Abstract
The molecular mechanisms that regulate invertebrate visual pigment absorption are poorly understood. Studies of amphioxus Go-opsin have demonstrated that Glu-181 functions as the counterion in this pigment. This finding has led to the proposal that Glu-181 may function as the counterion in other invertebrate visual pigments as well. Here we describe a series of mutagenesis experiments to test this hypothesis and to also test whether other conserved acidic amino acids in Drosophila Rhodopsin 1 (Rh1) may serve as the counterion of this visual pigment. Of the 5 Glu and Asp residues replaced by Gln or Asn in our experiments, none of the mutant pigments shift the absorption of Rh1 by more than 6 nm. In combination with prior studies, these results suggest that the counterion in Drosophila Rh1 may not be located at Glu-181 as in amphioxus, or at Glu-113 as in bovine rhodopsin. Conversely, the extremely low steady state levels of the E194Q mutant pigment (bovine opsin site Glu-181), and the rhabdomere degeneration observed in flies expressing this mutant demonstrate that a negatively charged residue at this position is essential for normal rhodopsin function in vivo. This work also raises the possibility that another residue or physiologic anion may compensate for the missing counterion in the E194Q mutant.
Highlights
Rhodopsin absorption is regulated by interactions between the retinal chromophore and amino acids within the opsin apoprotein
The site corresponding to bovine rhodopsin Asn-111 is the one position that is not conserved in all Drosophila pigments and contains Gln in Rh3 and Rh4 and Glu or Asp in the other opsins
The principal result from this study is that substitution of Asp or Glu amino acids present at positions 96, 124, 147, 194, and 203 in the Drosophila Rhodopsin 1 (Rh1) visual pigment does not cause the dramatic spectral shifts that would be expected if one of these residues was the counterion
Summary
Rhodopsin absorption is regulated by interactions between the retinal chromophore and amino acids within the opsin apoprotein. The visual pigment rhodopsin is a unique G-protein-coupled receptor that is activated by the conformational change of a covalently attached chromophore rather than the binding of a diffusible transmitter, drug or hormone [1] In both vertebrates and invertebrates, rhodopsin consists of an 11-cis-retinal chromophore that is bound to the opsin apoprotein via a protonated Schiff base (Fig. 1a). The rhabdomeric photoreceptors of Drosophila and many other invertebrates depolarize in response to light, as the result of the activation of a heterotrimeric Gq protein, which activates phospholipase C and two classes of lightsensitive transient receptor potential channels, TRP and TRPL [16] that admit both Naϩ and Ca2ϩ In addition to their functional differences, phylogentic evidence suggests that the Goopsins diverged from the r-opsins prior to the cnidarian-bilaterian split, over 580 million years ago (Fig. 2a) [17,18,19]. None of the mutants display a dramatic shift in color sensitivity to shorter wavelengths, as would be expected for a mutation in a putative counterion
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
