Supramolecular Architecture of Rhodopsin in Native Photoreceptors Revealed by Cryo-Electron Tomography

Abstract

Vision begins with the absorption of photons by rhodopsin, the visual pigment in photoreceptors. Rhodopsin belongs to the family of G protein-coupled receptors (GPCRs). Some family members form functional dimers or oligomers. Whether rhodopsin forms oligomers and whether these oligomers are functionally relevant is controversial 1-3. We study rhodopsin organization in vitreous ultrathin cryo-sections of intact, dark-adapted photoreceptors by cryo-electron tomography. This technique ascertains that rhodopsin is preserved in a close-to-native state. Briefly: retina is fixed by high-pressure freezing, ultra-thin sectioned and visualized by cryo-electron tomography. In the reconstructed and processed tomograms the organization of rhodopsin molecules becomes visible. We identify three levels of hierarchical supramolecular organization. Rhodopsin forms dimers; the dimers form rows; and rows come in special pairs like rail tracks. Rows are aligned parallel to the disk incisure. A row is comprised of at least 10 dimers and is about 50 nm in length. The distance between rows within a track is 5 nm; and tracks are separated by 15 nm. We propose that rhodopsin tracks provide a template that organizes the spatio-temporal interaction of preassembled signalling components on the disk surface. Aligned rows of immobile rhodopsin renders photoreceptors highly dichroic and might provide the structural basis for detection of polarized light. We envisage that rhodopsin-like type A GPCRs, which are highly homologous, also entertain a supramolecular organization.References1 Chabre, M., Cone, R. & Saibil, H. R. Biophysics - is rhodopsin dimeric in native rods? Nature 426, 30-31 (2003).2 Chabre, M. & le Maire, M. Monomeric G-protein-coupled receptor as a functional unit. Biochemistry 44, 9395-94O3 (2005).3 Fotiadis, D. et al. Atomic-force microscopy: rhodopsin dimers in native disc membranes. Nature 421, 127-128 (2003).