Loss of the Effector Function in a Transducin-α Mutant Associated with Nougaret Night Blindness

A missense mutation, G38D, was found in the rod transducin α subunit (Gα t ) in individuals with the Nougaret form of dominant stationary night blindness. To elucidate the mechanism of Nougaret night blindness, we have examined the key functional properties of the mutant transducin. Our data show that the G38D mutation does not alter the interaction between Gα t and Gβγ t or activation of transducin by photoexcited rhodopsin (R*). The mutant Gα t has only a modestly (∼2.5-fold) reduced k cat value for GTP hydrolysis. The GTPase activity of Gα t G38D can be accelerated by photoreceptor r egulator of G protein s ignaling, RGS9. Analysis of the Gα t G38D interaction with cGMP phosphodiesterase revealed marked impairment of the mutant effector function. Gα t G38D completely fails to bind the inhibitory PDE γ subunit and activate the enzyme. Altogether, our results demonstrate a novel molecular mechanism in dominant stationary night blindness. In contrast to known forms of the disease caused by constitutive activation of the visual cascade, the Nougaret form has its origin in attenuated visual signaling due to loss of effector function by transducin G38D mutant.