Spatial and temporal instability of the leading edge in Syk mutants. (A) Microscopic images of dHL-60 cells transiently transfected with EGFP-Syk, EGFP-Syk K402R, or EGFP-Syk Y348F constructs. Adhesion and polarization on immobilized fibrinogen was stimulated by fMLP (100 nM). Time-lapse video microscopy of the transfectants. Images were recorded at intervals of 30 seconds. EGFP-Syk was enriched at the leading edge (arrow). In contrast, EGFP-Syk K402R and EGFP-Syk Y348F transfectants showed a homogenous distribution of Syk and formed multiple lamellipodia (arrowheads). Scale bar equals 10 μm. The series shown are representative of 3 independent experiments. (B) Quantitative analysis of polarized dHL-60 cells that formed more than 1 lamellipodium upon fMLP stimulation. n = 96 (EGFP-Syk); n = 113 (EGFP-Syk K402R); n = 56 (EGFP-Syk Y348F). Counted cells were taken from at least 3 independent experiments. Means ± SD are shown. NS indicates not significant; **P < .002.