Bacteria use type III secretion injectisomes to inject effector proteins into eukaryotic target cells. Recruitment of effectors to the machinery and the resulting export hierarchy involve the sorting platform. These conserved proteins form pod structures at the cytosolic interface of the injectisome but are also mobile in the cytosol. Photoactivated localization microscopy in Yersinia enterocolitica revealed a direct interaction of the sorting platform proteins SctQ and SctL with effectors in the cytosol of live bacteria. These proteins form larger cytosolic protein complexes involving the ATPase SctN and the membrane connector SctK. The mobility and composition of these mobile pod structures are modulated in the presence of effectors and their chaperones, and upon initiation of secretion, which also increases the number of injectisomes from ~5 to ~18 per bacterium. Our quantitative data support an effector shuttling mechanism, in which sorting platform proteins bind to effectors in the cytosol and deliver the cargo to the export gate at the membrane-bound injectisome.

细菌使用 III 型分泌注射体将效应蛋白注射到真核靶细胞中。向机器招募效应器以及由此产生的输出层次结构涉及分拣平台。这些保守的蛋白质在注射体的胞质界面形成豆荚结构，但也在胞质中移动。小肠结肠炎耶尔森氏菌的光激活定位显微镜揭示了分选平台蛋白 SctQ 和 SctL 与活细菌细胞质中效应子的直接相互作用。这些蛋白质形成更大的胞质蛋白复合物，涉及 ATP 酶 SctN 和膜连接器 SctK。这些移动豆荚结构的移动性和组成在效应子及其伴侣存在的情况下以及在分泌开始时受到调节，这也将每个细菌的注射体数量从〜5个增加到〜18个。我们的定量数据支持效应器穿梭机制，其中分选平台蛋白与胞质溶胶中的效应器结合，并将货物递送至膜结合注射体的出口门。