Ubiquitination is a post-translational modification that regulates many processes in plants. Several ubiquitin E3 ligases act as either positive or negative regulators of immunity by promoting the degradation of different substrates. StPUB17 is an E3 ligase that has previously been shown to positively regulate immunity to bacteria, fungi and oomycetes, including the late blight pathogen Phytophthora infestans. Silencing of StPUB17 promotes pathogen colonization and attenuates Cf4/avr4 cell death. Using yeast-2-hybrid and co-immunoprecipitation we identified the putative K-homology (KH) RNA-binding protein (RBP), StKH17, as a candidate substrate for degradation by StPUB17. StKH17 acts as a negative regulator of immunity that promotes P. infestans infection and suppresses specific immune pathways. A KH RBP domain mutant of StKH17 (StKH17^GDDG) is no longer able to negatively regulate immunity, indicating that RNA binding is likely required for StKH17 function. As StPUB17 is a known target of the ubiquitin E3 ligase, StPOB1, we reveal an additional step in an E3 ligase regulatory cascade that controls plant defense.

泛素化是一种翻译后修饰，调节植物中的许多过程。几种泛素 E3 连接酶通过促进不同底物的降解而充当免疫的正向或负向调节剂。 StPUB17 是一种 E3 连接酶，此前已被证明可以积极调节对细菌、真菌和卵菌（包括晚疫病病原体致病疫霉）的免疫力。 StPUB17的沉默可促进病原体定植并减弱 Cf4/avr4 细胞死亡。使用酵母 2 杂交和免疫共沉淀，我们鉴定了假定的 K 同源 (KH) RNA 结合蛋白 (RBP) StKH17，作为 StPUB17 降解的候选底物。 StKH17 作为免疫的负调节因子，促进致病疫霉感染并抑制特定的免疫途径。 StKH17 的 KH RBP 结构域突变体 (StKH17 ^GDDG ) 不再能够负向调节免疫，表明 RNA 结合可能是 StKH17 功能所必需的。由于 StPUB17 是泛素 E3 连接酶 StPOB1 的已知靶点，因此我们揭示了控制植物防御的 E3 连接酶调控级联中的另一个步骤。