X-ray structure of sensory rhodopsin II at 2.1-angstrom resolution

Sensory rhodopsins (SRs) belong to a subfamily of heptahelical transmembran e proteins containing a retinal chromophore. These photoreceptors mediate t he cascade of vision in animal eyes and phototaxis in archaebacteria and un icellular flagellated algae. Signal transduction by these photoreceptors oc curs by means of transducer proteins. The two archaebacterial sensory rhodo psins SRI and SRII are coupled to the membrane-bound HtrI and HtrII transdu cer proteins. Activation of these proteins initiates phosphorylation cascad es that modulate the flagellar motors, resulting in either attractant (SRI) or repellent (SRII) phototaxis. In addition, transducer-free SRI and SRII were shown to operate as proton pumps, analogous to bacteriorhodopsin. Here , we present the x-ray structure of SRII from Natronobacterium pharaonis (p SRII) at 2.1-Angstrom resolution, revealing a unique molecular architecture of the retinal-binding pocket. in particular, the structure of pSRII exhib its a largely unbent conformation of the retinal (as compared with bacterio rhodopsin and halorhodopsin), a hydroxyl group of Thr-204 in the vicinity o f the Schiff base, and an outward orientation of the guanidinium group of A rg-72. Furthermore, the structure reveals a putative chloride ion that is c oupled to the Schiff base by means of a hydrogen-bond network and a unique, positively charged surface patch for a probable interaction with HtrII. Th e high-resolution structure of pSRII provides a structural basis to elucida te the mechanisms of phototransduction and color tuning.