Protein of the Week: rhodopsin
This bacterial rhodopsin (PDB code 1JGJ, structure generated with PyMOL) serves as a model for the light receptors in our vision system (structures such as 2PED from cow are nearly identical). The receptor consists of seven membrane spanning α-helices and functions much like a β-adrenergic receptor, with the notable exception of its activation occuring via a bound cofactor rather than a ligand. Retinal (shown in the gray space filling representation in the middle of the molecule) is bound via a Schiff base to a lysine in the red helix. In the unstimulated form, we find the 11-cis-retinal configuation shown below:
Upon activation by a photon of suitable color, the retinal isomerizes at the 11 position, becoming all-trans-retinal:
Activated rhodopsin stimulates the exchange of GTP for GDP in the α-subunit of the G protein transducin shown below (1A0R; the α-subunit is shown in purple). The β-subunit, shown in green, comprises a β-propeller and stays associated with the γ-subunit, shown in cyan.
When the α-subunit binds GTP, it dissociates from the other two subunits and interacts with a phosphodiesterase, which catalyzes the hydrolysis of cGMP. Ion channels which had been held open by cGMP now slam shut, initiating hyperpolarization across the membrane and the transmission of a nerve signal.
All of the vision receptors, whether they be in the low light rods or the color sensing cones, use retinal to sense photons. Amino acids within rhodopsin's core tune each receptor for its designated color.