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J Biol Chem, Vol. 273, Issue 12, 6731-6735, March 20, 1998
From the Department of Physiology and Biophysics, University of
Iowa College of Medicine, Iowa City, Iowa 52242
Members of the RGS family serve as
GTPase-activating proteins (GAPs) for heterotrimeric G-proteins and
negatively regulate signaling via G-protein-coupled receptors. The
recently resolved crystal structure of RGS4 bound to
Gi
1 suggests two potential mechanisms
for the GAP activity of RGS proteins as follows: stabilization of the
Gi
1 switch regions by RGS4 and the catalytic
action of RGS4 residue Asn128. To elucidate a role of the
Asn residue for RGS GAP function, we have investigated effects of the
synthetic peptide corresponding to the G
binding domain of human
retinal RGS (hRGSr) containing the key Asn at position 131, and we have
carried out mutational analysis of Asn131. Synthetic
peptide hRGSr-(123-140) retained its ability to bind the
AlF4
-complexed
transducin
-subunit,
Gt
·AlF4
,
but failed to elicit stimulation of Gt
GTPase activity. Wild-type hRGSr stimulated Gt
GTPase activity by ~10-fold with
an EC50 value of 100 nM. Mutant hRGSr proteins
with substitutions of Asn131 by Ser and Gln had a
significantly reduced affinity for Gt
but were capable
of substantial stimulation of Gt
GTPase activity, 80 and
60% of Vmax, respectively. Mutants
hRGSr-Leu131, hRGSr-Ala131, and
hRGSr-Asp131 were able to accelerate Gt
GTPase activity only at very high concentrations (>10
µM) which appears to correlate with a further decrease of
their affinity for transducin. Two mutants, hRGSr-His131
and hRGSr-
131, had no detectable binding to transducin.
Mutational analysis of Asn131 suggests that the
stabilization of the G-protein switch regions rather than catalytic
action of the Asn residue is a key component for the RGS GAP
action.
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