Phototropin (phot) is a blue light (BL) receptor kinase involved in the BL responses of several species, ranging from green algae to higher plants. Phot converts BL signals from the environment into biochemical signals that trigger cellular responses. In phot, the LOV1 and LOV2 domains of the N-terminal region utilize BL for cyclic photoreactions and regulate C-terminal serine/threonine kinase (STK) activity. LOV2–STK peptides are the smallest functional unit of phot and are useful for understanding regulation mechanisms. The combined analysis of spectroscopy and STK activity assay in Arabidopsis phots suggests that the decay speed of the photo-intermediate S390 in LOV2 is one of the factors contributing to light sensitive kinase activity. LOV2 and STK are thought to be adjacent to each other in LOV2–STK with small angle scattering (SAXS). BL irradiation induces LOV2–STK elongation, resulting in LOV2 shifting away from STK. The N- and C-terminal lateral regions of LOV2, A′α-helix, Jα-helix, and A′α/Aβ gap are responsible for the propagation of the BL signal to STK via conformational changes. The comparison between LOV2–STK and full-length phot from Chlamydomonas suggests that LOV1 is directly adjacent to LOV2 in LOV2–STK; therefore, LOV1 may indirectly regulate STK. The molecular mechanism of phot is discussed.
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