Rhodopsin (G protein coupled receptor that is activated by light)
Rhodopsin (also known as visual purple): a light sensitive pigment in the rod cells of the retina involved in visual photo transduction coupled to transducin.
The human retina contains two types of photoreceptors: rods and cones (primary recipients of visual stimulation).
- Cones are involved in color vision while rods are stimulated by weak light like moonlight over a range of wavelengths.
Photoreceptor cells have two parts: the inner segment and the outer segment
- Outer segments of the rod photoreceptor contain a stack of membrane bound discs that contain the photosensitive pigment rhodopsin.
Rhodopsin consists of two components:
1) Protein molecule called opsin (specifically scotopsin)
- Light sensitive G protein coupled receptor that embeds in the lipid bilayer of cell membranes using seven protein transmembrane domains.
- The amino-terminal domain projects into the disk, and the carboxyl-terminal domain faces the cytosol of the outer segment.
2) Covalently bound cofactor called retinal (carotenoid).
- Form of retinal bound to opsin: 11-cis-retinal.
The light absorbing pigment 11-cis-retinal (Chromophore) is covalently bound to the amino group of a lysine residue in the seventh transmembrane domain of the opsin protein.
When a photon is absorbed by the retinal component of rhodopsin, the energy causes a photochemical change. 11-cis-retinal (bent form) is converted to all trans– retinal (linear isomer).
- Opsin is covalently bound to all-trans-retinal is called metarhodopsin II, or activated opsin.
Analogous to other G protein coupled receptors, light-activated form of rhodopsin interacts with and activates an associated G protein (Gt = transducin).
Transducin (Heterotrimeric G protein)
Transducin (Gt): The trimeric G protein coupled to rhodopsin
- Gt is found only in rod cells.
- Transducin present near to rhodopsin on the cytoplasmic face of the disk membrane.
- Transducin belongs to the same family of heterotrimeric GTP-binding proteins as Gs and Gi.
- Consists of three subunit: α subunit, β subunit and ϒ subunit.
- Alpha subunit of transducin (Gt) can bind either GDP or GTP.
When GDP is bound to Gtα (inactive form) : All three subunits stay together
When GTP is bound to Gtα (active form): Alpha subunit dissociate from trimeric complex.