Retinal (of which vitamin A is the alcohol) is the chromophore of the rhodopsin family of proteins. Haloarchaea contain four members of this family, two of which, halorhodopsin and bacteriorhodopsin, convert solar energy (green-orange light) into transmembrane gradients of chloride and protons, respectively. Luecke et al. describe the crystal structure of a third member, known as sensory rhodopsin II, which mediates avoidance of more energetic and potentially harmful blue light and coordinated repression of bacteriorhodopsin and halorhodopsin under environmental conditions that allow the use of other sources of nutrients and energy. Three points now become clear: (i) how the local environment of the retinal chromophore is tuned to optimize the absorbance of shorter wavelength light; (ii) why this family member does not transport ions; and (iii) how this sensor communicates with downstream signaling components.
H. Luecke, B. Schobert, J. K. Lanyi, E. N. Spudich, J. L. Spudich, Crystal structure of sensory rhodopsin II at 2.4 Angstroms: Insights into color tuning and transducer interaction. Science 293, 1499-1503 (2001). [Abstract] [Full Text]