PHOSPHATIDYLINOSITOL 3-KINASE BINDS TO ALPHA-ACTININ THROUGH THE P85 SUBUNIT

Phosphatidylinositol 3-kinase (PI 3-kinase) has been shown to play an important role in the signal transduction of cell growth. It is also s uggested that it is involved in cytoskeletal reorganization. We have f ound that alpha-actinin copurifies with PI 3-kinase from bovine thymus . The antibody against PI 3-kinase 85 kDa subunit (p85) also co-immuno precipitates alpha-actinin from lysates of NIH/3T3 cells. In addition, anti-alpha-actinin antibody coprecipitates PI 3-kinase activity. This coprecipitation was observed even after depolymerization of actin fib res, suggesting that PI 3-kinase binds directly to alpha-actinin. As a lpha-actinin is a phosphatidylinositol 4,5-bisphosphate (PI4,5P(2))-bi nding protein, bindingexperiments using various constructs of truncate d p85 were carried out in the presence or absence of PI4,5P(2). In the absence of PI4,5P(2), chicken gizzard alpha-actinin binds only to the whole p85 construct, but it binds to the proline-rich region of p85 f ragments in the presence of PI4,5P(2). This binding is enhanced with i ncreased concentrations of PI4,5P(2) up to 10 mu M, whereas phosphatid ylinositol and phosphatidylinositol 4-phosphate were not good activato rs of alpha-actinin binding. These results suggest that PI 3-kinase bi nds to alpha-actinin and regulates cytoskeletal reorganization.