Molecular structure and regulation of phototropin kinase by blue light.

Phototropin (phot) is a blue light photoreceptor in plants that mediates phototropism, chloroplast movement, stomata opening and leaf expansion. The phot molecule has two photoreceptive domains, LOV 1 and 2, in the N-terminal half and the C-terminal half forms Ser/Thr kinase. Phot acts as a blue light-regulated protein kinase. Each LOV domain binds a FMN and undergoes a unique cyclic reaction upon blue light absorption that induces conformational changes in the protein moiety and leads to regulation of the kinase activity, in which LOV2 plays a predominant role in the switching and LOV1 acts to attenuate the light sensitivity. Phot kinase is classified into the AGC kinase group since the consensus amino acid residues and the motifs are well conserved except for the lack of the hydrophobic motif and the presence of additional amino acid sequence in the activation loop. Secondary structure prediction and 3D structure simulation show a alpha/beta fold of the phot kinase similar to that of the catalytic subunit of PKA. The additional sequence forms an extra helix and loops. Docking simulation of the LOV2 domain with phot kinase provided useful information regarding the molecular mechanism underlying the photoregulation of phot kinase.