# Phytochromes: Structural Perspectives on Photoactivation and Signaling

> **NIH NIH R01** · WASHINGTON UNIVERSITY · 2021 · $292,600

## Abstract

PROJECT SUMMARY
Most organisms employ an array of photoreceptors to detect their light environment. Arguably the most influential are the
phytochromes (Phys), a diverse group essential for plant growth and development, and widely distributed in many
bacterial, fungal, and algal genera. By reversible photointerconversion of their bilin chromophores between a red light-
absorbing Pr state and a far-red light-absorbing Pfr state, Phys act as photoswitches in various signaling cascades
responsive to light intensity, direction, duration, and spectral quality. Moreover, through the thermal reversion of Pfr back
to Pr, some Phys sense temperature through enthalpic effects on the rate of this reaction, and possibly perceive time via
the nighttime depletion of Pfr. The cumulative effects of this Pr/Pfr interconversion impact numerous physiological
processes important to agriculture and the biology of harmful plant and human pathogens. In addition, their unique
photochemistries have recently provided invaluable optogenetic tools, including novel fluorophores for tissue imaging,
and engineered photoswitches that can regulate cellular events with remarkable temporal and spatial precision.
Recently, we and others have made great strides in understanding how Phys signal through studies on the photosensing
region. An emerging toggle model posits that a light-triggered isomerization of the bilin yields angstrom-scale
rearrangements within the bilin-binding pocket that is ultimately transduced into large-scale conformational changes in
the dimeric photoreceptor. While the model helps clarify gross changes required for endstate conversion, the
intermediates of photoexcitation and ensuing structural changes necessary for a signaling-competent Pfr state are
uncertain. It is also unclear how well the model applies to plant Phys given their distinctive modular architectures.
The objective of this proposal is to complete this picture through continued structural and biochemical analyses of
representative Phys in their Pr and Pfr states, and in combination with their downstream effectors. Specific aims are to:
(1) use x-ray crystallography and cryo-electron microscopy to develop more comprehensive structures of plant and
bacterial Phys, including models of full-length dimeric photoreceptors with theirs signal output modules; (2) define how
Phys transduce the light signal through association with their downstream partners; (3) exploit serial femtosecond x-ray
crystallography to structurally define the intermediates generated after photon absorption; (4) use steady-state and surface
mapping methods to better understand the protein surface dynamics during photoconversion; and (5) appreciate how
diversity within the plant Phy family is used to enhance thermal perception through the biochemical and structural
analyses of the PhyB isoform that employs a predicted intrinsically disorder region at its N-terminus to sense temperature.
Taken together, this project will provide an...

## Key facts

- **NIH application ID:** 10242010
- **Project number:** 5R01GM127892-04
- **Recipient organization:** WASHINGTON UNIVERSITY
- **Principal Investigator:** RICHARD DAVID VIERSTRA
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $292,600
- **Award type:** 5
- **Project period:** 2018-09-01 → 2022-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10242010

## Citation

> US National Institutes of Health, RePORTER application 10242010, Phytochromes: Structural Perspectives on Photoactivation and Signaling (5R01GM127892-04). Retrieved via AI Analytics 2026-05-21 from https://api.ai-analytics.org/grant/nih/10242010. Licensed CC0.

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