Electron crystallographic analysis of two-dimensional crystals of sensory rhodopsin II: A 6.9 angstrom projection structure

Sensory rhodopsins, phototaxis receptors in Haloarchaea, were purified and reconstituted into halobacterial Lipids to form photoactive two-dimensional crystals. Images of vitreous ice-embedded, flattened, tubular crystals of sensory rhodopsin II (SRII) of Natronobacterium pharaonis were recorded usi ng a held emission gun electron cryo-microscope. Fourier components for the SRII structure were determined either from the separated image transforms from single layers that formed each side of flattened tubes, or by a deconv olution procedure when two layers were stacked in register so that they gen erated a single crystal lattice by superposition. Most micrographs showed s ignificant diffraction to 6.9 A after computer processing, and the results provide the first intermediate-resolution information obtained for an archa eal sensory rhodopsin. The projection structure of SRII indicates that the helix positions match the seven-helix arrangement of the archaeal transport rhodopsins rather than that of the eukaryotic visual pigments. The structu ral similarity of SRII to the transport rhodopsins supports models in which the transport and signalling mechanisms of archaeal rhodopsins derive from the same retinal-driven changes in protein conformation. (C) 2001 Academic Press.