Figure 2.

Mechanistic models of outer membrane translocation by autotransporters. A. Hairpin model: In this model the C-terminal part of the passenger domain inserts itself as an unfolded polypeptide in the barrel of the TU, forming a hairpin. The extracellular folding of the autochaperone domain then pulls the remainder of the passenger domain. B. Threading model. This model is similar to the hairpin model but postulates that the N-terminal part of the passenger domain is inserted first, without hairpin. C. Multimeric model. In this model, multiple TUs are assembling in an oligomer forming a big central pore. Folded and unfolded polypeptides could then cross the outer membrane through this pore. D. Omp85 model. In this model, a number of periplasmic and outer membrane proteins organized around Omp85 are involved in the insertion of the TU and also the translocation across the outer membrane of the passenger domain.