Deprotonation of the Schiff base of rhodopsin is obligate in the activation of the G protein.

Photolysis of rhodopsin leads to the formation of an activated intermediate that activates a G protein, thus beginning the visual cascade. This activated form of rhodopsin appears coincident in time with the spectroscopically defined intermediate, metarhodopsin II. Metarhodopsin I, the precursor of metarhodopsin II, contains a protonated Schiff base, whereas metarhodopsin II does not. The question of whether the deprotonation of the protonated Schiff base is obligate in the formation of activated rhodopsin was addressed by monomethylating the active-site lysine of permethylated rhodopsin and determining whether this pigment can activate the G protein upon photolysis. The photolysis of the new pigment, which absorbs at 520 nm, led to the formation of a relatively stable metarhodopsin I-like intermediate with a lambda max of approximately equal to 485 nm, with no apparent formation of either metarhodopsin II- or metarhodopsin III-like intermediates. The only probe available to detect formation of the active form of rhodopsin is G protein activation. Photolysis of the pigment in the presence of the G protein did not lead to measurable activation of the GTPase activity of the latter. These studies establish a functional link between Schiff base deprotonation and activation of the G protein. It is concluded that proton transfer from the protonated Schiff base of rhodopsin is obligate for the initiation of visual transduction.