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Changes in active site histidine hydrogen bonding trigger cryptochrome activation.

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Cryptochrome (CRY) light sensing in insects involves flavin photoreduction. This process, coupled with His378 protonation, triggers conformational changes in the C-terminal tail, regulating interactions with the Timeless (TIM) protein.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Chronobiology

Background:

  • Cryptochrome (CRY) is the primary light sensor for insect circadian clocks.
  • CRY's function relies on light-induced conformational changes of its flavin cofactor and associated protein regions.
  • The C-terminal tail (CTT) of Drosophila CRY (dCRY) inhibits interactions with target proteins like Timeless (TIM) in its resting state.

Purpose of the Study:

  • To elucidate the molecular mechanism by which light induces conformational changes in dCRY.
  • To investigate the role of the conserved His378 residue in mediating light-dependent signaling.
  • To understand how these structural changes regulate dCRY's interaction with TIM and subsequent cellular functions.

Main Methods:

  • All-atom molecular dynamics (MD) simulations to observe CTT dynamics.
  • Poisson-Boltzmann calculations to determine pKa changes.
  • Replica-exchange MD simulations to explore conformational landscapes.
  • Site-directed mutagenesis (His378 variants) and cellular assays.

Main Results:

  • Flavin photoreduction to anionic semiquinone (ASQ) destabilizes the CTT, leading to its release within 25 ns.
  • CTT release is strongly correlated with the protonation state of His378, whose pKa increases upon flavin reduction.
  • His378 variants exhibit altered pH dependence of photoreduction and differential effects on TIM activation and dCRY degradation.
  • His378Arg/Lys variants activate TIM but resist light-induced degradation, suggesting distinct conformational outcomes.

Conclusions:

  • The dCRY photosensory mechanism involves light-induced flavin reduction coupled to His378 protonation.
  • Altered hydrogen bonding at His378 upon protonation drives CTT release and regulates dCRY function.
  • Different conformational states may be involved in TIM interaction versus self-degradation, highlighting functional plasticity.