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

by Alexander V. Kolesnikov, Elena Lobysheva, Jaya P. Gnana-Prakasam, Vladimir J. Kefalov, Oleg G. Kisselev

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 Gγ₁ (Gngt1) and Gγ_c (Gngt2) isoforms, respectively. The only other farnesylated G-protein γ-subunit, Gγ₁₁ (Gng11), is expressed in multiple tissues but not retina. To determine whether Gγ₁ regulates uniquely rod phototransduction, we generated transgenic rods expressing Gγ₁, Gγ_c, or Gγ₁₁ in Gγ₁-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 Gγ₁-deficient rods. Electroretinography showed restoration of visual function in all three transgenic Gγ₁-deficient lines. Recordings from individual transgenic rods showed that photosensitivity impaired in Gγ₁-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 Gγ₁-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 Gγ₁₁ transgenic mice occurred at significantly dimmer background light. Consistent with this, transretinal ERG recordings revealed gradual response recovery in moderate background illumination in Gγ₁₁ transgenic mice but not in Gγ₁ 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 Gγ₁₁ affects the ability of rods to adapt to background light.