Polymerizing actin fibers position integrins primed to probe for adhesion sites.

Migrating cells extend protrusions, probing the surrounding matrix in search of permissive sites to form adhesions. We found that actin fibers polymerizing along the leading edge directed local protrusions and drove synchronous sideways movement of beta1 integrin adhesion receptors. These movements lead to the clustering and positioning of conformationally activated, but unligated, beta1 integrins along the leading edge of fibroblast lamellae and growth cone filopodia. Thus, rapid actin-based movement of primed integrins along the leading edge suggests a "sticky fingers" mechanism to probe for new adhesion sites and to direct migration.