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J. Biol. Chem., Vol. 260, Issue 7, 4156-4168, Apr, 1985
N Bennett and Y Dupont
The mechanism of interaction of the G-protein of retinal rods with
rhodopsin and with nucleotides has been investigated using two independent
techniques, light-scattering and direct binding measurements with labeled
nucleotides. Binding of photoexcited rhodopsin (R*) and nucleotides are
shown to be antagonist, and three conformations of the G-protein are
described, each of which is proposed to be related to a different level of
light-scattering, as follows: (a) the "dark" state, stable in the absence
of photoexcited rhodopsin, in which the nucleotide site is poorly
accessible and has a high affinity (dissociation constants, 0.1 microM for
GDP and 0.01 microM for GppNHp); (b) the R*-bound state in which the
nucleotide site is rapidly accessible with a lower affinity (dissociation
constants, about 20 microM for GDP and GTP; 20-100 microM for GppNHp).
Binding of R* to the G-protein therefore enables rapid binding or exchange
of the nucleotide; this in turn reduces the affinity of the G-protein for
R* (dissociation constants, 0.2 microM for G-protein with GDP bound and 2-
10 microM for G-protein with GppNHp bound, compared to 1 nM in absence of
bound nucleotide); and (c) the third state, the activator of the
phosphodiesterase. In the presence of GTP, an additional irreversible and
fast step, which is proposed to be the dissociation of alpha-GTP from beta
gamma, is shown to occur; a steady state equilibrium is obtained, and the
dissociation constant measured between GTP and this third state of the
G-protein in the presence of R* is an apparent constant which depends on
the rate of transconformation between the first two states and on the rate
of GTP hydrolysis. The minimum value of this apparent dissociation constant
for GTP (0.05-0.1 (microM) is obtained at high levels of illumination.
Finally, some results (number of nucleotide sites and saturation of the
rate of the light-scattering signal) suggest an oligomeric association of
the G-protein.
The G-protein of retinal rod outer segments (transducin). Mechanism of interaction with rhodopsin and nucleotides
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