Cdc42 activation in human NK cells upon binding to K562 orantibody-coated P815 target cells. Human NK cells were allowed to bind to paraformaldehyde-prefixed K562 (A), P815 alone, or P815 plus anti-CD16 (B73.1) (B) target cells (E/T ratio, 5:1) for the indicated time periods at 37°C. Cell lysates were incubated with the GST-PAK fusion protein, and bound, active GTP-Cdc42 molecules were evaluated using Western blot analysis with an anti-Cdc42 mAb (top). Cell lysates probed for total Cdc42 are shown as loading control (bottom). These results represent 1 of 3 independent experiments.