# Engineered bacteriophytochrome heterodimers for research and applications

## TL;DR

Scientists engineered bacteriophytochrome proteins to form light-sensitive heterodimers, enabling new tools for controlling cellular processes with red light.

## Contribution

The novel contribution is creating monomeric bacteriophytochrome modules that form stable heterodimers, enabling light-controlled kinase activity.

## Key findings

- Modified bacteriophytochrome PSMs form stable heterodimers and control histidine kinase activity in response to red light.
- Dimerization is essential for kinase activity of FixL but not for phosphatase activity of DrBphP.
- Heterodimeric variants were applied to a red light-regulated gene expression system, demonstrating combined control of cellular events.

## Abstract

Many proteins are dimeric, functioning as complexes of two identical or different subunits. Bacteriophytochromes are homodimeric photoreceptor proteins that sense red/far-red light with a photosensory module (PSM) and convert it to a biological response via an output module, usually a histidine kinase (HK). Here, we generate monomeric bacteriophytochrome PSMs that form stable heterodimers once mixed by modifying two salt bridges at the dimerization interface of the Deinococcus radiodurans phytochrome (DrBphP). We confirm that these heterodimeric PSMs can control output HK module activity in response to red light and reveal that dimerization is required for kinase activity of the model HK FixL, but not necessarily for phosphatase activity of DrBphP. By applying the heterodimeric variants to a red light-regulated gene expression tool, we exemplify the combined control of cellular events using both heterodimerization and light. These results pave the way for new heterodimeric systems, for example, in receptor protein research and optogenetics.