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Ultrafast structural changes direct the first molecular events of vision

The visual pigment rhodopsin plays a critical role in the process of low-light vision in vertebrates. It is present in the disk membranes of rod cells in the retina and is responsible for transforming the absorption of light into a physiological signal. Rhodopsin has a unique structure that consists of seven transmembrane (TM) α-helices with an 11-cis retinal chromophore covalently bound to the Lysine sidechain of 7th TM helix. A negatively charged amino acid (glutamate) forms a salt bridge with the protonated Schiff base (PSB) of the chromophore to stabilize the receptor in the resting state.

Rhodopsin transforms the absorption of light into a physiological signal through conformational changes that activate the intracellular G protein transducin—a member of the Gi/o/t family—initiating a signaling cascade, resulting in electrical impulses sent to the brain and ultimately leading to visual perception. Although previous studies have provided valuable insights into the mechanism of signal transduction in rhodopsin, methods that provide both a high spatial and temporal resolution are necessary to fully understand the activation mechanism at the atomic scale from femtoseconds to milliseconds. This study presents the first experimentally-derived picture of the rhodopsin activation mechanism at the atomic scale using time-resolved serial femtosecond crystallography in association with hybrid quantum mechanics/molecular mechanics (QM/MM) simulations. The results show that light-induced structural changes in rhodopsin occur on a timescale of hundreds of femtoseconds, and they reveal new details about the conformational changes that occur during activation.

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