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

Abstract

The molecular mechanism of transmembrane signal transduction is still a pertinent question in cellular biology. Generally, a receptor can transfer an external signal via its cytoplasmic surface as found for GPCRs like rhodopsin or via the membrane domain like it is utilized by sensory rhodopsin II (SRII) in complex with its transducer HtrII. In the absence of HtrII SRII functions as a proton pump. Here, we report on the crystal structure of the active state of SRII from Natronomonas pharaonis (NpSRII). The problem of a dramatic loss of the diffraction quality upon loading of the active state was overcome by growing better crystals and reducing the occupancy of the state. The determined conformational changes at the region comprising helices F and G are similar to those observed for the NpSRII-transducer complex but they are much more pronounced. Meaning of these differences for proton pumping ability and understanding of the signal transduction by NpSRII is discussed.

Light-Induced Helix Movements in Channelrhodopsin-2

“…Light-induced differences in the location of TMH6 and TMH7 of ChR2 are consistent with similar movements of the corresponding helices in bR [7,27], SRII [28][29][30][31] and HR [32], which are thus a common feature of conformational dynamics in microbial-type rhodopsins. However, in the complex of SRII with its transducer, the TMH6 movement is inhibited and replaced by a rotation of one of the transducer TMHs [29,33].…”

Transmembrane Signal Transduction in Two‐Component Systems: Piston, Scissoring, or Helical Rotation?

“…The sensory rhodopsin is a seven‐helical retinal‐binding photoactive membrane protein. Upon illumination, it undergoes conformational changes, which are then passed to the transducer and then, as in chemotaxis systems, to class II HK CheA . Sensory rhodopsin transducers are 2TM membrane proteins similar to chemoreceptors.…”

“…Comparison of the ground and active state structures of the complex reveals minuscule rearrangements, in which conformational changes in Np SRII are roughly two times smaller than those in the protein not bound to the transducer, and conformational changes in Np HtrII are smaller than variation between the ground state structures in the space groups I2 1 2 1 2 1 , P2 1 2 1 2, and P6 4 . It is possible that the light‐induced conformational changes are restricted by crystal contacts, or that the signal transduction mechanism is not evident due to the lack of downstream HtrII domains.…”

Ground state structure of D75N mutant of sensory rhodopsin II in complex with its cognate transducer

“…According to the current understanding of the signal transduction mechanism [11] retinal isomerizes upon absorption of a photon and the hydrogen bond between Schiff base and the adjacent water molecule W1 is broken resulting in its disappearance from the active site. Side chains of the residues in the active site become mobile as can be seen from increased B-factors in active states [11,27]. The carboxylic group of Asp75 is rotated by 90° with respect to its ground-state position thereby losing its connection to the water molecule W3, which becomes disordered.…”