SurA is the major chaperone of outer membrane proteins (OMPs) in the periplasm. The molecular mechanism when SurA performs its chaperoning function is still unclear. Here, a purification-after-crosslinking (PAC) procedure was combined with single-molecule fluorescence resonance energy transfer (smFRET) to probe the conformations of SurA and OmpC in their complex. We found that SurA in the free state rearranges itself based on the crystal structure, except that the P2 domain moves towards the core domain with two major positions, forming a clamp-like conformation to accommodate OmpC. The obvious rearrangement of the P2 domain of SurA helps SurA to hold OmpC. OmpC attaches to SurA randomly and has the propensity to be near the middle part of the crevice. The noncollapsed and disordered conformations of OMPs provided by the OMPs·SurA complex are important to the subsequent delivery and folding process.

SurA是周质中外膜蛋白（OMP）的主要伴侣。SurA履行其伴侣功能的分子机制仍不清楚。在此，将交联后纯化（PAC）程序与单分子荧光共振能量转移（smFRET）结合使用，以检测其复合物中SurA和OmpC的构象。我们发现，处于游离状态的SurA会根据晶体结构进行自我重排，只是P2结构域以两个主要位置移向核心结构域，形成了钳形构象以容纳OmpC。SurA P2结构域的明显重排有助于SurA保持OmpC。OmpC随机附着在SurA上，倾向于靠近缝隙的中部。