Espinosa Avelina, Guo Ming, Tam Vincent C, Fu Zheng Qing, Alfano James R
Plant Science Initiative, University of Nebraska--Lincoln, NE 68588-0660, USA.
Mol Microbiol. 2003 Jul;49(2):377-87. doi: 10.1046/j.1365-2958.2003.03588.x.
The bacterial plant pathogen Pseudomonas syringae possesses a type III protein secretion system that delivers many virulence proteins into plant cells. A subset of these proteins (called Avr proteins) is recognized by the plant's innate immune system and triggers defences. One defence-associated response is the hypersensitive response (HR), a programmed cell death (PCD) of plant tissue. We have previously identified HopPtoD2 as a type III secreted protein from P. s. pv. tomato DC3000. Sequence analysis revealed that an N-terminal domain shared homology with AvrPphD and a C-terminal domain was similar to protein tyrosine phosphatases (PTPs). We demonstrated that purified HopPtoD2 possessed PTP activity and this activity required a conserved catalytic Cys residue (Cys(378)). Interestingly, HopPtoD2 was capable of suppressing the HR elicited by an avirulent P. syringae strain on Nicotiana benthamiana. HopPtoD2 derivatives that lacked Cys(378) no longer suppressed the HR indicating that HR suppression required PTP activity. A constitutively active MAPK kinase, called NtMEK2DD, is capable of eliciting an HR-like cell death when transiently expressed in tobacco. When NtMEK2DD and HopPtoD2 were co-delivered into plant cells, the HR was suppressed indicating that HopPtoD2 acts downstream of NtMEK2DD. DC3000 hopPtoD2 mutants were slightly reduced in their ability to multiply in planta and displayed an enhanced ability to elicit an HR. The identification of HopPtoD2 as a PTP and a PCD suppressor suggests that the inactivation of MAPK pathways is a virulence strategy utilized by bacterial plant pathogens.
细菌性植物病原菌丁香假单胞菌拥有一种III型蛋白分泌系统，该系统可将多种毒力蛋白输送到植物细胞中。这些蛋白中的一部分（称为Avr蛋白）被植物的先天免疫系统识别并触发防御反应。一种与防御相关的反应是超敏反应（HR），即植物组织的程序性细胞死亡（PCD）。我们之前已将HopPtoD2鉴定为来自番茄丁香假单胞菌致病型DC3000的III型分泌蛋白。序列分析表明，其N端结构域与AvrPphD具有同源性，C端结构域与蛋白酪氨酸磷酸酶（PTP）相似。我们证明，纯化的HopPtoD2具有PTP活性，且该活性需要一个保守的催化半胱氨酸残基（Cys(378)）。有趣的是，HopPtoD2能够抑制无毒丁香假单胞菌菌株在本氏烟草上引发的HR。缺乏Cys(378)的HopPtoD2衍生物不再抑制HR，这表明抑制HR需要PTP活性。一种组成型激活的促分裂原活化蛋白激酶（MAPK激酶），称为NtMEK2DD，当在烟草中瞬时表达时能够引发类似HR的细胞死亡。当NtMEK2DD和HopPtoD2共同导入植物细胞时，HR受到抑制，这表明HopPtoD2在NtMEK2DD的下游起作用。DC3000 hopPtoD2突变体在植物体内繁殖的能力略有降低，并表现出引发HR的能力增强。将HopPtoD2鉴定为一种PTP和PCD抑制剂表明，MAPK途径的失活是细菌性植物病原菌利用的一种毒力策略。
