THE TRANSDUCER PROTEIN HTRII MODULATES THE LIFETIMES OF SENSORY RHODOPSIN-II PHOTOINTERMEDIATES

We studied the photochemical reaction cycle of sensory rhodopsin II (S RII) by flash photolysis of Halobacterium salinarum membranes genetica lly engineered to contain or to lack its transducer protein HtrII. Fla sh photolysis data from membranes containing HtrII were fit well in th e 10 mu s-10 s range by three rate constants and a linear unbranched p athway 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 o f wild-type membranes, Data from membranes devoid of HtrII exhibited s imilar M and O intermediates but with altered kinetics, and a third in termediate absorbing maximally near 470 nm (N) was present in an equil ibrium 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, t he acceleration of O decay, is attributable to a large decrease in act ivation enthalpy. Based on comparison of SRII photoreactions to those of sensory rhodopsin I and bacteriorhodopsin, we interpret this kineti c effect to indicate that HtrII interacts with SRII so that it alters the reaction process involving deprotonation of Asp(73), the proton ac ceptor from the Schiff base.