Immune responses are triggered when pattern recognition receptors recognize microbial molecular patterns. The Arabidopsis (Arabidopsis thaliana) receptor-like cytoplasmic kinase BOTRYTIS-INDUCED KINASE1 (BIK1) acts as a signaling hub of plant immunity. BIK1 homeostasis is maintained by a regulatory module in which CALCIUM-DEPENDENT PROTEIN KINASE28 (CPK28) regulates BIK1 turnover via the activities of two E3 ligases. Immune-induced alternative splicing of CPK28 attenuates CPK28 function. However, it remained unknown whether CPK28 is under proteasomal control. Here, we demonstrate that CPK28 undergoes ubiquitination and 26S proteasome-mediated degradation, which is enhanced by flagellin treatment. Two closely related ubiquitin ligases, ARABIDOPSIS TÓXICOS EN LEVADURA31 (ATL31) and ATL6, specifically interact with CPK28 at the plasma membrane; this association is enhanced by flagellin elicitation. ATL31/6 directly ubiquitinate CPK28, resulting in its proteasomal degradation. Furthermore, ATL31/6 promotes the stability of BIK1 by mediating CPK28 degradation. Consequently, ATL31/6 positively regulate BIK1-mediated immunity. Our findings reveal another mechanism for attenuating CPK28 function to maintain BIK1 homeostasis and enhance immune responses.

中文翻译：

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钙依赖性蛋白激酶 CPK28 被泛素连接酶 ATL31 和 ATL6 靶向，用于蛋白酶体介导的降解以微调拟南芥中的免疫信号传导

当模式识别受体识别微生物分子模式时，就会触发免疫反应。拟南芥 (Arabidopsis thaliana) 受体样细胞质激酶 BOTRYTIS-INDUCED KINASE1 (BIK1) 充当植物免疫的信号枢纽。BIK1 稳态由调节模块维持，其中钙依赖性蛋白激酶28 (CPK28) 通过两个 E3 连接酶的活性调节 BIK1 转换。免疫诱导的 CPK28 可变剪接减弱 CPK28 功能。然而，CPK28 是否受蛋白酶体控制仍然未知。在这里，我们证明 CPK28 经历泛素化和 26S 蛋白酶体介导的降解，鞭毛蛋白处理增强了这种降解。两种密切相关的泛素连接酶 ARABIDOPSIS TÓXICOS EN LEVADURA31 (ATL31) 和 ATL6 在质膜上与 CPK28 特异性相互作用；鞭毛蛋白诱导增强了这种关联。ATL31/6 直接泛素化 CPK28，导致其蛋白酶体降解。此外，ATL31/6 通过介导 CPK28 降解来促进 BIK1 的稳定性。因此，ATL31/6 正向调节 BIK1 介导的免疫。我们的研究结果揭示了另一种减弱 CPK28 功能以维持 BIK1 稳态和增强免疫反应的机制。