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Daniel Tranchina, Frank S. Chen, Ching-Kang Chen, Michael L. Woodruff, Gordon L. Fain; Role Of Rhodopsin Kinase And Recoverin In Mouse Rod Photoreceptor Transduction. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5263.
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Light-activated rhodopsin (Rh*) is turned off by phosphorylation by rhodopsin kinase (RK), which is normally ~1% of Rh concentration. We wondered why changes in RK and recoverin (Rv) expression produce minimal changes in dark-adapted sensitivity but large changes in response recovery rate constants (tau_REC & tau_D).
Rv was knocked out (Rvko) and RK over (ox) and under (ux) expressed in stable transgenic mouse lines as previously described; electrical responses of single rods were recorded with suction electrodes.
From SIL+ (RK ox by 10X), Rvko, RK+/- and Bark/RK7 (RK ux by 7X), we studied the effect of RK expression over at least a 70-fold range and show: (1) RK ox and Rvko barely affect sensitivity: single-photon response (SPR) peak amplitudes of WT, SIL+, and Rvko rods were nearly identical. (2) But tau_REC & tau_D decreased from 185±15 ms & 173±10 ms in WT to 126±14 ms & 131±10 ms in SIL+ and 87±15 ms & 117±8 ms in Rvko. (3) PDE* decay remained rate-limiting in SIL+ rods: in SIL+ with GAP 6-fold ox (R9AP95), tau_REC & tau_D were further decreased both to ~50 ms and were indistinguishable from R9AP95 alone. (4) In RK+/- and Bark/RK7, SPR amplitude increased but much less than expected. (5) A simple model for Rh* deactivation in which RK competes with G-protein for Rh* so that phosphorylation rate is proportional to RK expression could not fit our results. (6) They could be fit if (a) Rh* must change conformation before phosphorylation; (b) once the change in conformation has occurred, RK binds and phosphorylates at a rate dependent on the RK expression level; and (c) RK also increases the rate of decay of light-activated PDE*.
We hypothesize that changes in RK expression minimally alter sensitivity because a required conformational change of Rh* before phosphorylation diminishes the dependence of Rh* decay on the rate of phosphorylation; and time constants of response decay are altered because the turnoff of light-activated PDE* is modulated through an interplay of RK and recoverin.
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