Fly Cryptochrome and the Visual System

Cryptochromes are flavoproteins structurally and evolutionarily related to photolyases, involved in the development, magnetoreception and temporal organization of a variety of organisms. Drosophila CRYPTOCHROME (dCRY) mediates light synchronization of the master circadian clock, and is an integral component of the circadian clocks in fly's peripheral tissues, where it works as a transcriptional repressor. The C-terminus of dCRY plays an important role in modulating light sensitivity and activity of the protein. The activation of dCRY by light requires a conformational change (1) but it has been suggested that it could be mediated/modulated also by specific “regulators” that bind the C-terminus of the protein and regulate the light-dependence of dCRY activity (2). This hypothesis was later supported by the observation that the C-terminus of dCRY is a potential hotspot for molecular interactions as it harbours several protein-protein interaction motifs (3). Here we show that some functional linear motifs are evolutionary conserved in the C-terminus of Cryptochromes and that class III PDZ binding sites are selectively maintained in animals. A co-immunoprecipitation assay followed by mass spectrometry analysis has revealed that dCRY interacts with Retinal DeGeneration A (RDGA) and Neither Inactivation Nor Afterpotential C (NINAC). Both proteins belong to a multi-protein complex (the Signalplex) that includes visual signaling molecules. We have found that dCRY interacts with proteins of the visual cascade through INAD (Inactivation No Afterpotential D) and that the interaction CRY-INAD, mediated by specific domains of the two proteins, is light-dependent. Moreover, we have observed an impairment of the visual behavior in flies mutants for dCRY, which suggests a role, direct or indirect, for this photoreceptor in fly vision.