Cell Contact–Dependent Outer Membrane Exchange in Myxobacteria: Genetic Determinants and Mechanism

Biofilms are dense microbial communities. Although widely distributed and medically important, how biofilm cells interact with one another is poorly understood. Recently, we described a novel process whereby myxobacterial biofilm cells exchange their outer membrane (OM) lipoproteins. For the first time we report here the identification of two host proteins, TraAB, required for transfer. These proteins are predicted to localize in the cell envelope; and TraA encodes a distant PA14 lectin-like domain, a cysteine-rich tandem repeat region, and a putative C-terminal protein sorting tag named MYXO-CTERM, while TraB encodes an OmpA-like domain. Importantly, TraAB are required in donors and recipients, suggesting bidirectional transfer. By use of a lipophilic fluorescent dye, we also discovered that OM lipids are exchanged. Similar to lipoproteins, dye transfer requires TraAB function, gliding motility and a structured biofilm. Importantly, OM exchange was found to regulate swarming and development behaviors, suggesting a new role in cell–cell communication. A working model proposes TraA is a cell surface receptor that mediates cell–cell adhesion for OM fusion, in which lipoproteins/lipids are transferred by lateral diffusion. We further hypothesize that cell contact–dependent exchange helps myxobacteria to coordinate their social behaviors.生物被膜（biofilm）是一类致密的微生物群落。尽管其分布广泛且具有重要医学价值，但学界对生物被膜内细胞间的相互作用机制仍知之甚少。近期本团队报道了一种全新的过程：黏细菌（myxobacteria）生物被膜内的细胞可交换其外膜（outer membrane, OM）脂蛋白。本次研究首次鉴定出两种参与该转运过程的蛋白TraAB。预测这两种蛋白定位于细胞被膜；其中TraA编码一个亲缘关系较远的PA14凝集素样结构域、一个富含半胱氨酸的串联重复区域，以及一个被命名为MYXO-CTERM的推定C端蛋白分选标签；而TraB则编码一个OmpA样结构域。重要的是，供体细胞与受体细胞均需表达TraAB才能完成交换过程，这提示该转运为双向过程。本研究还利用亲脂性荧光染料发现，外膜脂质同样可发生交换。与脂蛋白交换类似，染料标记的脂质转运同样依赖TraAB的功能、细菌的滑动运动能力以及结构完整的生物被膜。值得注意的是，外膜交换可调控细菌的群集运动与发育行为，这表明该过程在细胞间通讯中具有全新的功能。本研究提出的工作模型认为，TraA作为细胞表面受体介导细胞间黏附以促进外膜融合，脂蛋白与脂质可通过侧向扩散完成转运。本团队进一步推测，这种依赖细胞接触的交换过程有助于黏细菌协调其群体社会行为。