The transducer protein HtrII modulates the lifetimes of sensory rhodopsin II photointermediates

We studied the photochemical reaction cycle of sensory rhodopsin II (SRII) by flash photolysis of Halobacterium salinarum membranes genetically engineered to contain or to lack its transducer protein HtrII. Flash photolysis data from membranes containing HtrII were fit well in the 10 mu s-10 s range by three rate constants and a linear unbranched pathway from the unphotolyzed state with 487 nm absorption maximum to a species with absorption maximum near 350 nm (M) followed by a species with maximum near 520 nm (O), as has been found in previous studies of wild-type membranes, Data from membranes devoid of HtrII exhibited similar M and O intermediates but with altered kinetics, and a third intermediate absorbing maximally near 470 nm (N) was present in an equilibrium mixture with O. The modulation of SRII photoreactions by HtrII indicates that SRII and HtrII are physically associated in a molecular complex. Arrhenius analysis shows that the largest effect of HtrII, the acceleration of O decay, is attributable to a large decrease in activation enthalpy. Based on comparison of SRII photoreactions to those of sensory rhodopsin I and bacteriorhodopsin, we interpret this kinetic effect to indicate that HtrII interacts with SRII so that it alters the reaction process involving deprotonation of Asp(73), the proton acceptor from the Schiff base.