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Background:The plant photoreceptor "phototropin" is a light-regulated kinase containing two photosensory domains named LOV. Results: Light-induced conformational change related to the kinase activation was detected in full-length phototropin of Chlamydomonas. Conclusion: LOV1 may interact with LOV2 and modify the photosensitivity of the kinase regulation by LOV2. Significance: Configuration of LOV1, LOV2, and kinase domain in a phot molecule is first demonstrated.

Background: A plant photoreceptor "phototropin" is a light-dependent kinase containing the LOV photosensory domains. Results: Mutations in the N-terminal flanking region of LOV2 elevate kinase activity in darkness. Conclusion:The N-terminal flanking region is involved in intramolecular signaling from LOV2 to the kinase domain. Significance: This work provides insights into how the LOV domain can activate the kinase domain intramolecularly.

SummaryA blue-light photoreceptor in plants, phototropin, mediates phototropism, chloroplast relocation, stomatal opening, and leaf-flattening responses. Phototropin is divided into two functional moieties, the N-terminal photosensory and the C-terminal signaling moieties. Phototropin perceives light stimuli by the light, oxygen or voltage (LOV) domain in the N-terminus; the signal is then transduced intramolecularly to the C-terminal kinase domain. Two phototropins, phot1 and phot2, which have overlapping and distinct functions, exist in Arabidopsis thaliana. Phot1 mediates responses with higher sensitivity than phot2. Phot2 mediates specific responses, such as the chloroplast avoidance response and chloroplast dark positioning. To elucidate the molecular basis for the functional specificities of phot1 and phot2, we exchanged the N-and C-terminal moieties of phot1 and phot2, fused them to GFP and expressed them under the PHOT2 promoter in the phot1 phot2 mutant background. With respect to phototropism and other responses, the chimeric phototropin consisting of phot1 N-terminal and phot2 C-terminal moieties (P1n/2cG) was almost as sensitive as phot1; whereas the reverse combination (P2n/1cG) functioned with lower sensitivity. Hence, the N-terminal moiety mainly determined the sensitivity of the phototropins. Unexpectedly, both P1n/2cG and P2n/1cG mediated the chloroplast avoidance response, which is specific to phot2. Hence, chloroplast avoidance activity appeared to be suppressed specifically in the combination of N-and C-terminal moieties of phot1. Unlike the chloroplast avoidance response, chloroplast dark positioning was observed for P2G and P2n/1cG but not for P1G or P1n/2cG, suggesting that a specific structure in the N-terminal moiety of phot2 is required for this activity.