Figure 7.

Activated β₁ integrin increases FA turnover and promotes cell migration and spreading at low matrix density. The human WT β₁-integrin chains or activated mutant (D759A) were stably expressed in β₁ integrin–deficient GD25 cells, and their adhesive properties were analyzed. (A) The integrin affinity state for both cell types was measured with the FNIII7-10 binding assay as described in Fig. 6 A. (B) Confocal images of GD25/WT and D759A β₁ cells spread on 10 μg/ml FN and immunostained to visualize β₁ integrin, paxillin, and filamentous actin. β₁ integrin was labeled with the 4B7R mAb, which recognizes the human β₁-integrin chain. (C) Cell spreading and migration assays of GD25/WT and D759A β₁ cells. Spreading of both cells types with or without 0.5 mM MnCl₂ treatment was measured as described in Fig. 6 B. Migration of both cells types was analyzed using time-lapse phase-contrast video microscopy and cell tracking as described in Fig. 2 B. (D) GD25 cells stably expressing EGFP-zyxin and either WT β₁ or D759A integrin were spread on 5 μg/ml FN and monitored by time-lapse video microscopy at 4-min intervals for 6 h. FA dynamics were assessed as described in Fig. 4. Four parameters for FA located at the leading edge of migrating cells were measured: assembly and disassembly speed, steady-state duration, and lifetime. Horizontal bars are the mean of all FAs. Error bars indicate SD. *, P < 10⁵. Bar, 20 μm.