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Wednesday 26 January 2005

Light detection

The initial events of phototransduction occur on the physically separate disc membranes in the outer segment portion of the cell.

Photoactivation of rhodopsin catalyzes the activation of hundreds of heterotrimeric G-proteins.

Nucleotide exchange on the G alpha-subunit is accelerated, and the dissociated Galpha-GTP subunit then interacts with the PDE6 holoenzyme (PDE).

Displacement of the PDE6 italic gamma subunit from the catalytic site by Galpha-GTP relieves the inhibition of catalysis at one catalytic subunit, resulting in rapid hydrolysis of cytoplasmic cGMP.

The drop in cGMP levels in the outer segment causes dissociation of cGMP from the cGMP-gated ion channels in the plasma membrane, causing their closure.

The reduced entry of cations into the outer segment causes membrane hyperpolarization, and ultimately, generation of the receptor potential at the photoreceptor synapse.


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