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

Abstract The heterotrimeric G protein transducin mediates visual signaling in vertebrate photoreceptor cells. Many aspects of transducin’s function were learned from knockout 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 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 subunit(s) naturally expressed in photoreceptors. In contrast, the second study reported the same light-sensitivity loss but a much lesser, only ∼6-fold reduction of Gα t and proposed that 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 knockout rods, we analyzed both mouse lines side-by-side. We first determined that outer segments of both mice have identical Gα t content, which is reduced ∼65-fold from the wild type 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 knockout rods and suggest that Gβ 1 γ 1 is not unique in its ability to mediate vertebrate phototransduction. Significance Statement Phototransduction has been a valuable system for understanding the basic principles of G protein signaling. One question that has remained unanswered is whether the G protein α-subunit can support signaling without its cognate βγ partner complex. Previous studies investigating this question in photoreceptors of Gγ 1 knockout mice came to mutually exclusive conclusions. We now resolve this controversy by showing that phototransduction in this knockout is supported by alternative βγ complexes rather than α-subunit alone. Most importantly, this study highlights functional interchangeability of different γ-subunits in the context of an intact in vivo system.