Abstract
Purpose: :
Light causes massive transducin translocation from the rod outer segments. In rods, this phenomenon is observed only when light intensity reaches a critical threshold level, at which the amount of activated transducin exceeds the capacity of the RGS9-Gβ5-R9AP GTPase activating complex to rapidly return transducin in its inactive state. Decreasing or increasing the activity of this complex shifts the threshold to a lower or higher light intensity, respectively. Despite significant similarity between rod and cone transducin -subunits, transducin translocation in cones is undetectable under physiological lighting conditions. Here we addressed the mechanisms preventing transducin from translocation in cones.
Methods: :
Anesthetized mice (wild type, rhodopsin kinase knockout and R9AP knockout) with dilated pupils were illuminated by light of various intensities from a calibrated light source and the distribution of transducin subunits was analyzed by immunostaining retina cross-sections.
Results: :
The exposure of wild type mice to light intensities up to ~100 000 lux on the cornea surface did not cause transducin translocation in cones, in agreement with most published reports. To test whether cone transducin does not translocate because the capacity of the GTPase complex in cones is never exhausted, we analyzed cone transducin translocation in R9AP knockout mice and found that robust translocation is observed at light intensities over ~10,000 lux. Furthermore, the same phenomenon takes place in rhodopsin kinase knockout mice.
Conclusions: :
Our data suggest that the lack of transducin translocation in cones is unlikely to be explained by any specific physico-chemical properties of cone transducin subunits, but rather by exceptionally high efficiency of biochemical mechanisms responsible for inactivation of the phototransduction cascade. These results emphasize major differences in kinetics of rod and cone phototransduction and indicate that these cells use completely different mechanisms to adapt to bright light.
Keywords: photoreceptors • opsins • signal transduction