Many virulence factors secreted from pathogenic Gram-negative bacteria are autotransporter proteins. The final step of autotransporter secretion is passage across the outer membrane (OM), mediated by a cotranslated C-terminal porin domain. The native structure is formed only after this final secretion step, which requires neither ATP nor a proton gradient. Sequence analysis reveals that, despite size, sequence, and functional diversity among autotransporter passenger domains, >97% are predicted to form parallel Ì_å_-helices, indicating this structural topology may be important for secretion. We report the folding behavior of pertactin, an autotransporter passenger domain from Bordetella pertussis. The pertactin Ì_å_-helix folds reversibly in isolation, but folding is much slower than expected based on size and native-state topology. Surprisingly, pertactin is not prone to aggregation during folding, even though folding is extremely slow. Interestingly, equilibrium denaturation results in the formation of a partially folded structure, a stable core comprising the C-terminal half of the protein. Examination of the pertactin crystal structure does not reveal any obvious reason for the enhanced stability of the C-terminus. Crystallographic data of the partially folded state implies a native like structure for the C-terminus. Interestingly, the C-terminus forms a dimer with a non-native interface. This interface has a relatively low KD ~ 0.3 ÌâåµM. In vivo, slow folding would prevent premature folding of the passenger domain in the periplasm, before OM secretion. Moreover, the extra stability of the C-terminal rungs of the Ì_å_-helix might serve as a template for the formation of native protein during OM secretion; hence, vectorial folding of the Ì_å_-helix could contribute to the energy-independent translocation mechanism. We show here that the C-terminus is the first part of the pertactin passenger domain reaching the OM, and that the C-terminus can adopt a stable structure outside the cell, prior to the completion of OM secretion. Coupled with the sequence analysis, these results suggest a general mechanism for autotransporter secretion. The combination of this data, including the lack of pertactin aggregation, could lead to new insights into the formation and prevention of protein aggregation in vivo.