Rhodopsin (A531)

Rhodopsin is the protein in the mammalian retina responsible for the light sensitivity of rod cells, which are responsible for vision in low light levels. The rhodopsin protein is a member of the G protein-coupled receptor (GPCR) rhodopsin-like family. It exists with a the ligand bound subsatnce named retinal. Retinal can exist in two isomeric forms, 11-cis and 11-trans retinal. In the dark rhodopsin is associated with 11-cis retinal, but light causea the 11-cis form to flip to the 11-trans form, and this causes an alteration in the structure of the rhodopsin making it catalytically active resulting in a reduced rate of photoreceptor synaptic release. Activity is shut off by phosphorylation under the influence of rhodopsin kinase, the activity of which results in binding of visual arrestin (a.k.a. arrestin-1 and S-antigen), which prevents rhodopsin from interacting with and activating more transducin molecules.

Defects in Rhodopsin are the cause of retinitis pigmentosa type 4. Patients typically have night vision blindness and loss of midperipheral visual field. As their condition progresses, they lose their far peripheral visual field and eventually central vision as well.