DISTINCT FUNCTIONS OF INTEGRIN-ALPHA AND INTEGRIN-BETA SUBUNIT CYTOPLASMIC DOMAINS IN CELL SPREADING AND FORMATION OF FOCAL ADHESIONS

Integrin-mediated cell adhesion often results in cell spreading and th e formation of focal adhesions. We exploited the capacity of recombina nt human alpha(IIb)beta3 integrin to endow heterologous cells with the ability to adhere and spread on fibrinogen to study the role of integ rin cytoplasmic domains in initiation of cell spreading and focal adhe sions. The same constructs were also used to analyze the role of the c ytoplasmic domains in maintenance of the fidelity of the integrin repe rtoire at focal adhesions. Truncation mutants of the cytoplasmic domai n of alpha(IIb) did not interfere with the ability Of alpha(IIb)beta3 to initiate cell spreading and form focal adhesions. Nevertheless, del etion of the alpha(IIb) cytoplasmic domain allowed indiscriminate recr uitment of alpha(IIb)beta3 to focal adhesions formed by other integrin s. Truncation of the beta3 subunit cytoplasmic domain abolished cell s preading mediated by alpha(IIb)beta3 and also abrogated recruitment of alpha(IIb)beta3 to focal adhesions. This truncation also dramatically impaired the ability of alpha(IIb)beta3 to mediate the contraction of fibrin gels. In contrast, the beta3 subunit cytoplasmic truncation di d not reduce the fibrinogen binding affinity of alpha(IIb)beta3. Thus, the integrin beta3 subunit cytoplasmic domain is necessary and suffic ient for initiation of cell spreading and focal adhesion formation. Fu rther, the beta3 cytoplasmic domain is required for the transmission o f intracellular contractile forces to fibrin gels. The alpha subunit c ytoplasmic domain maintains the fidelity of recruitment of the integri ns to focal adhesions and thus regulates their repertoire of integrins .