Zhang Zhibin, Liu Yanan, Huang Hao, Gao Minghui, Wu Di, Kong Qing, Zhang Yuelin
Department of Botany, University of British Columbia, Vancouver, BC, Canada.
National Institute of Biological Sciences, Beijing, China.
EMBO Rep. 2017 Feb;18(2):292-302. doi: 10.15252/embr.201642704. Epub 2016 Dec 16.
MAP kinase signaling is an integral part of plant immunity. Disruption of the MEKK1-MKK1/2-MPK4 kinase cascade results in constitutive immune responses mediated by the NLR protein SUMM2, but the molecular mechanism is so far poorly characterized. Here, we report that SUMM2 monitors a substrate protein of MPK4, CALMODULIN-BINDING RECEPTOR-LIKE CYTOPLASMIC KINASE 3 (CRCK3). Similar to SUMM2, CRCK3 was isolated from a suppressor screen of mkk1 mkk2 and is required for the autoimmunity phenotypes in mekk1, mkk1 mkk2, and mpk4 mutants. In wild-type plants, CRCK3 is mostly phosphorylated. MPK4 interacts with CRCK3 and can phosphorylate CRCK3 in vitro In mpk4 mutant plants, phosphorylation of CRCK3 is substantially reduced, suggesting that MPK4 phosphorylates CRCK3 in vivo Further, CRCK3 associates with SUMM2 in planta, suggesting SUMM2 senses the disruption of the MEKK1-MKK1/2-MPK4 kinase cascade through CRCK3. Our study suggests that a MAP kinase substrate is used as a guardee or decoy for monitoring the integrity of MAP kinase signaling.
丝裂原活化蛋白激酶（MAP激酶）信号传导是植物免疫的一个组成部分。MEKK1-MKK1/2-MPK4激酶级联的破坏会导致由NLR蛋白SUMM2介导的组成型免疫反应，但迄今为止其分子机制仍不清楚。在这里，我们报告SUMM2监测MPK4的一个底物蛋白，即钙调素结合类受体细胞质激酶3（CRCK3）。与SUMM2类似，CRCK3是从mkk1 mkk2的抑制子筛选中分离出来的，并且是mekk1、mkk1 mkk2和mpk4突变体自身免疫表型所必需的。在野生型植物中，CRCK3大多处于磷酸化状态。MPK4与CRCK3相互作用，并能在体外磷酸化CRCK3。在mpk4突变体植物中，CRCK3的磷酸化显著降低，这表明MPK4在体内磷酸化CRCK3。此外，CRCK3在植物中与SUMM2结合，这表明SUMM2通过CRCK3感知MEKK1-MKK1/2-MPK4激酶级联的破坏。我们的研究表明，一种MAP激酶底物被用作被监测物或诱饵来监测MAP激酶信号传导的完整性。
