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Figure 1. Vertebrate phototransduction Rhodopsin (R), the visual pigment of the rod cell, consists of the transmembrane-spanning protein, opsin, chemically linked to the chromophore 11-cis-retinal at Lys 296. R is localized specifically to a morphologically distinct part of the cell, the outer segment. Photon absorption by the chromophore leads to its isomerization to the all-trans- configuration along with accompanying changes in the protein moiety. Photolyzed R, R*, is catalytically active, binding and activating a G-protein (also known as transducin, T). In turn, R* activates the membrane-associated phosphodiesterase (PDE). Cation channels are directly gated by cGMP and control the influx of ions across photoreceptor plasma membranes. The hydrolysis of cGMP by PDE results in a change in cGMP-gated channel conformation, resulting in channel closure. Channel closure decreases the conductance of the plasma membrane to cations, and results in the hyperpolarization of the plasma membrane, inhibition of neurotransmitter release, and signaling of the light stimulus to adjacent neurons. Ca2+ also plays a key role in the recovery of the dark state of photoreceptors through the regulation of guanylyl cyclase (GC), the enzyme that catalyzes the conversion of GTP to cGMP. In the dark, [Ca2+] is high, ~500 nM and GC activity is low. After photoactivation, closure of the plasma membrane channels reduces the influx of cations, including Ca2+. However, the cell's Na+/Ca2+-K+ exchanger continues to extrude Ca2+, and as a result, the [Ca2+] decreases, activating GCs to produce cGMP by specific Ca2+-binding proteins, GCAPs in the Ca2+-free forms. Reprinted fromTrends in Neurosciences, vol. 19(12), by Polans A, Baehr W, and Palczewski K: Turned on by Ca2+! The physiology and pathology of Ca2+-binding proteins in the retina. Pages 547-554, © 1996, with permission from Elsevier Science. |
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