图1. PBL2^UMP诱导的ZAR1抗性小体的激活与装配
野油菜黄单胞菌的效应蛋白AvrAC以尿苷酰化修饰拟南芥PBL2激酶, 尿苷酰化的PBL2^UMP作为配体通过与RKS1互作而被ZAR1-RKS1复合物招募。RKS1的激活片段在与PBL2^UMP的2个尿苷基部分(球形)相互作用后变得稳定(橙色表面), 并与ZAR1^NBD结构域发生立体碰撞, 导致后者向外旋转, 从而释放ADP, 形成可与dATP/ATP结合的中间状态ZAR1-RKS1-PBL2^UMP复合体。该复合物结合dATP/ATP后, 诱导ZAR1结构重塑和折叠转换, 隐藏在非活性ZAR1-RKS1复合物中的ZAR1的N顶末端α-螺旋(α1, 红色)暴露在溶剂中, 导致ZAR1完全激活(激活态ZAR1-RKS1-PBL2^UMP), 继而通过多聚化形成五聚轮状结构的ZAR1抗性小体(紫色方框内突出显示形成的漏斗状结构)。CC、NBD、HD1、WHD和LRR为ZAR1的不同结构域。

Figure 1. PBL2^UMP-induced activation and assembly of the ZAR1 resistosome
Arabidopsis PBL2 is modified by uridylyl transferase AvrAC, which is an effector protein from Xanthomonas campestris. The uridylylated PBL2 (PBL2^UMP) as a ligand is then recruited by the ZAR1-RKS1 complex through interaction with the pseudokinase RKS1. The activation segment of RKS1 becomes stabilized (orange surface) after interacting with the two uridylyl moieties (in sphere) of PBL2^UMP, and sterically clashes with ZAR1^NBD, causing the latter to rotate outward and consequently release ADP, forming an intermediate ZAR1-RKS1-PBL2^UMP complex which allows it to bind dATP/ATP. Binding of dATP/ATP induces structural remodeling and fold switching of ZAR1. The very N-terminal helix (α1, red) of ZAR1 buried in the inactive ZAR1-RKS1 complex becomes solvent-exposed in the activated ZAR1-RKS1-PBL2^UMP complex, forming a ZAR1 resistosome pentameric structure through polymerization (a funnel-shaped structure highlighted within the purple frame). CC, NBD, HD1, WHD and LRR are different structural domains of ZAR1.