Development of the signal in sensory rhodopsin and its transfer to the cognate transducer

Abstract

The microbial phototaxis receptor sensory rhodopsin II (NpSRII, also named phoborhodopsin) mediates the photophobic response of the haloarchaeon Natronomonas pharaonis by modulating the swimming behaviour of the bacterium. After excitation by blue-green light NpSRII triggers, by means of a tightly bound transducer protein (NpHtrII), a signal transduction chain homologous with the two-component system of eubacterial chemotaxis. Two molecules of NpSRII and two molecules of NpHtrII form a 2:2 complex in membranes as shown by electron paramagnetic resonance and X-ray structure analysis. Here we present X-ray structures of the photocycle intermediates K and late M (M2) explaining the evolution of the signal in the receptor after retinal isomerization and the transfer of the signal to the transducer in the complex. The formation of late M has been correlated with the formation of the signalling state. The observed structural rearrangements allow us to propose the following mechanism for the light-induced activation of the signalling complex. On excitation by light, retinal isomerization leads in the K state to a rearrangement of a water cluster that partly disconnects two helices of the receptor. In the transition to late M the changes in the hydrogen bond network proceed further. Thus, in late M state an altered tertiary structure establishes the signalling state of the receptor. The transducer responds to the activation of the receptor by a clockwise rotation of about 15 degrees of helix TM2 and a displacement of this helix by 0.9 A at the cytoplasmic surface.

Active State of Sensory Rhodopsin II: Structural Determinants for Signal Transfer and Proton Pumping

“…The same is observed in BR M state 46 , as well as in NpSRII/NpHtrII complex M state 9 . There are also conformational changes of up to 1 Å in the cytoplasmic channel, probably associated with the charge redistribution.…”

“…A comparative analysis of structural changes in NpSRII alone and in NpSRII/NpHtrII necessitated crystals, grown under conditions close to those reported for the complex (as described in Gordeliy et al 5,9 and has been discussed by Reshetnyak et al 13 ). We managed to obtain the crystals at similar conditions.…”

“…The functionally important conformational step occurs during the M1→M2 transition which leads to the signaling state 7,8 . During this transition the signal is transferred through the interface comprising helices F and G to NpHtrII, which results in a rotary motion of transmembrane helix TM2 6,9 . This activation occurs at the level of the membrane.…”

“…This activation occurs at the level of the membrane. How this signal is then transmitted to signal domain thereby modulating the activity of the histidine kinase CheA and triggering the cytoplasmic two component signaling cascade is not known yet 2,5,9,10 .…”

“…is formed at room temperature in the nanosecond range 25 , as well as for the NpSRII/HtrII complex, for which data of the ground state and its K-and M-intermediate (the active state) are available 5,9 . Despite several attempts the structures of the late (active) states of NpSRII were not obtained yet.…”