Center for Biomembrane Physics (MEMPHYS), Department of Physics, Chemistry and Pharmacy (FKF), University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark.
Soft Matter. 2016 Jun 21;12(23):5164-71. doi: 10.1039/c6sm00464d. Epub 2016 Apr 12.
The bacterial Shiga toxin is composed of an enzymatically active A-subunit, and a receptor-binding homopentameric B-subunit (STxB) that mediates intracellular toxin trafficking. Upon STxB-mediated binding to the glycolipid globotriaosylceramide (Gb3) at the plasma membrane of target cells, Shiga toxin is internalized by clathrin-dependent and independent endocytosis. The formation of tubular membrane invaginations is an essential step in the clathrin-independent STxB uptake process. However, the mechanism by which STxB induces these invaginations has remained unclear. Using a combination of all-atom molecular dynamics and Monte Carlo simulations we show that the molecular architecture of STxB enables the following sequence of events: the Gb3 binding sites on STxB are arranged such that tight avidity-based binding results in a small increment of local curvature. Membrane-mediated clustering of several toxin molecules then creates a tubular membrane invagination that drives toxin entry into the cell. This mechanism requires: (1) a precise molecular architecture of the STxB binding sites; (2) a fluid bilayer in order for the tubular invagination to form. Although, STxB binding to the membrane requires specific interactions with Gb3 lipids, our study points to a generic molecular design principle for clathrin-independent endocytosis of nanoparticles.
细菌志贺毒素由一个具有酶活性的 A 亚单位和一个介导细胞内毒素转运的受体结合五聚体 B 亚单位（STxB）组成。STxB 通过与靶细胞膜上的糖脂神经节苷脂Gb3 介导结合，志贺毒素通过网格蛋白依赖和非依赖的内吞作用被内化。管状膜内陷的形成是网格蛋白非依赖 STxB 摄取过程中的一个重要步骤。然而，STxB 诱导这些内陷的机制仍不清楚。我们使用全原子分子动力学和蒙特卡罗模拟的组合表明，STxB 的分子结构使以下事件序列成为可能：STxB 上的 Gb3 结合位点的排列方式使得基于紧密亲和力的结合导致局部曲率的微小增量。然后，几个毒素分子的膜介导聚集会产生管状膜内陷，从而驱动毒素进入细胞。这种机制需要：（1）STxB 结合位点的精确分子结构；（2）形成管状内陷所需的流体双层。尽管 STxB 与膜的结合需要与 Gb3 脂质的特定相互作用，但我们的研究指出了一种用于网格蛋白非依赖性纳米颗粒内吞作用的通用分子设计原则。
