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Updated: Jul 4, 2025
Analysis of Circadian Photoresponses in Drosophila Using Locomotor Activity
Dissecting the Interaction between Cryptochrome and Timeless Reveals Underpinnings of Light-Dependent Recognition
Connor M Schneps1, Robert Dunleavy1, Brian R Crane1
1Department of Chemistry & Chemical Biology, Cornell University, Ithaca, New York 14853, United States.
Abstract:
Circadian rhythms are determined by cell-autonomous transcription-translation feedback loops that entrain to environmental stimuli. In the model circadian clock of Drosophila melanogaster, the clock is set by the light-induced degradation of the core oscillator protein timeless (TIM) by the principal light-sensor cryptochrome (CRY). The cryo-EM structure of CRY bound to TIM revealed that within the extensive CRY:TIM interface, the TIM N-terminus binds into the CRY FAD pocket, in which FAD and the associated phosphate-binding loop (PBL) undergo substantial rearrangement. The TIM N-terminus involved in CRY binding varies in isoforms that facilitate the adaptation of flies to different light environments. Herein, we demonstrate, through peptide binding assays and pulsed-dipolar electron spin resonance (ESR) spectroscopy, that the TIM N-terminal peptide alone exhibits light-dependent binding to CRY and that the affinity of the interaction depends on the initiating methionine residue. Extensions to the TIM N-terminus that mimic less light-sensitive variants have substantially reduced interactions with CRY. Substitutions of CRY residues that couple to the flavin rearrangement in the CRY:TIM complex have dramatic effects on CRY light activation. CRY residues Arg237 on α8, Asn253, and Gln254 on the PBL are critical for the release of the CRY autoinhibitory C-terminal tail (CTT) and subsequent TIM binding. These key light-responsive elements of CRY are well conserved throughout Type I cryptochromes of invertebrates but not by cryptochromes of chordates and plants, which likely utilize a distinct light-activation mechanism.
Insights
The circadian clock protein timeless (TIM) binds light-sensor cryptochrome (CRY) via its N-terminus. This interaction, crucial for light entrainment in fruit flies, is modulated by specific CRY residues and TIM N-terminal variations.
Area of Science:
- * Molecular and Cellular Biology
- * Chronobiology
- * Structural Biology
Background:
- * Circadian rhythms govern daily biological processes through internal clocks.
- * In Drosophila, light synchronizes the circadian clock via cryptochrome (CRY) and timeless (TIM).
- * Light induces TIM degradation mediated by CRY, setting the clock.
Purpose of the Study:
- * To elucidate the structural and functional mechanisms of CRY-TIM interaction in Drosophila.
- * To identify key residues and regions involved in light-dependent CRY activation and TIM binding.
- * To understand how TIM N-terminal variations affect light adaptation in flies.
Main Methods:
- * Cryo-electron microscopy (cryo-EM) to determine the structure of the CRY:TIM complex.
- * Peptide binding assays to assess TIM N-terminal peptide interactions with CRY.
- * Pulsed-dipolar electron spin resonance (ESR) spectroscopy to study light-dependent binding dynamics.
- * Site-directed mutagenesis of CRY residues to evaluate their role in light activation.
Main Results:
- * The TIM N-terminus binds within the CRY FAD pocket, inducing conformational changes.
- * Light-dependent binding of the TIM N-terminal peptide to CRY was confirmed, influenced by the initiating methionine.
- * Specific CRY residues (Arg237, Asn253, Gln254) are critical for releasing the CRY C-terminal tail and enabling TIM binding.
- * Alterations in TIM N-terminus length mimic less light-sensitive variants, reducing CRY interaction.
- * Conserved light-responsive elements in invertebrate CRYs differ from those in chordates and plants.
Conclusions:
- * The CRY:TIM interface and light-induced conformational changes are essential for circadian clock entrainment in Drosophila.
- * Specific amino acid residues in CRY and the TIM N-terminus dictate the light sensitivity and interaction dynamics.
- * These findings reveal conserved and divergent mechanisms of cryptochrome-mediated light sensing across species.
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