Tyrosine phosphorylation of pp125^FAK.

The indicated CHO cells were seeded onto plastic dishes that had been precoated with 10 μg/mL fibrinogen. After incubation for 30, 60, and 90 minutes at 37°C, plates were washed twice with ice-cold PBS. Adherent cells were then lysed and subject to pp125^FAKimmunoprecipitation and antiphosphotyrosine (upper panel) or anti-pp125^FAK (lower panel) Western blot. Note that there was a small degree of pp125^FAK phosphorylation in cells expressing “activated” integrins. FAK phosphorylation is thought to be downstream, rather than upstream, of integrin receptor activation, raising the possibility that postreceptor occupancy events may be facilitated in these cells.