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M. L. Woodruff, G. L. Fain, F. S. Chen, H. Shim, C.-K. J. Chen; Substitution of Cone for Rod Transducin Decreases Sensitivity and Accelerates Response Turnoff. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2042.
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Cone light responses are about 25 times less sensitive per Rh* than rod responses and recover more rapidly after brief light exposure. Since rods and cones have different transducin molecules (i.e., GNAT1 and GNAT2), we asked whether the species of transducin plays any role in the difference in photoreceptor response properties.
We made a stable line of transgenic mice expressing cone transducin in rods and mated this line into a GNAT1ko background; suction-electrode recordings from single rods in the so called GNAT2C mouse line were made with standard techniques.
Expression of cone transducin in GNAT2C rods was at a level similar to that of GNAT1 in WT rods; levels of other transduction proteins were not affected. No degeneration of rods was observed, and bright light caused movement of GNAT2 from OS to IS in GNAT2C rods but not in WT cones. Responses of GNAT2C rods were about a factor of 5 less sensitive than WT rods; amplification constants averaged 5.3 + 0.3 s-2, somewhat larger than for WT cones (Nikonov et al JGP 127:359) but a factor of two smaller than for WT rods (11.6 + 0.5 s-2). Even more striking was the difference in the rate of decay of the response: values for τREC and integration times were more than a factor of 2 smaller than for WT rods, and τD averaged 88 + 9 ms (WT was 185 + 11 ms). These results suggested that cone transducin may speed either the formation of the GAP complex or GTP hydrolysis. We therefore mated GNAT2C mice into a R9AP95 background with 6 times the normal concentration of GAP proteins. Sensitivity and activation constants were not statistically different in GNAT2C/R9AP95 and GNAT2C rods, but both τREC and τD were smaller (τD was 50 + 4 ms). The values of τREC and τD were however indistinguishable for R9AP95 rods and GNAT2C/R9AP95 rods. Thus cone transducin accelerated response decay at a normal GAP concentration but had no effect when GAP was over-expressed.
Substitution of cone transducin for rod transducin produces about a 5-fold decrease in sensitivity, at least in part the result of a decrease in the rate of PDE activation. Cone transducin also speeds turnoff of the light response, and it apparently does this by accelerating the formation of the GAP complex. Since our mean value of τD in GNAT2C rods is similar to that of WT mouse cones, the species of transducin appears to play a major role in the acceleration of cone response kinetics.
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