Researchers studied the molecular mechanism of cryptochrome (CRY) 1, which lengthens circadian period to cause delayed sleep phase disorder (DSPD), and discovered that the photolyase homology region (PHR)-binding epitope in exon 11 disrupts the interaction between CRY1 and the subunit circadian locomotor cycles output kaput (CLOCK), which would otherwise inhibit the activity of CLOCK:BMAL1 and close the core feedback loop. Findings were published in the Proceedings of the National Academy of Sciences of the Unites States of America (PNAS),
In their study, the researchers wanted to find out how exon 11 regulates binding between the CRY1 PHR and CLOCK: BMAL1. They used fluorescence polarization (FP) assays to explore binding of a fluorescently labeled human CRY1 tail to the PHR domain in trans and mapped the PHR-binding epitopes on the tail using nuclear magnetic resonance (NMR) spectroscopy. Using NMR data, they studied structural characteristics of the isolated CRY1 tail and conducted a quantitative comparison of CRY1 tail NMR data to the chemical shift index.
They found that the Δ11 version of the human CRY1 tail has much lower affinity in binding to the PHR domain and that both CRY1 Δ11 and the isolated PHR have a higher affinity for the CLOCK: BMAL1 PAS domain core than CRY1 does.
When they compared the CRY1 tail NMR data to the chemical shift index, they found that the entire tail lacks secondary structure if the PHR domain is absent. The Δ11 had no impact on the CRY1 PHR: CLOCK PAS-B Complex.
Exon 11 is crucial for regulation of the interaction between CLOCK PAS-B and the CRY1 PHR domain.
“As an alternative to regulation by phototransduction, our data suggest that the association of other proteins or posttranslational modifications likely play direct roles in regulating the CRY1 PHR–tail interaction,” the researchers said. “Use of a flexibly tethered autoinhibitory domain to tune CRY1 activity in the clock is also consistent with a growing appreciation that intrinsically disordered regions offer a powerful means to modulate protein activity via regulated alternate splicing.”
Parico GCG, Perez I, Fribourgh JL, Hernandez BN, Lee HW, Partch CL. The human CRY1 tail controls circadian timing by regulating its association with CLOCK:BMAL1. PNAS. 2020;117(45)27971-27979. doi: 10.1073/pnas.1920653117