A 49-Kilodalton Phosphoprotein in theDrosophilaPhotoreceptor Is an Arrestin Homolog

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Abstract
The gene encoding the 49-kilodalton protein that undergoes light-induced phosphorylation in theDrosophilaphotoreceptor has been isolated and characterized. The encoded protein has 401 amino acid residues and a molecular mass of 44,972 daltons, and it shares approximately 42 percent amino acid sequence identity with arrestin (S-antigen), which has been proposed to quench the light-induced cascade of guanosine 3′,5′-monophosphate hydrolysis in vertebrate photoreceptors. Unlike the 49-kilodalton protein, however, arrestin, which appears to bind to phosphorylated rhodopsin, has not itself been reported to undergo phosphorylation. In vitro, Ca2+was the only agent found that would stimulate the phosphorylation of the 49-kilodalton protein. The phosphorylation of this arrestin-like protein in vivo may therefore be triggered by a Ca2+signal that is likely to be regulated by light-activated phosphoinositide-specific phospholipase C.