Summary of Phototransduction:
The light sensing cells in the eyes are located on the retina and are composed of two types of cells: rods and cones. Cones cells are responsible for high intensity light, color vision, and allow us to see fast moving objects. Rod cells are responsible for the sensing of low intensity light (as low as a single photon), yet have an enormous dynamic range (covering nearly 7 orders of magnitude in light intensity). Somewhat
counter intuitively, these cells are located in the posterior of the eye, with the cell bodies of multiple
neuronal layers located anterior to them. Despite this fact, light is still able to pass through these layers and into the photoreceptors.
Shown
above is a cartoon of the biochemical pathway responsible for phototransduction.
On the left is the rod cell, with the outer and inner segments labeled.
The process of signal transduction takes place on a series of stacked membranous
discs. The pathway begins when a photon of light activates the
photoreceptor protein called rhodopsin (R) which is a transmembrane protein
found embedded in the disc membranes. Once rhodopsin is activated (R*), it
acts to catalyze the GDP for GTP exchange on the heterotrimeric G protein called
transducin. Transducin, like all G proteins, is composed of a guanine
nucleotide binding a-subunit
which is "on" when bound to GTP and "off" when bound to
GDP. The GTP to GDP hydrolysis by Ga
acts as a clock which dictates the duration of the signaling cascade. Once
transducin is activated, via GDP to GTP exchange, it separates from its bg
subunits and proceeds to activate its downstream effector, the cGMP
phosphodiesterase (PDE), by binding to and displacing the g
inhibitory of PDE. Once transducin hydrolyzes its bound GTP, it releases
from PDE (allowing for g to once again inhibit PDE) and then re-associates with
its bg binding
partner. The hydrolysis rate of transducin will help determine the
recovery rate of the cell, allowing it to register additional photons. The
regulators of G protein signaling (RGS) family of proteins act as GTPase
acclerating proteins (GAPs) for Ga
subunits. As the name implies, RGS proteins down-regulate the signaling
cascade, leading to faster turn-off rates and recovery. In the case of rod
cells, the RGS protein is RGS9-1 and is found in a tight 1:1 complex with a G
protein b subunit, Gb5L.
The reason for RGS9-1's association with Gb5L
is not known, although both proteins are required for each other's
stability. From in vitro experiments, it has been shown that PDEg
can enhance the activity of RGS9-1, allowing for even faster transducin
turn-off.