TABLE 1.

Effect of effector region mutants on GEF activity of the catalytic domain of Cdc25^Mma

Protein type	Rate (10−5 s−1)		Fold stimulation
	Intrinsic	Stimulated
Wild type	0.7	47	67
D30K/E31K	0.4	13	37
E31K	0.4	29	71
T35A	0.9	18	20
T35S	0.6	23	38
E37G	2.2	64	29
D38A	0.4	4	10
D38E	0.4	11	28
Y40C	4.4	2,558	581

^aRas-3′mdGDP (100 nM) was incubated with Cdc25^Mm285 (50 nM) in standard buffer at 20°C, after which the exchange reaction was started by adding GDP (20 μM). The reaction was monitored for approximately 8,000 s and the data were fitted to a single exponential function. Variations between experiments (n = 2) were approximately 10% for the stimulated nucleotide dissociation rate and 20 to 50% for the low intrinsic dissociation rate.