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

Abstract

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 Gγc(Gngt2) isoforms, respectively. The only other farnesylated G-protein γ-subunit, Gγ11(Gng11), is expressed in multiple tissues but not retina. To determine whether Gγ1regulates uniquely rod phototransduction, we generated transgenic rods expressing Gγ1, Gγc, or Gγ11in 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 Gαt1expression, 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, Gαt1was targeted to the outer segments, reversing its diffuse localization found in Gγ1-deficient rods. Bright illumination triggered Gαt1translocation from the rod outer to inner segments in all three transgenic strains. However, Gαt1translocation in Gγ11transgenic mice occurred at significantly dimmer background light. Consistent with this, transretinal ERG recordings revealed gradual response recovery in moderate background illumination in Gγ11transgenic mice but not in Gγ1controls. Thus, while farnesylated Gγ subunits are functionally active and largely interchangeable in supporting rod phototransduction, replacement of retina-specific Gγ isoforms by the ubiquitous Gγ11affects the ability of rods to adapt to background light.