Imaging experiments were conducted on an Olympus FluoView 1000 laser scanning confocal microscope and an Olympus IX81-ZDC inverted microscope. Biosensor imaging was performed as previously described8 (link),29 . Simultaneous biosensor imaging and activation of PA-Rac was achieved using a MAG Biosystems FRAP-3D add-on (Photometrics) for galvanometer control of laser position. Detailed materials and methods are included in the supplementary information.