Visual response in barnacle photoreceptors is not initiated by transitions to and from metarhodopsin

ABSORPTION of a photon by the visual pigment of a photo-receptor results in a cascade of changes in the pigment molecule. At some time during the later stages of this cascade, a visual response appears, in the form of a change of the ionic conductance of the cell. In order to understand the mechanism of the process coupling the pigment changes to the modulation of the conductance, one should know from which state(s) or transition(s) of the pigment cascade the coupling process originates. In the vertebrate, it is commonly assumed ^1,2 that the fourth transition in the cascade, that from the state called metarhodopsin I to the state metarhodopsin II, is the predominant source of the coupling, mainly because this is the first transition involving a major molecular conformational change and the last temporally before the appearance of the visual response. Here we show that in the barnacle photoreceptor, transitions to and from the state called metarhodopsin (although its relation to vertebrate metarhodopsin is unclear) cannot be primarily involved in originating the coupling process, which must arise from earlier stages, not later than the second transition of the cascade.