Paxillin binding to a conserved sequence motif in the alpha 4 integrin cytoplasmic domain.

alpha(4)beta(1) integrin-mediated cell adhesion results in increased cell migration, reduced cell spreading, and focal adhesion formation relative to other beta(1) integrins. Paxillin, a signaling adapter protein, binds tightly to the alpha(4) cytoplasmic domain and is implicated in alpha(4) integrin signaling. We now report the mapping of a paxillin-binding site in the alpha(4) cytoplasmic domain and an assessment of its role in the alpha(4) tail-specific integrin functions. By using truncation mutants and a peptide competition assay, we found that a region of 9 amino acid residues (Glu(983)-Tyr(991)) within the alpha(4) cytoplasmic domain contains a minimal sequence sufficient for paxillin binding. Alanine scanning of this region implicated Tyr(991) and Glu(983) as critical residues. The role of these residues was confirmed by introducing these Ala substitutions into the full-length alpha(4) tail sequence. Y991A or E983A substitution disrupted the interaction of alpha(4) integrins with paxillin. These same two point mutations reversed the effects of the alpha(4) tail on cell spreading. The key features of the identified paxillin-binding sequence are present in all alpha(4) integrins sequenced to date, including that from Xenopus laevis. The maintenance of this sequence motif suggests that paxillin binding is an evolutionarily conserved function of alpha(4) integrins.