Wallmeroth Niklas, Anastasia Anna Katharina, Harter Klaus, Berendzen Kenneth Wayne, Mira-Rodado Virtudes
Center for Plant Molecular Biology (ZMBP), Department of Plant Physiology, University of Tübingen, Auf der Morgenstelle 32, D-72076, Tübingen, Germany.
Center for Plant Molecular Biology (ZMBP), Central Facilities, University of Tübingen, Auf der Morgenstelle 32, D-72076, Tübingen, Germany.
Protoplasma. 2017 Jan;254(1):597-601. doi: 10.1007/s00709-016-0944-4. Epub 2016 Jan 15.
Cytokinin signaling in Arabidopsis is carried out by a two-component system (TCS) multi-step phosphorelay mechanism that involves three different protein families: histidine kinases (AHKs), phosphotransfer proteins (AHPs), and response regulators (ARRs) that are in turn, subdivided into A-, B- and C-type ARRs depending on their function and structure. Upon cytokinin perception, AHK proteins autophosphorylate; this phosphate is then transferred from the AHKs to the AHPs to finally reach the ARRs. When B-type ARRs are activated by phosphorylation, they function as transcription factors that regulate the expression of cytokinin-dependent genes such as the A-type ARRs, among many others. In cytokinin signaling, while A- and B-type ARR function is well understood, it is still unclear if C-type ARRs (ARR22 and ARR24) play a role in this mechanism. Here, we describe a novel method suitable to study TCS activity natively as an in vivo system. We also show that ARR22 inhibits gene transcription of an A-type ARR upon cytokinin treatment in vivo. Consequently, we propose that ARR22, by acting as a phosphatase on specific AHPs, disrupts the TCS phosphorelay and prevents B-type ARR phosphorylation, and thus their activation as transcription factors, explaining the observed deactivation of cytokinin-responsive genes.
拟南芥中的细胞分裂素信号传导是通过一个双组分系统（TCS）多步磷酸化中继机制来实现的，该机制涉及三个不同的蛋白质家族：组氨酸激酶（AHKs）、磷酸转移蛋白（AHPs）和响应调节因子（ARRs），根据其功能和结构，ARRs又可细分为A、B和C型。在感知到细胞分裂素后，AHK蛋白会进行自身磷酸化；然后该磷酸基团从AHKs转移到AHPs，最终到达ARRs。当B型ARRs通过磷酸化被激活时，它们作为转录因子发挥作用，调节许多依赖细胞分裂素的基因的表达，如A型ARRs。在细胞分裂素信号传导中，虽然A型和B型ARR的功能已被充分了解，但C型ARRs（ARR22和ARR24）是否在该机制中发挥作用仍不清楚。在这里，我们描述了一种适合在体内系统中天然研究TCS活性的新方法。我们还表明，在体内细胞分裂素处理后，ARR22会抑制A型ARR的基因转录。因此，我们提出，ARR22通过对特定的AHPs起磷酸酶的作用，破坏了TCS磷酸化中继，并阻止了B型ARR的磷酸化，从而阻止了它们作为转录因子的激活，这就解释了观察到的细胞分裂素响应基因的失活现象。
