Transducin β-Subunit Can Interact with Multiple G-Protein γ-Subunits to Enable Light Detection by Rod Photoreceptors

The heterotrimeric G-protein transducin mediates visual signaling in vertebrate photoreceptor cells. Many aspects of the function of transducin were learned from knock-out mice lacking its individual subunits. Of particular interest is the knockout of its rod-specific γ-subunit (Gγ(1)). Two studies using independently generated mice documented that this knockout results in a considerable >60-fold reduction in the light sensitivity of affected rods, but provided different interpretations of how the remaining α-subunit (Gα(t)) mediates phototransduction without its cognate Gβ(1)γ(1)-subunit partner. One study found that the light sensitivity reduction matched a corresponding reduction in Gα(t) content in the light-sensing rod outer segments and proposed that Gα(t) activation is supported by remaining Gβ(1) associating with other Gγ subunits naturally expressed in photoreceptors. In contrast, the second study reported the same light sensitivity loss but a much lower, only approximately sixfold, reduction of Gα(t) and proposed that the light responses of these rods do not require Gβγ at all. To resolve this controversy and elucidate the mechanism driving visual signaling in Gγ(1) knock-out rods, we analyzed both mouse lines side by side. We first determined that the outer segments of both mice have identical Gα(t) content, which is reduced ∼65-fold from the wild-type (WT) level. We further demonstrated that the remaining Gβ(1) is present in a complex with endogenous Gγ(2) and Gγ(3) subunits and that these complexes exist in wild-type rods as well. Together, these results argue against the idea that Gα(t) alone supports light responses of Gγ(1) knock-out rods and suggest that Gβ(1)γ(1) is not unique in its ability to mediate vertebrate phototransduction.