Trp fluorescence reveals an activation‐dependent cation‐π interaction in the Switch II region of Gαi proteins

Crystal structures of Gα_i (and closely related family member Gα_t) reveal much of what we currently know about G protein structure, including changes which occur in Switch regions. Gα_t exhibits a low rate of basal (uncatalyzed) nucleotide exchange and an ordered Switch II region in the GDP‐bound state, unlike Gα_i, which exhibits higher basal exchange and a disordered Switch II region in Gα_iGDP structures. Using purified Gα_i and Gα_t, we examined the intrinsic tryptophan fluorescence of these proteins, which reports conformational changes associated with activation and deactivation of Gα proteins. In addition to the expected enhancement in tryptophan fluorescence intensity, activation of GαGDP proteins was accompanied by a modest but notable red shift in tryptophan emission maxima. We identified a cation‐π interaction between tryptophan and arginine residues in the Switch II of Gα_i family proteins that mediates the observed red shift in emission maxima. Furthermore, amino‐terminal myristoylation of Gα_i resulted in a less polar environment for tryptophan residues in the GTPase domain, consistent with an interaction between the myristoylated amino terminus and the GTPase domain of Gα proteins. These results reveal unique insights into conformational changes which occur upon activation and deactivation of G proteins in solution.