ABSTRACT
Arabidopsis thaliana phototropin1 (phot1) is a blue-light photoreceptor, i.e. a blue-light-activated Ser/Thr-protein kinase that mediates various light responses including phototropism. Phot1 functions in hypocotyl phototropism dependent on the light induction of ROOT PHOTOTROPISM2 (RPT2) proteins within a broad range of blue light intensities. It is not yet known however how RPT2 contributes to the photosensory adaptation of phot1 to high intensity blue light and the second positive phototropism. We here show that RPT2 suppresses the activity of phot1. Yeast two-hybrid analysis indicated RPT2 binding to the LOV1 (light, oxygen or voltage sensing 1) domain of phot1 required for its high photosensitivity. Our biochemical analyses revealed that RPT2 inhibits the autophosphorylation of phot1, suggesting that it suppresses the photosensitivity and/or kinase activity of phot1 through the inhibition of LOV1 function. We found for the first time that RPT2 proteins are degraded via a ubiquitin-proteasome pathway when phot1 is inactive and stabilized under blue-light conditions in a phot1-dependent manner. We propose that RPT2 is a molecular rheostat that maintains a moderate activation level of phot1 under any light intensity conditions.
