Distinct functions of integrin α and β subunit cytoplasmic domains in cell spreading and formation of focal adhesions

Integrin-mediated cell adhesion often results in cell spreading and the formation of focal adhesions. We exploited the capacity of recombinant human α_IIbβ₃ integrin to endow heterologous cells with the ability to adhere and spread on fibrinogen to study the role of integrin cytoplasmic domains in initiation of cell spreading and focal adhesions. The same constructs were also used to analyze the role of the cytoplasmic domains in maintenance of the fidelity of the integrin repertoire at focal adhesions. Truncation mutants of the cytoplasmic domain of α_IIb did not interfere with the ability of α_IIbβ₃ to initiate cell spreading and form focal adhesions. Nevertheless, deletion of the α_IIb cytoplasmic domain allowed indiscriminate recruitment of α_IIbβ₃ to focal adhesions formed by other integrins. Truncation of the ft subunit cytoplasmic domain abolished cell spreading mediated by α_IIbβ₃ and also abrogated recruitment of α_IIbβ₃ to focal adhesions. This truncation also dramatically impaired the ability of α_IIbβ₃ to mediate the contraction of fibrin gels. In contrast, the β₃ subunit cytoplasmic truncation did not reduce the fibrinogen binding affinity of α_IIbβ₃. Thus, the integrin β₃ subunit cytoplasmic domain is necessary and sufficient for initiation of cell spreading and focal adhesion formation. Further, the β₃ cytoplasmic domain is required for the transmission of intracellular contractile forces to fibrin gels. The α subunit cytoplasmic domain maintains the fidelity of recruitment of the integrins to focal adhesions and thus regulates their repertoire of integrins.