细菌的“第六种分泌系统”(T6SS) 是负责将一系列毒性效应物分子转位到细菌和真核猎物细胞中去的一种细胞器。这篇文章描述了来自PAAR-repeat超级家族的蛋白是怎样在VgrG噬菌体尾尖（在穿透猎物细胞中所涉及的一种蛋白复合物）上形成一个尖的锥形延伸的，同时也描述了它们何以在吸收效应物中也起一个作用。这些数据支持关于T6SS的一个新模型：变细的尾尖被多种效应物所装饰，它们被一个由收缩驱动的转位事件一起输送到目标细胞内。（生物谷Bioon.com）
生物谷推荐英文摘要：
Nature doi: 10.1038/nature12453
PAAR-repeat proteins sharpen and diversify the type VI secretion system spike
Mikhail M. Shneider, Sergey A. Buth,Brian T. Ho,Marek Basler, John J. Mekalanos & Petr G. Leiman
The bacterial type VI secretion system (T6SS) is a large multicomponent, dynamic macromolecular machine that has an important role in the ecology of many Gram-negative bacteria. T6SS is responsible for translocation of a wide range of toxic effector molecules, allowing predatory cells to kill both prokaryotic as well as eukaryotic prey cells1, 2, 3, 4, 5. The T6SS organelle is functionally analogous to contractile tails of bacteriophages and is thought to attack cells by initially penetrating them with a trimeric protein complex called the VgrG spike6, 7. Neither the exact protein composition of the T6SS organelle nor the mechanisms of effector selection and delivery are known. Here we report that proteins from the PAAR (proline-alanine-alanine-arginine) repeat superfamily form a sharp conical extension on the VgrG spike, which is further involved in attaching effector domains to the spike. The crystal structures of two PAAR-repeat proteins bound to VgrG-like partners show that these proteins sharpen the tip of the T6SS spike complex. We demonstrate that PAAR proteins are essential for T6SS-mediated secretion and target cell killing by Vibrio cholerae and Acinetobacter baylyi. Our results indicate a new model of the T6SS organelle in which the VgrG–PAAR spike complex is decorated with multiple effectors that are delivered simultaneously into target cells in a single contraction-driven translocation event.
