α1-antitrypsin binds to and interferes with functionality of EspB from atypical and typical enteropathogenic Escherichia coli strains

Enteropathogenic Escherichia coli (EPEC), including diffusely adhering atypical E. coli, strains use a type III secretion system to deliver effector proteins into the membrane and cytoplasm of infected cells. The E. coli secreted proteins A, B, and D (EspA, EspB, and EspD) are required for the formation of the characteristic attaching and effacing (A/E) lesions. EspB and EspD are thought to form a translocation pore in the host cell membrane through which effector proteins are injected into the host cytosol. Besides its function in pore formation, EspB has been found in the cytosol and implicated to function as an effector protein. We screened for putative host cell proteins interacting with EspB of atypical EPEC strains and identified alpha(1)-antitrypsin (AAT) as a binding partner for EspB. AAT binds to EspB in pull-down and overlay experiments and also to EspD in overlay experiments. In agreement with the role of EspB and EspD in pore formation, EPEC-mediated hemolysis of red blood cells is strongly reduced by AAT in a concentration-dependent manner, indicating that AAT interferes with type III secretion by inhibiting the formation of the translocation pore. This is further supported by a decreased actin polymerization after infection of HeLa or CaCo-2 cells with EPEC in the presence of physiologically relevant concentrations of AAT. In this study, we identify AAT as a new binding partner for EspB and EspD, suggesting a previously unappreciated role for AAT in host cell defense against EPEC infections and potentially also against other bacterial pathogens.