Functional interchangeability of rod and cone transducin α-subunits

Rod and cone photoreceptors use similar but distinct sets of phototransduction proteins to achieve different functional properties, suitable for their role as dim and bright light receptors, respectively. For example, rod and cone visual pigments couple to distinct variants of the heterotrimeric G protein transducin. However, the role of the structural differences between rod and cone transducin alpha subunits (T alpha) in determining the functional differences between rods and cones is unknown. To address this question, we studied the translocation and signaling properties of rod T alpha expressed in cones and cone T alpha expressed in rods in three mouse strains: rod T alpha knockout, cone T alpha GNAT2(cpfl3) mutant, and rod and cone T alpha double mutant rd17 mouse. Surprisingly, although the rod/cone T alpha are only 79% identical, exogenously expressed rod or cone T alpha localized and translocated identically to endogenous T alpha in each photoreceptor type. Moreover, exogenously expressed rod or cone T alpha rescued electroretinogram responses (ERGs) in mice lacking functional cone or rod T alpha, respectively. Ex vivo transretinal ERG and single-cell recordings from rd17 retinas treated with rod or cone T alpha showed comparable rod sensitivity and response kinetics. These results demonstrate that cone T alpha forms a functional heterotrimeric G protein complex in rods and that rod and cone T alpha couple equally well to the rod phototransduction cascade. Thus, rod and cone transducin alpha-subunits are functionally interchangeable and their signaling properties do not contribute to the intrinsic light sensitivity differences between rods and cones. Additionally, the technology used here could be adapted for any such homologue swap desired.