Binding of Paxillin to the α9 Integrin Cytoplasmic Domain Inhibits Cell Spreading

Abstract

α9β1 integrin is a member of the β1 integrin family, plays an important role in extravasation of neutrophils at sites of acute inflammation, and is required for the normal development of the lymphatic system. The α9 and α 4 integrin subunits are most closely related and form a subfamily of integrin α subunits. Previously, we have reported that the α 4 cytoplasmic domain directly and tightly binds paxillin, an intracellular signaling adaptor molecule. This interaction accounts for some of the unusual functional responses to α4 integrin-mediated cell adhesion, including stimulation of cell migration and inhibition of cell spreading and focal adhesion formation. In the current studies, we have examined the interaction between the α9 cytoplasmic domain and paxillin. Here we report that the α9 cytoplasmic domain binds paxillin directly and tightly and that the α9-paxillin association inhibits cell spreading. We have identified amino acid residues in the α9 cytoplasmic domain, Trp999 and Trp 1001, that are critical for paxillin binding, and alanine substitution of either Trp999 or Trp1001 blocks paxillin binding. Furthermore, these mutations also reverse the effect of the α9 cytoplasmic domain on cell spreading. Thus, the α 9 and α4 integrin subunits form a paxillin-binding subfamily of integrin α subunits, and direct binding of paxillin to the α9 cytoplasmic domain mediates some of the biological activities of the α9β1 integrin.