Skp and SurA are both periplasmic chaperones involved in the biogenesis of Escherichia coli β-barrel outer membrane proteins (OMPs). It is commonly assumed that SurA plays a major role whereas Skp is a minor factor. However, there is no molecular evidence for whether their roles are redundant. Here, by using different dilution methods, we obtained monodisperse and aggregated forms of OmpC and studied their interactions with Skp and SurA by single-molecule fluorescence resonance energy transfer and fluorescence correlation spectroscopy. We found that Skp can dissolve aggregated OmpC while SurA cannot convert aggregated OmpC into the monodisperse form and the conformations of OmpC recognized by the two chaperones as well as their stoichiometries of binding are different. Our study demonstrates the functional distinctions between Skp and SurA. In particular, the role of Skp is not redundant and is probably more significant under stress conditions.

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