State Key Laboratory of Plant Genomics, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, No.1 West Beichen Road, Chaoyang District, Beijing 100101, China; University of Chinese Academy of Sciences, No. 19 (A) Yuquan Road, Shijingshan District, Beijing 100049, China.
Biology Department, Queen's University, Kingston, ON K7L 3N6, Canada.
Mol Cell. 2018 Feb 1;69(3):493-504.e6. doi: 10.1016/j.molcel.2017.12.026. Epub 2018 Jan 18.
Plant pattern recognition receptors (PRRs) perceive microbial and endogenous molecular patterns to activate immune signaling. The cytoplasmic kinase BIK1 acts downstream of multiple PRRs as a rate-limiting component, whose phosphorylation and accumulation are central to immune signal propagation. Previous work identified the calcium-dependent protein kinase CPK28 and heterotrimeric G proteins as negative and positive regulators of BIK1 accumulation, respectively. However, mechanisms underlying this regulation remain unknown. Here we show that the plant U-box proteins PUB25 and PUB26 are homologous E3 ligases that mark BIK1 for degradation to negatively regulate immunity. We demonstrate that the heterotrimeric G proteins inhibit PUB25/26 activity to stabilize BIK1, whereas CPK28 specifically phosphorylates conserved residues in PUB25/26 to enhance their activity and promote BIK1 degradation. Interestingly, PUB25/26 specifically target non-activated BIK1, suggesting that activated BIK1 is maintained for immune signaling. Our findings reveal a multi-protein regulatory module that enables robust yet tightly regulated immune responses.
植物模式识别受体 (PRRs) 感知微生物和内源性分子模式，以激活免疫信号。细胞质激酶 BIK1 作为多个 PRRs 的下游作用元件，作为限速组件，其磷酸化和积累是免疫信号转导的核心。先前的工作确定钙依赖性蛋白激酶 CPK28 和异三聚体 G 蛋白分别是 BIK1 积累的负向和正向调节剂。然而，这种调节的机制尚不清楚。在这里，我们表明植物 U-box 蛋白 PUB25 和 PUB26 是同源的 E3 连接酶，它们将 BIK1 标记为降解，以负调控免疫。我们证明异三聚体 G 蛋白抑制 PUB25/26 活性以稳定 BIK1，而 CPK28 特异性地磷酸化 PUB25/26 中的保守残基以增强其活性并促进 BIK1 降解。有趣的是，PUB25/26 特异性地靶向非激活的 BIK1，这表明激活的 BIK1 被维持用于免疫信号。我们的研究结果揭示了一个多蛋白调节模块，该模块能够实现强大但又严格调控的免疫反应。
