Conformational dynamics of rhodopsins revealed through time-resolved X-ray structural studies

Abstract

Light plays a crucial role in shaping biological processes, influencing growth, behavior, and survival from microorganisms to complex animals. At the molecular level, specialized proteins detect and respond to light, driving essential biological functions. Rhodopsins, a notable group of these proteins, are found throughout all domains of life. They are categorized into microbial and animal families, sharing a common structure of seven transmembrane α-helices and a bound retinal chromophore. Upon exposure to visible light, the retinal undergoes isomerization, triggering structural changes within the protein that are necessary for the diverse functions of rhodopsins. These functions range from pumping ions across cellular membranes and propagating signals in phototaxis to initiating the complex process of vision. Understanding the mechanisms behind these light-induced changes provides critical insights into cellular signaling, energy conversion, and sensory perception. Given the central role of rhodopsins in these fundamental processes, studying their structure and dynamics is of great importance. Advanced techniques, such as time-resolved serial X-ray crystallography and time-resolved X-ray solution scattering are invaluable for capturing the rapid conformational changes in rhodopsins, allowing us to visualize the precise molecular events that underlie their function. These methods not only illuminate the complex workings of rhodopsins but also contribute to our broader understanding of protein dynamics and their implications in health and disease. In this thesis, we conducted serial synchrotron X-ray crystallography studies on sensory rhodopsin II in complex with its transducer HtrII to study its structure and light-induced conformational changes. We developed an approach for modeling membrane proteins in time-resolved X-ray solution scattering studies, addressing the cross term between the protein and the surrounding detergent micelle. We applied this approach to various rhodopsins, including bacteriorhodopsin, the sensory rhodopsin II-transducer complex, channelrhodopsin-2, and visual rhodopsin, enabling us to track their conformational dynamics as they carry out their specific biological functions.