| Summary: | 3-D images of marine mammal Opn4 proteins (teal) were aligned with squid Rh1 (green) to determine relative positions of helices and residues. (A) squid rhodopsin (Protein Data Bank accession number 2Z73) showing the relative position of the ten nonconservative amino acid substitutions from Fig 3 . A proposed protonated Schiff base counterion (Glu214) [ 77 – 79 ] is shown in orange. Panels B-D highlight nonconservative amino acids potentially involved in spectral tuning and their relative predicted distance to the retinal Schiff base lysine (red) terminal NZ atom or the β-ionone ring of the chromophore (yellow): (B) manatee Thr123 (9.5 Å); (C) Yangtze river dolphin Thr128 (11.8 Å); and (D) Weddell seal Ala311 (9.3 Å); Weddell seal and walrus Thr315 (11.2 Å). Opn4 numbering is from mouse.
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