Interplay Between Chromophore-Binding and Domain Assembly by the B12-Dependent Photoreceptor Protein, CarH

Ines Camacho, Rachelle Black, Derren Heyes, Linus O Johannissen, Lennart A. I. Ramakers, Bruno Bellina, Perdita Barran, Sam Hay, Alex R Jones

Research output: Contribution to journalArticlepeer-review


Organisms across the natural world respond to their environment through the action of photoreceptor proteins. The vitamin B12-dependent photoreceptor, CarH, is a bacterial transcriptional regulator that controls the biosynthesis of carotenoids to protect against photo-oxidative stress. Binding of B12 to CarH monomers in the dark results in formation of a homotetramer that complexes with DNA; B12 photochemistry results in tetramer dissociation, releasing DNA for transcription. Although details of the response of CarH to light are beginning to emerge, the biophysical mechanism of B12-binding in the dark and how this drives domain assembly is poorly understood. Here – using a combination of molecular dynamics simulations, native ion mobility mass spectrometry and time-resolved spectroscopy – we reveal a complex picture that varies depending on the availability of B12. When B12 is in excess, its binding drives structural changes in CarH monomers that result in the formation of head-to-tail dimers. The structural changes that accompany these steps mean they are rate-limiting. The dimers then rapidly combine to form tetramers. Strikingly, when B12 is scarcer, as is likely in nature, tetramers with native-like structures can form without a B12 complement to each monomer, with only one apparently required per head-to-tail dimer. We thus show how a bulky chromophore such as B12 shapes protein/protein interactions and in turn function, and how a protein can adapt to a sub-optimal availability of resources. This nuanced picture should help guide the engineering of B12-dependent photoreceptors as light-activated tools for biomedical applications.
Original languageEnglish
JournalChemical Science
Early online date5 May 2021
Publication statusPublished - 5 May 2021

Research Beacons, Institutes and Platforms

  • Photon Science Institute
  • Manchester Institute of Biotechnology


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