Small Protein Domains as Potential Spin Labels for In Vitro, Cellular, and Light-Induced Dipolar EPR Spectroscopy

This study explores the potential of small light-oxygen-voltage (LOV) domains for utilization as protein spin labels in different dipolar EPR spectroscopy methods. The distinctive photochemical properties of selected LOV domain variants are exploited to generate a variety of (meta)stable flavin mononucleotide (FMN) radicals upon blue light absorption. Three different radicals, FMN^·-, FMNH^·, and an FMN-methionine radical, and an excited FMN triplet species, were generated. The FMN radicals were generated in LOV single domains and two model LOV1-LOV2 fusion proteins, and the latter proteins demonstrated that simple and effective orthogonal spin labeling can be performed. Subsequently, dipolar EPR experiments were conducted in aqueous solution and in cells with and without additional light excitation, in order to measure the distances between the FMN cofactor radicals, and to infer the structure and dynamics of the LOV domain proteins. Interestingly, all LOV1-LOV2 fusion proteins exhibit defined but largely distinct distances. This can be attributed to two factors: the respective LOV domains have different interactions with each other, and the presence of neutral FMN radicals leads to dimerization of the LOV1 domains. Nevertheless, using LOV domains as genetically encoded spin labels could offer numerous advantages. As a true molecular biology concept, labeling and measurements can be performed in any accessible cell type using light as the only stimulus. Additionally, the various paramagnetic FMN states enable the measurement of distances between two radicals, as well as between a radical and a triplet state.