In this chapter, molecular aspects of squid rhodopsin structure and function are reviewed, and some new results on its purification and reconstitution into phospholipid membranes are presented. The retina of the squid contains highly ordered arrays of photoreceptor microvilli, aligned in two orthogonal directions in the plane normal to the optic axis. The microvillar membranes themselves have ordered membrane–membrane and membrane–cytoskeleton contacts. Their major protein is the visual pigment, rhodopsin. This arrangement aligns the rhodopsin molecules, thereby conferring sensitivity to the plane of polarization of light absorbed by the retinula cells. The biological function of polarization sensitivity in squid is unknown, but could be related to detection of prey or navigation. The photostable metarhodopsin is regenerated to rhodopsin by a chromophore exchange system involving the retinal isomerase retinochrome, and a soluble retinal binding protein. Cephalopod rhodopsins contain a unique C-terminal sequence of proline repeats, which may be involved in the structural organization. Once light is absorbed by rhodopsin, it functions as the activator of a second messenger cascade, which includes, in those invertebrates studied, the GTP-binding protein Gq and the effector enzyme phospholipase C. Rhodopsins absorb light by virtue of their 11-ds-retinal chromophore, covalently bound to and enclosed by the protein. The wavelength of maximal sensitivity is determined mainly by the amino acid residues in the region of the chromophore, but also by the variant of retinal present.
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