Molecular Mechanism of Rhodopsin Photoactivation

Abstract

Rhodopsin is a highly specialized GPCR that is activated by the rapid photochemical isomerization of its covalently bound 11-cis retinal chromophore. Using two-dimensional solid-state NMR spectroscopy, we define the position of the retinal in the active metarhodopsin II intermediate and the protein conformational changes that couple retinal isomerization to breaking of the “ionic lock” between transmembrane (TM) helices H3 and H6. Retinal isomerization leads to steric strain within the retinal binding site between the β-ionone ring and helix H5, and between the C19/C20 methyl groups and EL2. These interactions trigger the displacement of EL2, deprotonation of the Schiff base nitrogen and protonation of Glu113. Motion of the β-ionone ring leads to rearrangement of the hydrogen bonding network centered on H5, while interactions of the C19 and C20 methyl groups are involved in rearrangement of the EL2. Motion of the β-ionone ring is also coupled to the motion of Trp265, which triggers the shift of helices H6 and H7 into active conformations and the rearrangement of the hydrogen bonding network centered on the conserved NPxxY sequence. Motion of helices H5, H6 and H7, in turn, is coupled to the rearrangement of electrostatic interactions involving the conserved ERY sequence at the cytoplasmic end of H3, exposing the G protein binding site on the cytoplasmic surface of the protein. The location of the retinal and reorganization of the protein upon receptor activation provides a structural basis for understanding the action of agonists and antagonists in the large family of class A GPCRs.