Regulation of rod photoreceptor function by farnesylated G-protein γ-subunits

Heterotrimeric G-protein transducin, Gt, is a key signal transducer and amplifier in retinal rod and cone photoreceptor cells. Despite similar subunit composition, close amino acid identity, and identical posttranslational farnesylation of their Gγ subunits, rods and cones rely on unique Gγ(1) (Gngt1) and Gγ(c) (Gngt2) isoforms, respectively. The only other farnesylated G-protein γ-subunit, Gγ(11) (Gng11), is expressed in multiple tissues but not retina. To determine whether Gγ(1) regulates uniquely rod phototransduction, we generated transgenic rods expressing Gγ(1), Gγ(c), or Gγ(11) in Gγ(1)-deficient mice and analyzed their properties. Immunohistochemistry and Western blotting demonstrated the robust expression of each transgenic Gγ in rod cells and restoration of Gα(t1) expression, which is greatly reduced in Gγ(1)-deficient rods. Electroretinography showed restoration of visual function in all three transgenic Gγ(1)-deficient lines. Recordings from individual transgenic rods showed that photosensitivity impaired in Gγ(1)-deficient rods was also fully restored. In all dark-adapted transgenic lines, Gα(t1) was targeted to the outer segments, reversing its diffuse localization found in Gγ(1)-deficient rods. Bright illumination triggered Gα(t1) translocation from the rod outer to inner segments in all three transgenic strains. However, Gα(t1) translocation in Gγ(11) transgenic mice occurred at significantly dimmer background light. Consistent with this, transretinal ERG recordings revealed gradual response recovery in moderate background illumination in Gγ(11) transgenic mice but not in Gγ(1) controls. Thus, while farnesylated Gγ subunits are functionally active and largely interchangeable in supporting rod phototransduction, replacement of retina-specific Gγ isoforms by the ubiquitous Gγ(11) affects the ability of rods to adapt to background light.