Opsin of Calliphora peripheral photoreceptors R1-6. Homology with Drosophila Rh1 and posttranslational processing.

The rhodopsin and metarhodopsin states of two very distantly related fly species (Calliphora and Drosophila) are found to exhibit no species-specific differences in their absorbance spectra. Isolation and characterization of cDNAs encoding the major opsin of Calliphora reveal a high (86%) degree of amino acid identity with the corresponding Drosophila visual pigment. Completely conserved is the third cytoplasmic loop which displays the major structural differences with the vertebrate photopigments. Other conserved motifs are six potential phosphorylation sites in the C-terminal region of the molecule and two potential glycosylation sites in the extracellular domains at positions Asn18 and Asn194, respectively. Interestingly, unlike vertebrate visual pigments, only newly synthesized fly opsin is N-glycosylated, while the mature protein is not. The conserved structure of the cytoplasmic loops suggests that the molecular mechanism for the activation of the transduction cascade is precisely the same in Drosophila and in Calliphora. Thus, data obtained by investigating the biochemistry of rhodopsin-related processes in larger flies may be integrated with the results of genetic experiments in Drosophila into a common model of invertebrate phototransduction.