Protein-protein interactions (PPIs) mediate many fundamental cellular processes and their control through optically or chemically responsive protein domains has a profound impact on basic research and some clinical applications. Most available chemogenetic methods induce the association, i.e., dimerization or oligomerization, of target proteins, and the few available dissociation approaches either break large oligomeric protein clusters or heteromeric complexes. Here, we have exploited the

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... olled dissociation of a dimeric oxidoreductase from mycobacteria (MSMEG_2027) by its native cofactor, F420, which is not present in mammals, as a bioorthogonal monomerization switch. We found that in the absence of F420, MSMEG_2027 forms a unique domain-swapped dimer that occludes the cofactor binding site. Substantial remodelling of the intertwined N-terminal helix upon F420 binding results in the dissolution of the dimer. We then show that MSMEG_2027 can be expressed as fusion proteins in human cells and apply it as a tool to induce and release MAPK/ERK signalling downstream of a chimeric fibroblast growth factor receptor 1 (FGFR1) tyrosine kinase. This F420-dependent chemogenetic de-dimerization tool is stoichiometric, based on a single domain and presents a novel mechanism to investigate protein complexes in situ.