Chemical probing of the light-induced interaction between rhodopsin and G-protein. Near-infrared light-scattering and sulfhydryl modifications.

Abstract

Near-infrared light scattering and gel electrophoresis were used to monitor the interaction between rhodopsin and G-protein at photoreceptor disc membranes. It is found that substitution of one SH hydrogen at the G alpha-subunit by N-ethylmaleimide or thionitrobenzoate still allows dark binding of the G-unit to the membrane but blocks its light binding to rhodopsin. Repair of the modification restores the interaction of the proteins. Evidence is provided that substitution by the less bulky cyanide is also able to remove the hindrance to light binding. The rhodopsin-G interaction is reduced in proportion to bound N-ethylmaleimide, and the interaction kinetics remain constant over the measurable range. GTP/GDP exchange at the G-protein after interaction with rhodopsin does not reduce the accessibility of the relevant SH group. In contrast to the effect of G-protein modification, even exhaustive modification of rhodopsin has no effect on the dark and light binding of G-protein.