oa Higher-order organization of rhodopsin in native photoreceptors by cryo-electron tomography

Ashraf Al-Amoudi1,2, Monika Gunkel1, Weaam Alkhaldi1,2, Stephan Irsen1, U. Benjamin Kaupp1 1Center of Advanced European Studies and Research (caesar), Department of Molecular Sensory Systems, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany, 2German Center of Neurodegenerative Diseases, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany 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) that can form dimers or oligomers. Whether rhodopsin forms oligomers and whether these oligomers are functionally relevant is controversial 1-3. Here, we study rhodopsin organization by cryo-electron tomography (cryo-ET) of vitreous sections and sub-tomogram averaging of intact dark-adapted photoreceptors. This technique ascertains that rhodopsin is preserved in a close-to-native state. In combination with cryo-ET of vitreous sections, sub-tomogram averaging considerably enhances the signal-to-noise ratio thus allowing for in situ quantitative analysis at the molecular level. Briefly: retina is fixed by high-pressure freezing, ultra-thin sectioned and visualized by cryo-electron tomography and sub-tomogram averaging. In the reconstructed and processed tomograms the organization of rhodopsin molecules becomes visible. Our results show highly-organized structures of rhodopsin. 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. We propose that rhodopsin tracks provide a template that organizes the spatio-temporal interaction of preassembled signalling components on the disk surface. We envisage that some rhodopsin-like type A GPCRs, which are highly homologous, also entertain a supramolecular organization. Aligned rows of immobile rhodopsin renders photoreceptors highly dichroic and might provide the structural basis for detection of polarized light. References 1 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, 9395-9403 (2005). 3 Fotiadis, D. et al. Atomic-force microscopy: rhodopsin dimers in native disc membranes. Nature 421, 127-128 (2003).