Bacterial autotransporters are a large family of proteins that play key roles in the pathogenesis of many infectious diseases. The virulence functions of autotransporters depend on their correct assembly on the cell surface. To achieve this a C-terminal β-barrel must be correctly folded and inserted into the outer membrane to facilitate translocation of the N-terminal passenger domain to the cell exterior. Once at the surface the passenger domains of most autotransporters are folded into an elongated β-helix. In a cellular context, key molecules catalyze the assembly of the autotransporter β-barrel domain. However, how the passenger domain folds into its functional form is poorly understood. Here, we report on a previously unknown function of the autotransporter b-barrel domain: its role as a folding vector to nucleate folding of the passenger domain. We use mutational analysis on the autotransporter Pet to show that the passenger domain uses the b-hairpin structure of the fifth extracellular loop of the b-barrel domain as a structural template to fold into a b-helix. Bioinformatics and structural analyses, and mutagenesis of a homologous autotransporter suggest that this process is conserved among autotransporter proteins with b-helical passenger domain.