Binding of paxillin to alpha(4) integrins modifies integrin-dependent biological responses

The alpha(4) integrins are indispensable for embryogenesis, haematopoiesis and immune responses(1,2), possibly because a4 regulates cellular functions differently from other integrins through its cytoplasmic tail(3). We used novel mimics(4) of the alpha(4) tail to identify molecules that could accou nt for alpha(4)-specific signalling. Here we report that the alpha(4) tail, but not several other alpha-subunit tails, binds tightly to the signalling adaptor paxillin, Paxillin physically associated with alpha(4) integrins i n Jurkat T cells at high stoichiometry, and joining the alpha(4) tail to al pha(IIb) resulted in a complex of integrin alpha(IIb)beta(3) With paxillin. This association markedly enhanced the rates of alpha(IIb)beta(3)-dependen t phosphorylation of focal adhesion kinase and cell migration. It also redu ced cell spreading, focal adhesion and stress fibre formation. A point muta tion within the alpha(4) tail that disrupts paxillin binding reversed all o f these effects. Furthermore, alpha(4)beta(1)-dependent adhesion to VCAM-1 led to spreading of mouse embryonic fibroblasts derived from paxillin-null but not from wild-type mice. Thus, the tight association of paxillin with t he alpha(4) tail leads to distinct biochemical and biological responses to integrin-mediated cell adhesion.