ИСТИНА

The lecture will be focused on the application of small-angle neutron and X-ray scattering methods in the study of the structure, function and especially supramolecular organization of rhodopsin in photoreceptor membrane. Rhodopsin is a key molecule for vision. It is a prototypical member of a large G-protein-coupled receptor (GPCR) family. A unique feature of rhodopsin as a GPCR is that its incorporated ligand, 11-cis-retinal, acts as a highly efficient antagonist that enables to maintain rhodopsin in its dark state inactive. After cis-trans-photoisomerization the all-trans-retinal becomes a highly efficient agonist that triggers the process of phototransduction. As to photochemistry, the rate of 11-cis-retinal photoisomerization as a rhodopsin chromophore group is faster than 80 femtoseconds, its quantum yield is high (0.65). The protein environment plays a crucial role in visual pigment photochemistry. A great progress in determination of the rhodopsin tertiary structure was made in 2000 year. Rhodopsin was the first animal membrane protein that was crystallized. Its 3D structure was determined by X-ray diffraction analysis first with a resolution of 2.8 Å and then with a resolution of 2.2 Å. The knowledge of tertiary structure of rhodopsin opens new possibilities for understanding the intramolecular mechanisms of its main physiological functions: visual pigment spectral tuning (colour vision), unique photochemistry and interaction with other proteins. The supramolecular organization of the visual pigment rhodopsin in the photoreceptor membrane remains contentious. Specifically, whether this GPCR functions as a monomer or dimer remains unknown, as does the presence or absence of ordered packing of rhodopsin molecules in the photoreceptor membrane. Completely opposite opinions have been expressed on both issues. Herein, using small-angle neutron and X-ray scattering approaches, we performed a comparative analysis of the structural characteristics of the photoreceptor membrane samples in buffer, both in the outer segment of photoreceptor cells, and in the free photoreceptor disks. The average distance between the centers of two neighboring rhodopsin molecules was found to be ~5.8 nm in both cases. The results indicate an unusually high packing density of rhodopsin molecules in the photoreceptor membrane, but molecules appear to be randomly distributed in the membrane without any regular ordering.