Distinct Structural Dynamics of the Semiquinone State Define a Signalling Pathway in Avian Cryptochrome

TL;DR

This study uncovers how the semiquinone state in avian cryptochrome triggers specific structural changes, linking quantum photochemistry to cellular signalling for bird navigation.

Contribution

It reveals the unique conformational signature of the semiquinone state and its role in a high-fidelity signalling cascade in cryptochrome.

Findings

Semiquinone exhibits a transient destabilization of key loops.

Photochemical reduction induces allosteric structural transitions.

Distinct conformational dynamics connect quantum states to signalling.

Abstract

The light-dependent magnetic compass of night-migratory songbirds is widely hypothesized to rely on the radical pair mechanism within retinal cryptochrome. However, bridging the mechanistic gap between microsecond quantum spin dynamics and the long-lived, global protein conformational changes required for cellular signalling remains a formidable challenge. Here, we apply redox state-resolved hydrogen/deuterium-exchange mass spectrometry (HDX-MS) to map the conformational landscape of European robin cryptochrome 4a (ErCry4a) across its photocycle. We reveal that photochemical reduction drives robust, allosteric structural transitions across key functional nodes, including the phosphate-binding loop (PBL), protrusion loop (PL), FAD-proximal helix {\alpha}17, and the C-terminal {\alpha}22/{\alpha}23 network. Crucially, we isolate the structural fingerprint of the transient semiquinone, the presumed signalling species. Rather than acting as a linear structural stepping-stone, the semiquinone exhibits a distinct, non-monotonic conformational signature characterized by a transient destabilization of the PBL and PL, contrasting sharply with the global rigidification observed in the fully reduced state. These findings establish the semiquinone as a structurally unique and functionally competent biological entity. Our results provide direct biophysical evidence for a dedicated, high-fidelity structural signalling cascade, detailing how localized quantum-level photochemistry is translated into the precise conformational dynamics required for animal navigation.