Isolation and structure of an arrestin gene from Drosophila.

Abstract

A Drosophila gene encoding a homologue of vertebrate arrestin was isolated by subtractive hybridization and identified as a member of a set of genes that are preferentially expressed in the visual system. This gene encodes a 364-amino acid protein that displays >40% amino acid sequence identity with human and bovine arrestin. Interestingly, the Drosophila homologue lacks the C-terminal sequences that were postulated to interact with rhodopsin during the quenching of the phototransduction cascade in the vertebrate visual response. These findings are discussed in terms of invertebrate phototransduction. The Drosophila gene was mapped cytogenetically to chromosomal position 36D1-2, near the ninaD locus. However, the arrestin gene does not appear to be the ninaD locus, as sequence analysis of three ethylmethane sulfateinduced ninaD mutant alleles reveals no alteration in amino acid sequence.Phototransduction is the process that converts the energy of an absorbed photon into a change of the ionic permeabilities of the photoreceptor cell membrane. This light-induced change in ionic conductances gives rise to the receptor potential and synaptic activity of the photoreceptor cell. The mechanism of visual excitation in vertebrate photoreceptors is the best understood of all sensory transduction processes (1-4). Light activation of rhodopsin is the first step in the visual response. In the vertebrate, photoactivated rhodopsin molecules activate a guanine nucleotide-binding (G) protein, transducin, which in turn activates a cGMP phosphodiesterase. The reduction of intracellular levels of cGMP leads to the transient closure of a cGMP-gated cation-selective channel and hyperpolarization of the photoreceptor cell.Unlike vertebrates, the microvillar photoreceptors of invertebrates depolarize in response to light and thus open their cation-selective channels. The identity of the intracellular transmitter(s) that mediates excitation in invertebrates has eluded firm identification. Similarly, the enzyme cascade that triggers the visual response has not been defined. However, there is a large body of physiological, genetic, and biochemical work that has strongly implicated calcium and inositol phospholipid metabolism in excitation of dipteran and Limulus photoreceptors (5-7). It is believed that photoactivated rhodopsin interacts with a G protein, which in turn activates a phospholipase C. Phospholipase C would then catalyze the generation of the second messenger inositol 1,4,5-trisphosphate and the subsequent mobilization of calcium from intracellular storage sites. The transient increase in calcium levels (or inositol 1,4,5-trisphosphate) would then lead to the opening of a cation-selective channel and the generation of a depolarizing receptor potential. Strong support for this model was recently provided by the demonstration that the Drosophila no-receptor potential A (norpA) gene encodes a phospholipase C that is abundantly expressed in the adult retina (8,9

Phototransduction and retinal degeneration in Drosophila

“…It is proposed that the rate of arrestin binding determines the kinetics of rhodopsin inactivation in vivo [90]. In Drosophila, there are two arrestins expressed in photoreceptor cells, arrestin1 and arrestin2 (Arr 1 and Arr 2) [91][92][93]. Arr2, the major isoform, comprises ∼85% of the total arrestin, whereas the remaining 15% is Arr1 [94].…”

The mouse S‐antigen gene

“…Moreover, conservation of secondary structure is also high between the S-antigen of mammalian and Drosophila [26 -281. However, selected differences are apparent; Drosophilu arrestin lacks a C-terminus domain which is postulated to be a rhodopsin binding site [14,[26][27][28]. In addition, the presumptive phosphoryl binding sites are conserved well in a single exon in mammalian species but are diverged in Drosophila.…”

“…The identification and availability of a mammalian S-antigen gene would allow a detailed analysis of 11 31. its precise functional role in the phototransduction process. While the manuscript for this paper was in preparation, three laboratories reported the sequences of the Drosophilu S-antigen (arrestin) gene [26,271 and that for its 49-kDa homolog 1281. In this communication, we present the structural organization of the mouse S-antigen gene and compare it with the human gene [29] and with the Drosophila genes mentioned above.…”

Phototransduction in Drosophila

“…In this step, Arrestins plays a critical role by displacing Gq  subunit to bind with rhodoposin [38,52,53]. Drosophila photoreceptor cells contain two Arrestins, Arrestin1 and Arrestin2 (Arr 1 and Arr 2) [54][55][56]. Arr2, the major isoform of Arrestins, plays the predominant roles in the deactivation of rhodopsin [38], while Arr1 mediates light-dependent rhodopsin endocytosis [52].…”