Comparative Analysis of Cone and Rod Transducins Using Chimeric Gα Subunits

The molecular nature of transducin-alpha subunits (G alpha(t)) may contribute to the distinct physiology of cone and rod photoreceptors. Biochemical properties of mammalian cone G alpha(t2) subunits and their differences with rod G alpha(t1) are largely unknown. Here, we examined properties of chimeric G alpha(t2) in comparison with its rod counterpart. The key biochemical difference between the rod- and cone-like G alpha(t), was similar to 10-fold higher intrinsic nucleotide exchange on the chimeric G alpha(t2). Presented mutational analysis suggests that weaker interdomain interactions between the GTPase (Ras-like) domain and the helical domain in G alpha(t2) are in part responsible for its increased spontaneous nucleotide exchange. However, the rates of R*-dependent nucleotide exchange of chimeric G alpha(t2) and G alpha(t1) were equivalent. Furthermore, chimeric G alpha(t2) and G alpha(t1) exhibited similar rates of intrinsic GTPase activity as well as similar acceleration of GTP hydrolysis by the RGS domain of RGS9. Our results suggest that the activation and inactivation properties of cone and rod G alpha(t) subunits in an in vitro reconstituted system are comparable.