The use of GFP to localize Rho GTPases in living cells.
The green fluorescent protein (GFP) of the jellyfish Aequorea victoria has revolutionized the study of protein localization and dynamics. GFP fusions permit analysis of proteins in living cells and offer distinct advantages over conventional immunofluorescence. Among these are lower background, higher resolution, robust dual color colocalization, and avoidance of fixation artifacts. In the case of Ras and Rho family proteins, GFP fusions have allowed breakthroughs in the understanding of how CAAX proteins are targeted to specific cell membranes and how signaling at different membranes can result in different cellular responses. GFP-tagged Rho proteins have also been informative in analyzing the interactions with the cytosolic chaperone, RhoGDI. The major disadvantages of studying GFP fusion proteins is that they are generally overexpressed relative to endogenous proteins, and the GFP tag can, in principle, affect protein function. Fortunately, in the case of Ras and Rho family proteins, a GFP tag at the N terminus seems to have little effect on protein targeting and function. Nevertheless, it is prudent to confirm GFP fusion protein data with the study of the endogenous protein. This chapter describes the tagging of Rho proteins with GFP and the analysis of GFP-Rho protein localization by epifluorescence and confocal microscopy. It further describes methods of analyzing endogenous Rho proteins as confirmation of data acquired using GFP-Rho fusion proteins. These techniques will be useful for anyone studying Rho protein function and are widely applicable to many cell types and signal transduction systems.