The Arabidopsis (Arabidopsis thaliana) blue light photoreceptor phototropin1 (phot1) is a blue light–activated Ser/Thr protein kinase that mediates various light responses, including phototropism. The function of phot1 in hypocotyl phototropism is 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 phototropic responses under bright light conditions. We show that RPT2 suppresses the activity of phot1 and demonstrate that RPT2 binds to the PHOT1 light, oxygen or voltage sensing1 (LOV1) domain that is required for its high photosensitivity. Our biochemical analyses revealed that RPT2 inhibits autophosphorylation of phot1, suggesting that it suppresses the photosensitivity and/or kinase activity of phot1 through the inhibition of LOV1 function. We found that RPT2 proteins are degraded via a ubiquitin-proteasome pathway when phot1 is inactive and are stabilized under blue light 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.

拟南芥（Arabidopsis thaliana）蓝光感光体phototropin1（phot1）是一种蓝光激活的Ser / Thr蛋白激酶，可介导各种光响应，包括趋光性。phot1在下胚轴向光性中的功能取决于在广泛的蓝光强度范围内ROOT PHOTOTROPISM2（RPT2）蛋白的光诱导。但是，还不知道RPT2如何促进phot1对高强度蓝光的光感适应以及在明亮光条件下的光致响应。我们显示RPT2抑制phot1的活性，并证明RPT2结合到PHOT1的光，氧或电压传感1（LOV1）域，这是其高光敏性所必需的。我们的生化分析表明，RPT2抑制phot1的自磷酸化，提示它通过抑制LOV1功能来抑制phot1的光敏性和/或激酶活性。我们发现，当phot1处于非活动状态时，RPT2蛋白会通过泛素-蛋白酶体途径降解，并在phot1依赖的方式下在蓝光下稳定下来。我们建议RPT2是一种分子变阻器，在任何光强条件下都保持phot1的中等活化水平。