Putative components of phototransduction and chromatic antagonism in lizard parietal–eye photoreceptor

Abstract

Abstract: : Purpose:Lizard parietal eyes do not mediate image–forming vision but may detect dawn/dusk transitions. Unlike rods and cones, which give hyperpolarizing light responses, parietal–eye photoreceptors depolarize to light under dark–adapted conditions. In a green–light background, however, these photoreceptors hyperpolarize to superimposed blue light (Solessio & Engbretson, 1993). Thus, there are two chromatically antagonistic phototransduction pathways in the same cell, giving depolarizing and hyperpolarizing responses, respectively. Mechanistically, the two pathways appear to converge at a cGMP–phosphodiesterase, inhibiting and activating it, respectively (Xiong, et al., 1998). Our goal is to identify the components of these two antagonistic pathways and to understand the underlying mechanism. Methods: We generated a cDNA library from parietal eyes of side–blotched lizards, Uta stansburiana, and used homology–cloning to identify the putative phototransduction proteins. Subcellular localization of these proteins was examined by immunostaining with specific antibodies. Putative opsins were heterologously expressed and regenerated with 11–cis–retinal for spectral analysis. Results: We identified two opsins, one most similar to pinopsin and the other appears to be novel. Spectral analysis of the pinopsin–like protein remains to be done, although the published λ_max for American chameleon pinopsin is ∼482 nm (Kawamura & Yokoyama, 1997). The novel opsin, when heterologously expressed, showed a λ_max around 500 nm at pH 6.5, and converted to 380 nm upon photobleaching. Interestingly, this opsin contains a Gln instead of a Glu at its counterion position (corresponding to Glu–113 in bovine rhodopsin) – the only exception among all known vertebrate opsins. We also found two G–protein α subunits, G_αgust and G_αo, a cone–type phosphodiesterase and a rod–type cyclic–nucleotide–gated channel subunit (CNGA1). Immunostaining showed that these proteins were all expressed in the outer segment of the parietal–eye photoreceptor. Conclusions: The two opsins and the two G–protein α subunits may mediate the antagonistic phototransduction pathways in the parietal–eye photoreceptor. It remains to be examined which G_α activates, and which G_α inhibits, the phosphodiesterase, as well as the underlying mechanisms.

Keywords: signal transduction • opsins • photoreceptors