Bacterial autotransporters are proteins that contain a small C-terminal ‘β domain’ that facilitates translocation of a large N-terminal ‘passenger domain’ across the outer membrane (OM) by an unknown mechanism. Here we used EspP, an autotransporter produced by Escherichia coli 0157:H7, as a model protein to gain insight into the transport reaction. Initially we found that the passenger domain of a truncated version of EspP (EspPΔ1-851) was translocated efficiently across the OM. Blue Native polyacrylamide gel electrophoresis, analytical ultracentrifugation and other biochemical methods showed that EspPΔ1-851 behaves as a compact monomer and strongly suggest that the channel formed by the β domain is too narrow to accommodate folded polypeptides. Surprisingly, we found that a folded protein domain fused to the N-terminus of EspPΔ1-851 was efficiently translocated across the OM. Further analysis revealed that the passenger domain of wild-type EspP also folds at least partially in the periplasm. To reconcile these data, we propose that the EspP β domain functions primarily to target and anchor the protein and that an external factor transports the passenger domain across the OM.