病原细菌受体激酶PcrK特异识别寄主植物激素分子信号的机制

How bacteria employ their cellular receptors to recognize and respond to environmental stimuli is the prerequisite for survival and proliferation. This molecular process is also the cutting edge of the area of bacterial cell signaling. Our previous investigations have revealed that in the two-component signal transduction system PcrK-PcrR of Xanthomonas campestris pv. campestris (Xcc), the N-terminal sensor of receptor histidine kinase PcrK directly and physically binds to WF323, a cytokinin-family molecule acting as a plant hormone. In addition, WF323 significantly suppresses the phosphorylation level of PcrK. These evidences demonstrated that WF323 is the ligand of receptor PcrK. This project aims to deciphering the biochemical and molecular mechanisms of how Xcc uses receptor PcrK to detect plant hormone and its consequence in cell regulation. We will focus on the following three research contents, including: Interaction between cytokinin-PcrK and its subtle binding process; cytokinin drived PcrK-PcrR protein phosphorylation and transmembrane cell signaling; and investigation on the structure of PcrK-PcrR regulon, with special reference on the degradation of the second messenger c-di-GMP via PcrR. The results will increase our knowledge about the molecular basis of cross-kingdom communication between pathogenic bacteria and their hosts, and will also provide experimental evidence to understand the co-evolutionary process based on ligand-receptor interactions.

细菌如何利用细胞表面受体及时、准确、特异地识别来自于环境和寄主的分子信号是维持其生存繁殖最重要的前提，也是细菌信号转导研究领域的前沿热点。前期研究证明，在野油菜黄单胞菌双组分信号转导系统PcrK-PcrR中，受体组氨酸激酶PcrK的N端信号识别区（sensor）具有特异性结合植物细胞分裂素类分子WF323的能力，且PcrK的磷酸化水平受到WF323的抑制，表明WF323是受体PcrK的小分子配体。本项目拟阐明PcrK识别寄主植物激素分子的精细过程与调控意义。将分别从细胞分裂素配体－受体组氨酸激酶的分子互作与精细位点分析、细胞分裂素控制的PcrK-PcrR蛋白质可逆磷酸化过程与信号跨膜传递，及PcrR降解细胞第二信使c-di-GMP并调控下游基因表达进行分子分析。研究结果将加深我们对生物跨界（cross-kingdom）信号转导过程和病原细菌－宿主植物协同进化中配体－受体相互作用的理解。

识别宿主植物是病原细菌成功侵染的前提条件。然而，细菌如何识别植物信号，尤其是参与调控植物发育的信号分子是未知的。细菌细胞主要利用组氨酸激酶（histidine kinase, HK）感受外界信号刺激，它与反应调节蛋白(response regulator, RR)共同构成了细菌双组分信号转导系统（two component signal transduction system, TCS）。TCS是细菌中最重要的感应环境因子并调控细胞生理生化反应和细菌行为的信号转导系统。. 研究发现十字花科植物黑腐病的病原细菌-野油菜黄单胞菌（Xanthomonas campestris pv. campestris）的组氨酸激酶PcrK能特异性识别植物细胞分裂素－异戊烯基腺嘌呤2iP（2-isopentenyladenine）。2iP直接结合PcrK的胞外信号感应结构域CHASE，抑制PcrK的激酶活性。PcrK通过4步磷酸化过程，控制反应调节蛋白PcrR的磷酸化水平。2iP感应所介导的信号传递过程激活了PcrR的磷酸二酯酶活性，控制细菌降解胞内第二信使分子c-di-GMP (3′,5′-cyclic diguanylic acid)，并上调ctrA基因的表达，增强细菌抗氧胁迫的能力。该研究首次发现细菌细胞编码植物激素受体。研究揭示了一个进化上保守的跨界信号转导调控通路，在这条通路上，病原细菌通过截获植物激素信号增强自身的抗氧胁迫能力。