Photoproducts of rhodopsin in the disc membrane

SUMMARY After photoexcitation, rhodopsin undergoes relaxation through a series of photoproducts. The 380 nm product metarhodopsin II (MII), which arises in ms, is distinguished by its strong interaction with the native chemical environment of rhodopsin, the disc membrane. Concerning the phospholipid, MII-formation depends crucially on a high average degree of unsaturation of the hydrocarbons. Special polar head groups are not required. A specific interaction does exist with the G-protein which is peripherally membrane associated. The interaction is strong enough to stabilize MII and to favor it at the cost of both metarhodopsin I and III (MI/MIII). The active role of MII is to provide the binding conformation for the G-protein and to catalyse its GTP-dependent activation. Using the stabilization of MII (‘extra-MII’) as a monitor the thermal activation energy of G-activation could be determined. The photoproducts MII, Mill and free retinal are incorporated in a reaction scheme which describes their possible roles in activation, storage and deactivation. Finally, an intermediate MII § is discussed which has been inferred from a spectrally silent deprotonation of MII in seconds. G-protein stabilizes MII with respect to this transition, too. MII § might be involved in the rapid deactivation of MII, possibly providing the stage where the exposure of phosphate binding sites occurs.