A bacteriophytochrome Pr/Pfr heterodimer studied through single-particle time-resolved cryo-electron microscopy

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Phytochromes are light-sensitive proteins regulating cellular processes.
• Bacterial phytochromes (BphPs) utilize a two-component system involving histidine kinase (HK) and response regulator domains.
• Previous studies identified a stable Pr/Pfr heterodimer in Stigmatella aurantiaca BphP2 (SaBphP2).

Purpose of the Study:

• To investigate the structural dynamics of SaBphP2 during light-induced signal transduction.
• To elucidate the mechanism of light-triggered conformational changes in bacterial phytochromes.

Main Methods:

• Utilized the Spotiton technique for rapid cryo-electron microscopy (cryo-EM) sample preparation.
• Captured SaBphP2 particles within 10 milliseconds of illumination.
• Compared cryo-EM structures of Pr/Pfr heterodimers and Pr/Pr homodimers.

Main Results:

• Observed a stable Pr/Pfr heterodimer with distinct protomer conformations.
• Identified a significant 180° rotation of the HK domain in the Pfr state compared to the Pr state.
• Demonstrated that this rotation is driven by the unwinding of coiled-coil linkers between the photosensory and enzymatic domains.

Conclusions:

• The large-scale reorientation of the HK domain is crucial for light-triggered signal transduction in BphPs.
• Provides a mechanistic understanding of how light energy is converted into biochemical signals.
• Highlights the dynamic nature of phytochromes in response to light stimuli.