Activation-enhanced alpha(IIb)beta(3)-integrin-cytoskeleton interactions outside of focal contacts require the alpha-subunit.

Integrins link the cell's cytoskeleton to the extracellular matrix, as well as to receptors on other cells. These links occur not only at focal contacts but also at smaller integrin-containing protein complexes outside of focal contacts. We previously demonstrated the importance of focal contact-independent integrin-cytoskeleton interactions of beta(2) integrins: activation of adhesion resulted from a release of integrins from cytoskeletal constraints. To determine whether changes in integrin-cytoskeleton interactions were related to activation of the integrin, we used single particle tracking to examine focal contact-independent cytoskeletal associations of alpha(IIb)beta(3)-integrin, in which activation results in a large conformational change. Direct activation of alpha(IIb)beta(3) by mutation did not mimic activation of lymphocytes with phorbol ester, because it enhanced integrin-cytoskeleton interactions, whereas activation of lymphocytes decreased them. Using additional integrin mutants, we found that both alpha- and beta-cytoplasmic domains were required for these links. This suggests that 1) both beta(2)- and beta(3)-integrins interact with the cytoskeleton outside of focal contacts; 2) activation of a cell and activation of an integrin are distinct processes, and both can affect integrin-cytoskeleton interactions; and 3) the role of the alpha-subunit in integrin-cytoskeleton interactions in at least some circumstances is more direct than generally supposed.