Abstract
Phototropin (phot) is a blue-light photoreceptor for tropic responses, relocation of chloroplasts and stomata opening in plants. Phot has two chromophoric domains named LOV 1 and 2 in the N-terminus half, and a serine/threonine protein kinase domain in the C-terminus half. Phot has been shown to be autophosphorylated in response to blue light, however, no other substrate has been detected. Bacterially expressed Arabidopsis phot2 kinase domain (KD) with GST-tag (GST-KD) phoshorylated casein, a common in vitro substrate of ser/thr protein kinase. Using this in vitro analysis system, the roles of the each LOV domain were studied. GST-LOV2-KD phosphorylated casein in a light-dependent manner that was abolished by introduction of mutation blocking photochemical reaction of LOV2. GST-LOV1-LOV2-KD also showed light-activated protein kinase activity. Blocking the photochemical reaction of LOV1 had little effect on the kinase activity. In contrast, that of LOV2 brought significant decrease of the activity
