The cytoplasmic tyrosines of integrin subunit beta 1 are involved in focaladhesion kinase activation

We have previously shown that mutation of the two tyrosines in the cytoplas mic domain of integrin subunit beta 1 (Y783 and Y795) to phenylalanines mar kedly reduces the capability of beta 1A integrins to mediate directed cell migration. In this study, beta 1-dependent cell spreading was found to be d elayed in GD25 cells expressing beta 1A(Y783/795F) compared to that in wild -type GD25-beta 1A. Focal adhesion kinase (FAK) tyrosine phosphorylation an d activation were severely impaired in response to beta 1-dependent adhesio n in GD25-beta 1A(Y783/795F) cells compared to that in wild-type GD25-beta 1A or mutants in which only a single tyrosine was altered (beta 1A(Y783F) o r beta 1A(Y795F)). Phosphorylation site-specific antibodies selective for F AK phosphotyrosine 397 indicated that the defect in FAK phosphorylation via beta 1A(Y783/795F) lies at the level of the initial autophosphorylation st ep. Indeed, beta 1A-dependent tyrosine phosphorylation of tensin and paxill in was lost in the beta 1A(Y783/795F) cells, consistent with the impairment in FAK activation. in contrast, p130(CAS) overall tyrosine phosphorylation was unaffected by the beta 1 mutations. Despite the defect in beta 1-media ted FAK activation, FAK was still localized to focal adhesions, Taken toget her, the phenotype of the GD25-beta 1A(Y783/795F) cells resembles, but is d istinct from, the phenotype observed in FAK-null cells. These observations argue that tyrosines 783 and 795 within the cytoplasmic tail of integrin su bunit beta 1A are critical mediators of FAK activation and cell spreading i n GD25 cells.