Abstract
Mutation of Gly90, Glu113, Ala292, and Lys296 in the visual pigment rhodopsin constitutively activates the protein for activation of the G protein transducin. Three of these mutations have been shown to cause two different human diseases. Mutation of Gly90 and Ala292 results in complete night blindness, and mutation of Lys296 results in the degenerative disease retinitis pigmentosa. We show here that the mutants not only constitutively activate transducin but are also constitutively activated for phosphorylation by rhodopsin kinase. In addition, the phosphorylated mutants are shown to bind tightly to the inhibitory protein arrestin in a reaction that quenches the activity toward transducin. Thus the same mutations that result in constitutive activation of transducin also result in constitutive phosphorylation by rhodopsin kinase and binding of arrestin to inhibit the activity. This implies that the same conformational change may be responsible for activation of transducin and rhodopsin kinase. It also suggests that degeneration of photoreceptor cells in retinitis pigmentosa results indirectly from the activated state of the receptor, perhaps as a consequence of phosphorylation and persistent binding of arrestin.