Specific Binding of RGS9-G beta 5L to Protein Anchor in Photoreceptor Membranes Greatly Enhances Its Catalytic Activity

The complex between the short splice variant of the ninth member of the RGS protein family and the long splice variant of type 5 G protein beta subunit (RGS9-G beta 5L) plays a critical role in regulating the duration of the light response in vertebrate photoreceptors by activating the GTPase activity of the photoreceptor-specific G protein, transducin. RGS9-G beta 5L is tightly associated with the membranes of photoreceptor outer segments; however, the nature of this association remains unknown. Here we demonstrate that rod outer segment membranes contain a limited number of sites for high affinity RGS9-G beta 5L binding, which are highly sensitive to proteolysis. In membranes isolated from bovine rod outer segments, all of these sites are occupied by the endogenous RGS9-G beta 5L, which prevents the binding of exogenous recombinant RGS9-G beta 5L to these sites. However, treating membranes with urea or high pH buffers causes either removal or denaturation of the endogenous RGS9-G beta 5L, allowing for high affinity binding of recombinant RGS9-G beta 5L to these sites. This binding results in a striking ~70-fold increase in the RGS9-G beta 5L ability to activate transducin GTPase. The DEP (_d_isheveled/_E_GL- 10/_p_leckstrin) domain of RGS9 plays a crucial role in the RGS9-G beta 5L membrane attachment, as evident from the analysis of membrane-binding properties of deletion mutants lacking either N- or C-terminal parts of the RGS9 molecule. Our data indicate that specific association of RGS9-G beta 5L with photoreceptor disc membranes serves not only as a means of targeting it to an appropriate subcellular compartment but also serves as an important determinant of its catalytic activity.