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

Automated summary

This study demonstrates that Gγ subunits have functional activity and can be swapped in supporting rod phototransduction, but replacing retina-specific Gγ isoforms affects rods' ability to adapt to background light.