Table 1. Rates of GTPγS binding and GTP hydrolysis.

Rates of GTPγS binding were determined by in vitro [³⁵S]GTPγS binding assays calculated from the data shown in Fig. 2. Rates of GTP binding were determined by intrinsic fluorescence measurements. Rates of GTP hydrolysis were measured by the [γ-³²P]GTP hydrolysis assays shown in Figs. 2 and 3. The values of GTPγS binding and GTP hydrolysis (single-turnover) are shown as min⁻¹. The steady-state hydrolysis values are given as amount of GTP hydrolyzed/(min × Gα). G proteins for which no kinetic data were obtained are marked with an “x.”

	TvGα1*	TvGα2	TvGα4	TvGα5*	AtGPA1	EsGPA5	CoGα3	DdGα4	HsGαi1
GTPγS binding	0.20 ± 0.04	0.25 ± 0.07	1.15 ± 0.37	0.80 ± 0.17	1.15 ± 0.23	1.19 ± 0.38	0.020 ± 0.0071	0.029 ± 0.010	6.4 × 10−3 ± 3.1 × 10−4
Intrinsic fluorescence binding	1.62 ± 0.10	x	1.82 ± 0.21	6.53 ± 0.68	1.97 ± 0.25	0.95 ± 0.14	x	x	x
GTP hydrolysis	0.018 ± 0.0005	0.010 ± 0.001	3.8 × 10−3 ± 1.9 × 10−3	6.2 × 10−3 ± 1.0 × 10−3	0.058 ± 0.0053	0.18 ± 0.02	0.86 ± 0.54	0.19 ± 0.065	0.82 ± 0.16
Rate-limiting step†	Hydrolysis	Hydrolysis	Hydrolysis	Hydrolysis	Hydrolysis	Hydrolysis	Binding	Binding	Binding

^*The nucleotide exchange rates for TvGα1 and TvGα5 varied between the two methods, likely because of the inhibitory effect of Lubrol-PX on GDP dissociation in the [³⁵S] GTPγS assay (42).

^†The rate-limiting step was found by comparing the rate of single-turnover (or estimated single-turnover) of GTP hydrolysis to the rate of GTPγS binding. The GTP hydrolysis rates of TvGα1, TvGα2, and TvGα4 were inferred from steady-state turnover rates.