A conserved stable core structure in the passenger domain beta-helix of autotransporter virulence proteins

In Gram-negative bacteria, a wide variety of virulence factors are secreted via the autotransporter (AT) pathway. Intriguingly, there is no significant concentration of ATP in the periplasm, nor a proton gradient across the OM, so the energetic origin of efficient secretion of AT proteins is unknown. More than 97% of AT proteins are predicted to contain right-handed parallel beta-helical structure, and the three crystal structures available for AT passenger domains each contain a long right-handed parallel beta-helix. Previous studies have shown that pertactin, an AT from Bordetella pertussis, exhibits three-state folding and has a C-terminal stable core structure. Here, we show that Pet, an unrelated AT from Escherichia coli, also exhibits three-state unfolding and also has a stable core structure. Deletion mutants, mass spectrometry, and N-terminal sequencing demonstrate that the Pet stable core is also located near the C-terminus of the passenger domain. Moreover, sequence analysis suggests that three-state folding and a C-terminal stable core structure could be important general features of the biogenesis of AT proteins in vivo.