The C-terminal kinase fragment of Arabidopsis phototropin 2 triggers constitutive phototropin responses.

Phototropins mediate various blue-light responses such as phototropism, chloroplast relocation, stomatal opening and leaf flattening in plants. Phototropins are hydrophilic chromoproteins that are mainly bound to the plasma membrane. One of two phototropins in Arabidopsis thaliana, phot2, associates with the Golgi apparatus in a light-dependent manner. In this study, we analyzed the biological activities of the N-terminal photosensory and C-terminal kinase domains of phot2. For this purpose, these domains were fused to green fluorescent protein (GFP) and ectopically expressed in the wild-type and a phot1 phot2 double mutant of Arabidopsis. The kinase domain fused to GFP (P2CG) was localized to the plasma membrane and the Golgi apparatus, whereas the photosensory domain fused to GFP (P2NG) was uniformly localized in the cytosol. Hence, the kinase domain rather than the photosensory domain is responsible for the membrane association. Interestingly, the P2CG plants exhibited constitutive blue-light responses even in dark conditions, i.e. stomata were open and chloroplasts were in the avoidance position. By contrast, P2CG with a mutation that abolishes the kinase activity (P2C[D720/N]G) failed to exhibit these responses. phot2 kinase is therefore suggested to be correctly localized to functional sites in the cell and to trigger light signal transduction through its kinase activity. In contrast to P2CG, P2NG did not affect the phot2 responses, except for partial inhibition of the phototropic response caused by the endogenous phototropins.