Abstract
Phototropins (phots) are the autophosphorylating protein kinases that mediate phototropism, chloroplast movements, leaf flattening, and stomatal opening. However, the autophosphorylation sites of Arabidopsis phot have not been determined, and the physiological role of autophosphorylation remains unknown. Here, we identified eight phosphorylation sites in the N-terminus, Hinge1 region, kinase domain, and C-terminus in Arabidopsis phot1 by liquid chromatography-tandem mass spectrometry. We substituted the identified Ser and Thr with Ala in phot1 and analyzed their functions by inspecting the phot1-mediated responses after the transformation of the phot1 phot2 double mutant. We found that phosphorylation of Ser-851 in the kinase activation loop was required for all of the responses, whereas the phosphorylation of the other sites was not. The Ser-851 was rapidly autophosphorylated by blue light in fluence-dependent manners and dephosphorylated gradually upon darkness. These results demonstrate that autophosphorylation of the Ser-851 is a primary step for downstream signaling.
