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Outer membrane proteins (OMPs) of gram-negative bacteria are synthesized in the cytosol and must cross the periplasm before insertion into the outer membrane. The 17-kDa protein (Skp) is a periplasmic chaperone that assists the folding and insertion of many OMPs, including OmpA, a model OMP with a membrane embedded beta-barrel domain and a periplasmic alphabeta domain. Structurally, Skp belongs to a family of cavity-containing chaperones that bind their substrates in the cavity, protecting them from aggregation. However, some substrates, such as OmpA, exceed the capacity of the chaperone cavity, posing a mechanistic challenge. Here, we provide direct NMR evidence that, while bound to Skp, the beta-barrel domain of OmpA is maintained in an unfolded state, whereas the periplasmic domain is folded in its native conformation. Complementary cross-linking and NMR relaxation experiments show that the OmpA beta-barrel is bound deep within the Skp cavity, whereas the folded periplasmic domain protrudes outside of the cavity where it tumbles independently from the rest of the complex. This domain-based chaperoning mechanism allows the transport of beta-barrels across the periplasm in an unfolded state, which may be important for efficient insertion into the outer membrane.

The cavity-chaperone Skp protects its substrate from aggregation but allows independent folding of substrate domains