背景回顾：Immune responses are triggered when pattern recognition receptors (PRRs) 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. 

提出问题：However, it remained unknown whether CPK28 is under proteasomal control.

主要发现：Here, we demonstrate that CPK28 undergoes ubiquitination and 26S proteasome-mediated degradation, which is enhanced by flagellin treatment.

结果1-泛素连接酶ATL31/6与CPK28互作：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. 

结果2-ATL31/6介导CPK28的泛素化与降解：ATL31/6 directly ubiquitinate CPK28, resulting in its proteasomal degradation.

结果3-ATL31/6介导BIK1的稳定性：Furthermore, ATL31/6 promote the stability of BIK1 by mediating CPK28 degradation.

结果4-ATL31/6介导植物免疫：Consequently, ATL31/6 positively regulate BIK1-mediated immunity.

结论：Our findings reveal another mechanism for attenuating CPK28 function to maintain BIK1 homeostasis and enhance immune responses.

 摘 要 

模式识别受体PRRs识别到微生物分子模式后，会诱导免疫响应。拟南芥类受体胞质激酶BIK1作为植物免疫的信号中心发挥作用。BIK1内稳态是由一个调控模块所维持的，该模块中钙依赖性蛋白激酶CPK28通过两个E3连接酶的活性调控BIK1蛋白的周转。免疫诱导的CPK28基因可变剪切会减弱CPK28的功能。但是，对于CPK28是否受到泛素化的控制还不清楚。本文中，作者发现CPK28蛋白经历了泛素化和26S蛋白酶体介导的降解，该过程受到鞭毛蛋白处理的增强。两个近缘的泛素连接酶ATL31和ATL6会特异性地与CPK28蛋白在质膜上互作；该过程受到鞭毛蛋白诱导的增强。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的降解，促进BIK1的稳定性。最终，ATL31/6正向调控BIK1介导的免疫。本文的研究结果揭示了一种减弱CPK28功能，以维持BIK1内稳态，从而增强免疫响应的机制。