Sequential Rearrangement of Interhelical Networks Upon Rhodopsin Activation in Membranes: the Meta IIa Conformational Substate

Photon absorption by rhodopsin is proposed to lead to an activation pathway that is described by the extended reaction scheme Meta I ⇌ Meta IIa ⇌ Meta IIb ⇌ Meta IIbH+, where Meta IIbH+ is thought to be the conformational substate that activates the G protein transducin. Here we test this extended scheme for rhodopsin in a membrane bilayer environment by investigating lipid perturbation of the activation mechanism. We found that symmetric membrane lipids having two unsaturated acyl chains, such as 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), selectively stabilize the Meta IIa substate in the above mechanism. By combining FTIR and UV−visible difference spectroscopy, we characterized the structural and functional changes involved in the transition to the Meta IIa intermediate, which links the inactive Meta I intermediate with the Meta IIb states formed by helix rearrangement. Besides the opening of the Schiff base ionic lock, the Meta IIa substate is characterized by an activation switch in a conserved ...