Photoactivatable Rac1: a photosensor tilt by time-resolved crystallography and a partially open switch I when GDP-bound

Time-resolved serial crystallography shows that within 350 microseconds, about one-fifth of LOV2 photosensors form the flavin-cysteine adduct and tilt on Rac1 while keeping GTP and dark-state switches up to 1.85 ms.

• Why it matters: It remains unclear how the LOV2 light signal is transmitted to a fused effector protein, limiting mechanistic understanding of light-controlled Rac1 activation.
• What they did: Photoactivatable Rac1 was imaged by time-resolved serial crystallography at room temperature, with light data referenced to interleaved dark frames because dark crystals include a variable GDP-bound sub-population.
• The result: In the GDP-bound sub-population, switch I is only partially open and is held by a Ca2+ ion bridging LOV2 and Rac1, a conformation that can persist in simulations without the lattice.