Veela defines a molecular link between Cryptochrome and Timeless in the light-input pathway to Drosophila 's circadian clock

Abstract

Organisms use the daily cycles of light and darkness to synchronize their internal circadian clocks with the environment. Because they optimize physiological processes and behavior, properly synchronized circadian clocks are thought to be important for the overall fitness. In Drosophila melanogaster, the circadian clock is synchronized with the natural environment by light-dependent degradation of the clock protein Timeless, mediated by the blue-light photoreceptor Cryptochrome (Cry). Here we report identification of a genetic variant, Veela, which severely disrupts this process, because these genetically altered flies maintain behavioral and molecular rhythmicity under constant-light conditions that usually stop the clock. We show that the Veela strain carries a natural timeless allele (ls-tim), which encodes a less-light-sensitive form of Timeless in combination with a mutant variant of the F-box protein Jetlag. However, neither the ls-tim nor the jetlag genetic variant alone is sufficient to disrupt light input into the central pacemaker. We show a strong interaction between Veela and cryptochrome genetic variants, demonstrating that the Jetlag, Timeless, and Cry proteins function in the same pathway. Veela also reveals a function for the two natural variants of timeless, which differ in their sensitivity to light. In combination with the complex array of retinal and extraretinal photoreceptors known to signal light to the pacemaker, this previously undescribed molecular component of photic sensitivity mediated by the two Timeless proteins reveals that an unexpectedly rich complexity underlies modulation of this process.

Ramshackle (Brwd3) promotes light-induced ubiquitylation of Drosophila Cryptochrome by DDB1-CUL4-ROC1 E3 ligase complex

“…In addition to dTIM degradation, light also induces dCRY itself to degrade, thus enabling newly synthesized dTIM and dPER to accumulate and reestablish the repressive phase of the circadian clock circuitry (5). The ubiquitin-proteasome system (UPS) mediates the light-dependent degradation of dTIM (6,7) and light-independent degradation of dPER (8,9). These degradations occur through SCF (Skp1/Cullin/F-box protein) ubiquitin E3 ligase complexes [Cullin1-RING Finger E3 Ligase (CRL1)], which are responsible for targeting substrates to the UPS (10,11).…”

Even a stopped clock tells the right time twice a day: circadian timekeeping in Drosophila

“…Like cry b mutants, jet mutants are rhythmic in LL, have normal DD behaviour and show reduced responses to light pulses, suggesting a defect in the light input pathway [54]. This effect is dependent on the genetic background, as rhythmic behaviour is only observed in jet c mutants when they have one of two naturally occurring tim alleles that differ by 23 AA at the N-terminus [55,56]. The functional difference between these two TIM isoforms remains to be determined.…”

Timeless genes and jetlag

“…In the ''pregenomic'' era, analysis of a new mutant strain required meticulous attention to the phenotypic details of increasingly fine mapping crosses. As Peschel et al (2) demonstrate, such analyses still play a crucial role in the analysis of mutant lines. What at first seemed to be a contradictory anomaly -how can jet c /jet c show a normal light entrainment phenotype?…”

“…In their work, Peschel et al (2) describe another strain of Drosophila that exhibits strong rhythmicity in LL, which they named Veela. (One suspects that the authors so dubbed this strain, named for the siren-like creatures whose dancing mesmerizes Harry Potter and his friend Ron but who turn into vengeful, fireball-throwing bird-like creatures when spurned, to summarize the sequential attraction and difficulty in working with these flies.)…”

“…The remarkable implication of this discovery, that there are specific genes whose functions specifically subserve a general circadian timekeeping mechanism, has been spectacularly borne out with the discovery of at least a half-dozen other such ''clock factors'' in forward genetic screens and the ensuing construction of a coherent molecular model for circadian clock function. In a recent issue of PNAS, Peschel et al (2) showed that, 35 years later, classical forward genetic studies continue to yield important insights into the mechanisms of the circadian clock.…”