Talin is a structural component of focal adhesion sites and is thought to be engaged in multiple protein interactions at the cytoplasmic face of cell/matrix contacts. Talin is a major link between integrin and the actin cytoskeleton 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 found 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 with phospholipid bilayer, talin/phosphoinositide association was restricted to PI4,5P 2 . This association led to a conformational change of the protein. Moreover, the interaction between integrin and talin was greatly enhanced 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 necessary 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 propose that PI4,5P 2 -dependent signaling modulates assembly of focal adhesions by regulating integrin-talin complexes. These results demonstrate that activation of the integrinbinding activity of talin requires not only integrin engagement to the extracellular matrix but also the binding of PI4,5P 2 to talin, suggesting a possible role of lipid metabolism in organizing the sequential assembly of focal adhesion components.

The invasiveness of cells is correlated with the presence of dynamic actin-rich membrane structures called invadopodia, which are membrane protrusions that are associated with localized polymerization of sub-membrane actin filaments. Similar to focal adhesions and podosomes, invadopodia are cell-matrix adhesion sites. Indeed, invadopodia share several features with podosomes, but whether they are distinct structures is still a matter of debate. Invadopodia are built upon an N-WASP-dependent branched actin network, and the Rho GTPase Cdc42 is involved in inducing invadopodial-membrane protrusion, which is mediated by actin filaments that are organized in bundles to form an actin core. Actin-core formation is thought to be an early step in invadopodium assembly, and the actin core is perpendicular to the extracellular matrix and the plasma membrane; this contrasts with the tangential orientation of actin stress fibers anchored to focal adhesions. In this Commentary, we attempt to summarize recent insights into the actin dynamics of invadopodia and podosomes, and the forces that are transmitted through these invasive structures. Although the mechanisms underlying force-dependent regulation of invadopodia and podosomes are largely unknown compared with those of focal adhesions, these structures do exhibit mechanosensitivity. Actin dynamics and associated forces might be key elements in discriminating between invadopodia, podosomes and focal adhesions. Targeting actin-regulatory molecules that specifically promote invadopodium formation is an attractive strategy against cancer-cell invasion.