2.9-Å Resolution Structure of Anthrax Protective Antigen Pore Determined by Cryo Electron Microscopy

Abstract

Anthrax toxin, the major virulence factor of Bacillus anthracis causing high mortality in human and animals, comprises protective antigen (PA), lethal factor (LF) and edema factor (EF). PA plays a central role in the toxicity by translocating the enzymes LF and EF into the cytosol, which makes it not only a vaccine component and therapeutic target for anthrax infections but also an excellent model system for understanding the mechanism of protein translocation. PA first forms soluble oligomeric prepores on the host cell surface and then the endosomal acidification converts prepores into membrane-spanning pores that perform the protein translocation activity. Although atomic structures of PA prepores are available, how PA senses low pH, converts to active pore and translocates LF and EF are not well defined without a high-resolution structure of the PA pore. Here we report the structure of PA pore at 2.9-A resolution solved by cryo electron microscopy with direct electron counting. The structure reveals the details of the membrane-spanning translocation channel and the long-sought-after catalytic Φ-clamp. Supported by a large body of existing mutagenesis data and by comparisons with available PA prepore structures, our atomic model suggests mechanisms of the low-pH triggered conversion from prepore to pore and of proton-driven protein translocation through the PA pore.Acknowledgements: This work was supported in part by grants from the National Institutes of Health, the National Science Foundation, and the American Heart Association.