3D Structures of Plant Phytochrome A as Pr and Pfr From Solid-State NMR: Implications for Molecular Function

Area of Science:

• Structural Biology
• Plant Photobiology
• Spectroscopy

Background:

• Phytochromes are crucial plant photoreceptors regulating light responses.
• The Pfr signaling state of phytochrome is essential for its function but structurally poorly understood.
• Understanding phytochrome structure aids in deciphering plant photomorphogenesis.

Purpose of the Study:

• To determine the three-dimensional (3D) structure of oat phytochrome A (phyA3) in the Pfr signaling state.
• To elucidate the chromophore geometry and the molecular mechanisms of phytochrome photoconversion.
• To provide atomic-level insights into plant light perception and signaling.

Main Methods:

• Solid-state magic-angle spinning (MAS) NMR spectroscopy was employed to determine interatomic contacts.
• The photosensory module of oat phyA3 was auto-assembled in vitro with isotopically labeled phycocyanobilin (PCB) chromophore.
• Quantum mechanics/molecular mechanics (QM/MM) methods were used to refine 3D structural models based on NMR data.

Main Results:

• Definitive atomic assignments for the Pfr chromophore revealed a ZZEssa periplanar geometry with a β-facial disposition of the C ring.
• The Y268 side chain exhibits a significant shift compared to previous crystal structures to accommodate the chromophore.
• A detailed photoconversion sequence is proposed, involving ring flips, propionate swaps, salt bridge breakage, and domain refolding.

Conclusions:

• The determined Pfr structure and photoconversion mechanism challenge existing models.
• Structural changes, including NTE binding and tongue region refolding, are linked to phyA photoconversion.
• Findings provide a foundation for understanding phytochrome-mediated signaling and translocation within plant cells.