Conformation, localization, and integrin binding of talin depend on its interaction with phosphoinositides

Talin is a structural component of focal adhesion sites and is thought to b e engaged in multiple protein interactions at the cytoplasmic face of cell/ matrix contacts. Talin is a major link between integrin and the actin cytos keleton and was shown to play an important role in focal adhesion assembly. Consistent with the view that talin must be activated at these sites, we f ound that phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5 -bisphosphate (PI4,5P(2)) bound to talin in cells in suspension or at early stages of adhesion, respectively. When phosphoinositides were associated w ith phospholipid bilayer, talin/phosphoinositide association was restricted to PI4,5P(2). This association led to a conformational change of the prote in. Moreover, the interaction between integrin and talin was greatly enhanc ed by PI4,5P(2)-induced talin activation. Finally, sequestration of PI4,5P( 2) by a specific pleckstrin homology domain confirms that PI4,5P(2) is nece ssary for proper membrane localization of talin and that this localization is essential for the maintenance of focal adhesions. Our results support a model in which PI4,5P(2) exposes the integrin-binding site on talin, We pro pose that PI4,5P(2)-dependent signaling modulates assembly of focal adhesio ns by regulating integrin-talin complexes. These results demonstrate that a ctivation of the integrin-binding activity of talin requires not only integ rin engagement to the extracellular matrix but also the binding of PI4,5P(2 ) to talin, suggesting a possible role of lipid metabolism in organizing th e sequential assembly of focal adhesion components.