Squid Rhodopsin

In 2008, two structures of rhodopsin from the Japanese flying squid Todarodes pacificus were published. This was of wider significance than might first be supposed since invertebrate rhodopsin, unlike the previously solved bovine rhodopsin, signal through a Gq-type G protein rather than transducin. Protein was purified from rhabdomeric membranes isolated from squid retina and treated with proteases to remove the unique C-terminal proline-rich extension of squid rhodopsin. Shimamura et al. [37] crystallized the protein in dodecyl maltoside and obtained a 3.7 Â structure, while Murakami and Kouyama [38] used OG and obtained a structure to 2.5 Â. The overall structure and arrangement of the seven helices was similar to that previously found for bovine rhodopsin; however, a major difference was that helices TM5 and TM7 protrude an additional 25 Â further into the cytoplasm. In addition, as well as having H8 in the C-terminal tail, as is found in bovine rhodopsin, the squid structure contained an additional helix, H9, which is after H8 in the amino acid sequence and lies close to the extended helix TM6. Between helices H8 and H9 is a short helical loop, which dips into the hydrophobic membrane region. These additional cytoplasmic structures are likely to be involved in binding of Gq, and it is possible that other Gq-linked receptors may interact with their G protein via a similar mechanism perhaps with the G protein adjacent to the helices [39].

Another significant difference found between the squid and bovine rhodop-sin structures is the retinal binding pocket. The side chains interacting with retinal are altered compared to those in bovine rhodopsin, and the retinal polyene chain lies in a more linear fashion. Asn87 and Tyr111 replace Glu113 and Gly89 as the possible hydrogen bonding partner of the Schiff base in the dark state, while the putative counterion Glu180, which is highly conserved in other visual pigments, is too far away to have a direct interaction.

An important feature of the invertebrate eye is that it detects the polarization of light. In the Murakami and Kouyama structure, rhodopsin forms two intermembrane dimers arranged in a tetrameric structure in which four reti-nylidene chromophores are oriented parallel with one another. This might provide a mechanism for the detection of the polarization plane. The final difference is that in squid rhodopsin, covalently bound retinal is not released from the lysine side chain after photoisomerization but may be isomerized back to 11-cis configuration within the protein.

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