Generating and using the Cdc42 biosensor in living cells. (a) HeLa cells transfected with either CBD or mutant CBD (mCBD) constructs and co-transfected with Cdc42Q61L or Cdc42T17N (n ≥ 7 cells per condition). The Cdc42 inhibitor ML 14158 was added to cells for 1 hour (10 μM) prior to fluorescence measurements. (b) Migrating HeLa cells expressing CBD-TAG-EGFP and labeled in-cell with mero166 show Cdc42 activity at motile portions of the cell edge and in cell protrusions, consistent with previous observations (left panel). Cells containing the mutant biosensor with reduced affinity for Cdc42 (mCBD-TAG-EGFP, right panel) showed no such localized activity. White arrows indicate protrusions that led to cell translocation. (Scale bar 20 μm, dye excitation filter 545/50, dye emission filter 630/45, EGFP excitation filter 470/40, EGFP emission filter 630/45).