The type VI secretion system (T6SS) is a widespread protein export apparatus found in Gram-negative bacteria. The majority of T6SSs deliver toxic effector proteins into competitor bacteria. Yet, the structure, function, and activation of many of these effectors remains poorly understood. Here, we present the structures of the T6SS effector RhsA from Pseudomonas protegens and its cognate T6SS spike protein, VgrG1, at 3.3 Å resolution. The structures reveal that the rearrangement hotspot (Rhs) repeats of RhsA assemble into a closed anticlockwise β-barrel spiral similar to that found in bacterial insecticidal Tc toxins and in metazoan teneurin proteins. We find that the C-terminal toxin domain of RhsA is autoproteolytically cleaved but remains inside the Rhs ‘cocoon’ where, with the exception of three ordered structural elements, most of the toxin is disordered. The N-terminal ‘plug’ domain is unique to T6SS Rhs proteins and resembles a champagne cork that seals the Rhs cocoon at one end while also mediating interactions with VgrG1. Interestingly, this domain is also autoproteolytically cleaved inside the cocoon but remains associated with it. We propose that mechanical force is required to remove the cleaved part of the plug, resulting in the release of the toxin domain as it is delivered into a susceptible bacterial cell by the T6SS.

中文翻译：

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VI型分泌系统输出的细菌Rhs效应子的结构

类型VI小号ecretion小号ystem（T6SS）是在革兰氏阴性细菌中发现的一种普遍的蛋白输出装置。大多数 T6SS 将有毒效应蛋白传递到竞争细菌中。然而，许多这些效应器的结构、功能和激活仍然知之甚少。在这里，我们以 3.3 Å 的分辨率展示了来自Pseudomonas protegens的 T6SS 效应子 RhsA及其同源 T6SS 刺突蛋白 VgrG1 的结构。该结构揭示，[R earrangement ^ h OT小号RhsA 的锅 (Rhs) 重复组装成闭合的逆时针 β 桶螺旋，类似于细菌杀虫 Tc 毒素和后生动物特纽蛋白中发现的螺旋。我们发现 RhsA 的 C 端毒素域被 autoproteolytically 切割，但仍保留在 Rhs '茧'内，除了三个有序的结构元素外，大部分毒素都是无序的。N 端“插头”结构域是 T6SS Rhs 蛋白独有的，类似于香槟软木塞，在一端密封 Rhs 茧，同时还介导与 VgrG1 的相互作用。有趣的是，该结构域也在茧内自蛋白水解切割，但仍与之相关。我们建议需要机械力来移除插头的裂开部分，