Receptor kinases (RKs) are fundamental for extracellular sensing and regulate development and stress responses across kingdoms. In plants, leucine-rich repeat receptor kinases (LRR-RKs) are primarily peptide receptors that regulate responses to myriad internal and external stimuli. Phosphorylation of LRR-RK cytoplasmic domains is among the earliest responses following ligand perception, and reciprocal transphosphorylation between a receptor and its coreceptor is thought to activate the receptor complex. Originally proposed based on characterization of the brassinosteroid receptor, the prevalence of complex activation via reciprocal transphosphorylation across the plant RK family has not been tested. Using the LRR-RK ELONGATION FACTOR TU RECEPTOR (EFR) as a model, we set out to understand the steps critical for activating RK complexes. While the EFR cytoplasmic domain is an active protein kinase in vitro and is phosphorylated in a ligand-dependent manner in vivo, catalytically deficient EFR variants are functional in antibacterial immunity. These results reveal a noncatalytic role for EFR in triggering immune signaling and indicate that reciprocal transphoshorylation is not a ubiquitous requirement for LRR-RK complex activation. Rather, our analysis of EFR along with a detailed survey of the literature suggests a distinction between LRR-RKs with RD- versus non-RD protein kinase domains. Based on newly identified phosphorylation sites that regulate the activation state of the EFR complex in vivo, we propose that LRR-RK complexes containing a non-RD protein kinase may be regulated by phosphorylation-dependent conformational changes of the ligand-binding receptor, which could initiate signaling either allosterically or through driving the dissociation of negative regulators of the complex.

受体激酶 (RK) 是细胞外传感和调节跨界发育和应激反应的基础。在植物中，富含亮氨酸的重复受体激酶 (LRR-RK) 主要是调节对无数内部和外部刺激的反应的肽受体。LRR-RK 细胞质结构域的磷酸化是配体感知后最早的反应之一，并且受体与其辅助受体之间的相互转磷酸化被认为可激活受体复合物。最初基于油菜素类固醇受体的表征提出，尚未测试通过植物 RK 家族的相互转磷酸化进行的复杂激活的普遍性。使用 LRR-RK 延伸因子 TU 受体 (EFR) 作为模型，我们着手了解激活 RK 复合物的关键步骤。虽然 EFR 细胞质结构域在体外是一种活性蛋白激酶，并且在体内以配体依赖性方式磷酸化，但催化缺陷型 EFR 变体在抗菌免疫中发挥作用。这些结果揭示了 EFR 在触发免疫信号传导中的非催化作用，并表明相互转磷酸化并不是 LRR-RK 复合物激活的普遍要求。相反，我们对 EFR 的分析以及对文献的详细调查表明，具有 RD- 与非 RD 蛋白激酶结构域的 LRR-RK 之间存在区别。基于新发现的调节体内 EFR 复合物活化状态的磷酸化位点，我们提出含有非 RD 蛋白激酶的 LRR-RK 复合物可能受配体结合受体的磷酸化依赖性构象变化的调节，