PMC

(A) Rhodopsin (green) labeling only; (B) retinochrome (red) labeling only. (C) Overlap of rhodopsin and retinochrome labeling occurs at the junction between arm and each sucker, and appears yellow. Small cell bodies also labeled with rhodopsin and retinochrome are hair cells (see ). Blue shows autofluorescence of the tissue. Scale bar, 25μm.

(A) Rhodopsin (green) labeling only; (B) retinochrome (red) labeling only. (C) Overlap of rhodopsin and retinochrome labeling in large muscle bundles (mb) parallel to dermal tissue appears yellow. Blue shows autofluorescence of the tissue. Scale bar, 25μm.

Secondary structural model of retinochrome showing the location of Glu and Asp residues (white letters). For affinity purification with an anti-rhodopsin antibody, the amino acid sequence of monoclonal antibody Rho1D4 epitope (ETSQVAPA) is introduced to the C terminus of retinochrome. On the basis of sequence alignment, the bovine rhodopsin amino acid residue numbering system is used. The retinochrome numbering system is shown in parentheses.

The phylogenic relationship of opsins and the structural relationship of retinochrome and RGR. A: Opsins are subdivided into three large groups including ciliary opsins, rhabdomeric opsins, and group 4 opsins. In ciliary photoreceptors, phototransduction is mediated by ciliary visual opsin, which uses a G protein specifically G_t-mediated signaling cascade and elicits a hyperpolarization to light. In rhabdomeric photoreceptors, phototransduction is mediated by rhabdomeric opsin, which uses a G_q-mediated signaling cascade and elicits a depolarization to light. Retinochrome and RGR, which are the group 4 opsins, act as photoisomerases. However, a G-protein cascade associated with the activation of retinochrome or RGR remains possible. B: Homology models of RGR from Bos taurus (violet) and retinochrome from Todarodes pacificus (teal blue) were created using Swiss-model (). The proposed seven-transmembrane domain models of RGR and retinochrome were superimposed in PyMol editor. The chromophore-binding site at a Lys residue and the counterion Glu residue are highlighted as sticks.