Effects of Mutations in the Cytoplasmic Domain of Integrin β1 to Talin Binding and Cell Spreading

Integrins are transmembrane proteins linking the extracellular matrix or certain cell–cell contacts to the cytoskeleton. To study integrin–cytoskeleton interactions we wanted to relate talin–integrin interaction to integrin function in cell spreading and formation of focal adhesions. For talin-binding studies we used fusion proteins of glutathione S-transferase and the cytoplasmic domain of integrin β₁ (GST-cytoβ₁) expressed in bacteria. For functional studies chimeric integrins containing the extracellular and transmembrane parts of β₃ linked to the cytoplasmic domain of β₁ were expressed in CHO cells as a dimer with the α_IIb subunit. Point mutations in the amino acid sequence N⁷⁸⁵PIY⁷⁸⁸ of β₁ disrupted both the integrin–talin interaction and the ability of the integrin to mediate cell spreading. COOH-terminal truncation of β₁ at the amino acid position 797 disrupted its ability to mediate cell spreading, whereas the disruption of talin binding required deletion of five more amino acids (truncation at position 792). A synthetic peptide from this region of β₁ (W⁷⁸⁰DTGENPIYKSAV⁷⁹²) bound to purified talin and inhibited talin binding to GST-cytoβ₁. The ability of the mutants to mediate focal adhesion formation or to codistribute to focal adhesions formed by other integrins correlated with their ability to mediate cell spreading. These results confirm the previous finding that a talin-binding site in the integrin β₁ tail resides at or close to the central NPXY motif and suggest that the integrin–talin interaction is necessary but not sufficient for integrin-mediated cell spreading.