Institute of Biology, Department of Genetics, Martin Luther University Halle-Wittenberg, D-06099 Halle (Saale), Germany.
Mol Plant Microbe Interact. 2018 Jan;31(1):61-74. doi: 10.1094/MPMI-06-17-0138-R. Epub 2017 Sep 12.
Pathogenicity of the gram-negative bacterium Xanthomonas campestris pv. vesicatoria depends on a type III secretion (T3S) system, which translocates effector proteins into plant cells. Effector proteins contain N-terminal T3S and translocation signals and interact with the T3S chaperone HpaB, which presumably escorts effectors to the secretion apparatus. The molecular mechanisms underlying the recognition of effectors by the T3S system are not yet understood. In the present study, we analyzed T3S and translocation signals in the type III effectors XopE2 and XopJ from X. campestris pv. vesicatoria. Both effectors contain minimal translocation signals, which are only recognized in the absence of HpaB. Additional N-terminal signals promote translocation of XopE2 and XopJ in the wild-type strain. The results of translocation and interaction studies revealed that the interaction of XopE2 and XopJ with HpaB and a predicted cytoplasmic substrate docking site of the T3S system is not sufficient for translocation. In agreement with this finding, we show that the presence of an artificial HpaB-binding site does not promote translocation of the noneffector XopA in the wild-type strain. Our data, therefore, suggest that the T3S chaperone HpaB not only acts as an escort protein but also controls the recognition of translocation signals.
革兰氏阴性细菌野油菜黄单胞菌 pv.vesicatoria 的致病性取决于一种 III 型分泌（T3S）系统，该系统将效应蛋白易位到植物细胞中。效应蛋白含有 N 端 T3S 和易位信号，并与 T3S 伴侣 HpaB 相互作用，HpaB 可能将效应蛋白引导到分泌装置。T3S 系统识别效应蛋白的分子机制尚不清楚。在本研究中，我们分析了野油菜黄单胞菌 pv.vesicatoria 的 III 型效应子 XopE2 和 XopJ 的 T3S 和易位信号。这两种效应蛋白都含有最小的易位信号，只有在没有 HpaB 的情况下才被识别。额外的 N 端信号促进了 XopE2 和 XopJ 在野生型菌株中的易位。易位和相互作用研究的结果表明，XopE2 和 XopJ 与 HpaB 的相互作用以及 T3S 系统的预测细胞质底物对接位点不足以促进易位。与这一发现一致，我们表明，人工 HpaB 结合位点的存在并不能促进野生型菌株中无效应子 XopA 的易位。因此，我们的数据表明，T3S 伴侣 HpaB 不仅作为一种伴侣蛋白起作用，而且还控制着对易位信号的识别。
