Molecular adaptations of rod opsins in deep-sea fish

Purpose. The physical environment of the deep-sea places constraints on the vision of deep-sea fish. The object of the study was to investigate adaptations of visual pigments to light quality and high pressure. Methods. The four species studied, Hoplostethus mediterraneus, Catetyx laticeps, Gonostoma elongatum, and Histiobranchus batbybius, are recovered from depths ranging from 500 to almost 5000m. The rod opsin gene was PCR-ampIified and sequenced. Results. λ_max values for the rod pigment of the four deep-sea species varies from 480nm in H. mediterraneus and G. elongatum to 468nm in C. laticeps. Six candidate amino acid substitutions for spectral tuning are identified; all are located on the inner face of the chromphore-binding pocket formed by the seven transmembrane α-helices and involve either a charge change or gain/loss of an hydroxyl group. Two of these, at sites 83 and 292, are consistently substituted in a way that correlates with a spectral shift in all four species to 480nm. Shifts to shorter wavelengths may involve substitution at one or more of the remaining four sites. Opsin from H. batbybius, the species with the greatest depth range, has an increased proportion of hydroxyl-bearing and proline residues. These may be associated with adaptation to high pressures. Conclusions. A number of candidate amino acid substitutions are identified that can account for the spectral shifts to shorter wavelengths seen in the rod opsins of deep-sea fish. The hydroxyl content of deep-sea opsins is elevated. Such changes will result in an increase in the rigidity of the protein and are consistent with the elevated thermal stability seen in other proteins of deep-sea fish.