YtvA, a photosensory LOV (light-oxygen-voltage) protein from Bacillus subtilis, exists as a dimer that previously appeared to undergo surprisingly small structural changes after light illumination compared with other light-sensing proteins. However, we now report that light induces significant structural perturbations in a series of YtvA-LOV domain derivatives in which the Ja helix has been truncated or replaced. Results from native gel analysis showed significant mobility changes in these derivatives after light illumination; YtvA-LOV without the Ja helix dimerized in the dark state but existed as a monomer in the light state. The absence of the Ja helix also affected the dark regeneration kinetics and the stability of the flavin mononucleotide (FMN) binding to its binding site. Our results demonstrate an alternative way of photo-induced signal propagation that leads to a bigger functional response through dimer/monomer conversions of the YtvA-LOV than the local disruption of Ja helix in the As-LOV domain.

Rational design and synthetic feasibility are critical factors for the photoaffinity labelling (PAL) approach, which can identify protein targets of bioactive small molecules under native cellular conditions. In this study, we developed photoaffinity labelling probes derived from 1,3,4‐oxadiazin‐5(6H)‐ones (OPALs) for LL‐2003, a previously reported potential anticancer agent against IGF‐1R and Src. Our photoaffinity labelling strategy enabled successful photo‐crosslinking of the probes (OPAL‐6 and OPAL‐8) with the target proteins in both mammalian cell lysates and live MCF7, A549, HepG2 and HeLa cells in situ. In vitro and in situ labelling demonstrated different patterns and expression levels of the proteome, and the strongest band for Src appeared in the A549 cell line. An in‐gel fluorescence scan combined with MS/MS analysis of the IGF‐1R overexpressed insect proteome labelled by OPAL‐6 and OPAL‐8 identified the binding location of the synthesized probes.

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