Molecular mechanism of SurA’s chaperoning function to outer membrane proteins revealed by purification-after-crosslinking single-molecule FRET

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.