of human gelsolin, purified from plasma or as recombinant protein, mouse adseverin and Physarum fragminP. Dissociation of actin-containing complexes by LPA analyzed by gelfiltration indicated that LPA is active as a monomer, in contrast to PI 4,5-P₂. We further show that binding of LPA to these actin-regulatory proteins promotes their phosphorylation by pp60^c-src. A PI 4,5-P₂-binding peptide counteracted the effects mediated by LPA, suggesting that LPA binds to the same target region in these actin-binding proteins. When both LPA and PI 4,5-P₂ were used in combination we found that LPA reduced the threshold concentration at which PI 4,5-P₂ was active. Significantly, LPA promoted the release of gelsolin from barbed actin filaments in octylglucoside-permeabilized human platelets. These results suggest that lysophosphatidic acid could act as an intracellular modulator of actin-binding proteins. Our findings can also explain agonist-induced changes in the actin cytoskeleton that are not mediated by polyphosphoinositides.