Abstract
Purpose::
Retinal rods and cones have homologous but distinct heterotrimeric G proteins (transducins). Rod transducin-α (Gαt1), but not cone transducin-α (Gαt2), undergoes light-dependent translocation. We studied this difference and the importance of subunit composition of transducin in mediating light responses.
Methods::
We expressed human Gαt2 (GNAT2+) in mouse rods on a Gαt1 null (gnat1-/-) or heterozygous-knockout (gnat1+/-) background. The properties of Gαt2 in mouse rods were evaluated by analyzing the interactions of transgenic Gαt2 with endogenous rhodopsin and Gß1γ1, the light-dependent translocation of Gαt2 and Gß1γ1, and the light-response properties of the GNAT2+gnat1+/- and GNAT2+gnat1-/- rods.
Results::
Gαt2 was able to substitute for Gαt1 in coupling rhodopsin to the effector (cGMP-phosphodiesterase), but with low efficiency. Transgenic Gαt2 showed light-dependent translocation in GNAT2+gnat1-/- rods, suggesting that the cellular environment rather than the properties of Gαt2 dictate translocation. Unlike in wild-type rods, Gß1γ1 in GNAT2+gnat1-/- rods failed to return to the rod outer segment (ROS) upon dark-adaptation due to their low affinity for Gαt2. Most interestingly, the decline of the light response of GNAT2+gnat1-/- rods was much prolonged. In GNAT2+gnat1-/- mice born and raised in constant darkness, most Gß1γ1 were located in the ROS, and the decline of the light response became almost normal.
Conclusions::
Gßγ subunits are required for the normal termination of rod photoresponse.
Keywords: photoreceptors • transgenics/knock-outs