Two‐state folding of the outer membrane protein X into a lipid bilayer membrane

Folding and insertion of β‐barrel membrane proteins into native membranes is efficiently catalyzed by β‐barrel assembly machineries. Understanding this catalysis requires a detailed description of the corresponding uncatalyzed folding mechanisms, which however have so far remained largely unclear. Here, we resolve folding and membrane insertion of the E. coli outer membrane protein X (OmpX) into 1,2‐didecanoyl‐sn‐glycero‐3‐phosphocholine (PC10:0) membranes at the atomic level. By combining four different experimental techniques, we correlate global folding kinetics with global and local hydrogen bond formation kinetics. Under a well‐defined reaction condition, these processes follow single‐exponential velocity laws, with rate constants identical within experimental error. The data thus establish at atomic resolution that OmpX folds and inserts into the lipid bilayer of PC10:0 liposomes by a two‐state mechanism.

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