Rhodopsin, light activation

Farrens DL, Altenbach C, Yang K, Hubbell WL, Khorana HG. Requirement of rigid-body motion of transmembrane helices for light activation of rhodopsin. Science 1996 274(5288) 768-770. 

The interaction between the receptor and the G-protein is transient and rapidly reversible. This is indicated, for example, by the fact that a single light-activated rhodopsin molecule may activate 500 to 1000 transducin molecules during its 1 to 3 sec lifetime. Hence, the interaction should, in the endpoint, be governed by the normal laws of chemical interaction and expressible in terms of association and dissociation rate constants and binding affinity. The question then arises as to whether the affinity of different receptors for different G-proteins varies. That is, is there specificity in receptor-G-protein coupling, and, if so, what determines this ... 

Ovchinnikov 234 237) has shown that bovine rhodopsin, although quite different in amino acid sequence (348 residues), also forms seven transmembrane helices. This structural similarity between bacterial and mammalian light activated membrane proteins is remarkable. Since the two amino acid sequences have little in common it would appear that the necessary requirement is seven transmembrane helices to form a channel which is specific for proton migration. For example it has been suggested that a similar arrangement and function is performed by the lactose permease of E. coli237). 

Brown MF, Salgado GFJ, Struts AV (2010) Retinal dynamics during light activation of rhodopsin revealed by solid-state NMR spectroscopy. BBA-Biomembranes 1798 177-193... 

S ATP -I- 338-SKTETSQVAPA-348 <1, 12> (<1, 12> peptide containing the last 11 amino acids of the C-terminal of bovine rhodopsin [20, 24] <1> phosphorylated at Ser-343, about 11% of the rate with rhodopsin, photo-activated rhodopsin-dependent, soluble active kinase catalyzes photoacti-vated rhodopsin-independent peptide phosphorylation [20] <12> only in the presence of photoactivated rhodopsin, which activates RK for peptide phosphorylation, also activated by metarhodopsin III, but not by opsin, up to 60% of the rate with photoactivated rhodopsin, light-dependent phosphorylation [24]) (Reversibility <1,12> [20,24]) [20, 24]... 

In rod and cone cells of the retina, light activates rhodopsin, which stimulates replacement of GDP by GTP on the G protein transducin. The freed a subunit of transducin activates cGMP phosphodiesterase, which lowers [cGMP] and thus closes cGMP-dependent ion channels in the outer segment of the neuron. The resulting hyperpolarization of the rod or cone cell carries the signal to the next neuron in the pathway, and eventually to the brain. 

Because one molecule of activated cGMP phosphodiesterase can hydrolyze more than 105 molecules of cGMP per second the light response is highly amplified. There is also an earlier stage of amplification. Each molecule of light-activated rhodopsin (R ) is able to catalyze the exchange of GTP for GDP on hundreds of molecules of... 

Figure 23-43 The light-activated transducin cycle. In step a photoexcited rhodopsin (R ) binds the GDP complex of the heterotrimeric transducin (T ). After GDP—GTP exchange (step b) the activated transducin T GTP reacts with the inhibited phosphodiesterase (PDEapY2) to release the activated phosphodiesterase (PDEap). Based on scheme by Stryer528 and other information.

Cai, K., Itoh, Y., and Khorana, H. G. (2001). Mapping of contact sites in complex formation between transducin and light-activated rhodopsin by covalent crosslinking Use of a photoactivatable reagent. Proc. Natl. Acad. Sci. USA 98, 4877-4882. 

Mazzoni, M. R., and Hamm, H. E. (1996). Interaction of transducin with light-activated rhodopsin protects it from proteolytic digestion by trypsin. / Biol. Chem. 271,... 

Several groups have demonstrated the use of trNOESY experiments to determine the structure of peptides bound to their receptor. For example, Kisselev et al. determined the 3D solution structure of the transductin a-subunit bound to light-activated rhodopsin (51). They found that the peptide IKENLKDCGLF formed an a-helix terminated by an open reverse turn (Fig. 11), while in contrast the conformation of the peptide remained disordered when in contact with nonactivated rhodopsin. Their findings led to the development of derivatives of the peptide that maintained the binding conformation and had improved affinity (73). Importantly, in their studies they used rhodopsin in a membrane environment (extracted from bovine rods), emphasizing the capabilities of the method for the study of integral membrane receptor directed inhibitors in their natural environment. 

Kisselev, O. G., Kao, J., Ponder, J. W Fann, Y. C., Gautam, N. and Marshall, G. R. (1998) Light-activated rhodopsin induces structural binding motif in G protein a subunit. Proc. Natl. Acad. Sci. 95,4270 1275. 

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