YscX was discovered as an essential part of the Yersinia type III secretion system about 20 years ago. It is required for substrate secretion and is exported itself. Despite this central role, its precise function and mode of action remain unknown. In order to address this knowledge gap, this present study refocused attention on YscX to build on the recent advances in the understanding of YscX function. Our experiments identified an N-terminal secretion domain in YscX promoting its secretion, with the first five codons constituting a minimal signal capable of promoting secretion of the signal less β-lactamase reporter. Replacing the extreme YscX N-terminus with known secretion signals of other Ysc-Yop substrates revealed that the YscX N-terminal segment contains non-redundant information needed for YscX function. Further, both in cis deletion of the YscX N-terminus in the virulence plasmid and ectopic expression of epitope-tagged YscX variants again lead to stable YscX production but not type III secretion of Yop effector proteins. Mislocalisation of the needle components, SctI and SctF, accompanied this general defect in Yops secretion. Hence, a coupling exists between YscX secretion permissiveness and the assembly of an operational secretion system.

中文翻译：

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假结核耶尔森菌的 III 型分泌依赖于真正的 N 端 YscX 分泌域

YscX 被发现是耶尔森氏菌的重要组成部分大约 20 年前的 III 型分泌系统。它是底物分泌所必需的，并自行输出。尽管有这个核心作用，但它的确切功能和作用方式仍然未知。为了解决这一知识差距，本研究将注意力重新集中在 YscX 上，以建立对 YscX 功能理解的最新进展。我们的实验确定了 YscX 中促进其分泌的 N 端分泌结构域，前五个密码子构成最小信号，能够促进信号较少的 β-内酰胺酶报告基因的分泌。用其他 Ysc-Yop 底物的已知分泌信号替换极端 YscX N 末端显示 YscX N 末端片段包含 YscX 功能所需的非冗余信息。此外，在顺式毒力质粒中 YscX N 末端的缺失和表位标记的 YscX 变体的异位表达再次导致稳定的 YscX 生产，但不会导致 Yop 效应蛋白的 III 型分泌。针成分 SctI 和 SctF 的错误定位伴随着 Yops 分泌的这种普遍缺陷。因此，YscX 分泌许可与可操作分泌系统的组装之间存在耦合。