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Crystal structure of a complex between anthrax toxin and its host cell receptor

Abstract

Anthrax toxin consists of the proteins protective antigen (PA), lethal factor (LF) and oedema factor (EF)1. The first step of toxin entry into host cells is the recognition by PA of a receptor on the surface of the target cell. Subsequent cleavage of receptor-bound PA enables EF and LF to bind and form a heptameric PA63 pre-pore, which triggers endocytosis. Upon acidification of the endosome, PA63 forms a pore that inserts into the membrane and translocates EF and LF into the cytosol2. Two closely related host cell receptors, TEM8 and CMG2, have been identified. Both bind to PA with high affinity and are capable of mediating toxicity3,4. Here, we report the crystal structure of the PA–CMG2 complex at 2.5 Å resolution. The structure reveals an extensive receptor–pathogen interaction surface mimicking the non-pathogenic recognition of the extracellular matrix by integrins5. The binding surface is closely conserved in the two receptors and across species, but is quite different in the integrin domains, explaining the specificity of the interaction. CMG2 engages two domains of PA, and modelling of the receptor-bound PA63 heptamer6,7,8 suggests that the receptor acts as a pH-sensitive brace to ensure accurate and timely membrane insertion. The structure provides new leads for the discovery of anthrax anti-toxins, and should aid the design of cancer therapeutics9.

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Figure 1: Structure of the PA–CMG2 complex.
Figure 2: The MIDAS motifs of the PA–CMG2 complex (a) and the collagen–integrin α2β1 complex5 (b).
Figure 3: Intermolecular contacts between PA domains II and IV and CMG2.
Figure 4: Key elements of the PA–CMG2 interaction a, Solvent-accessible surface of the PA domain IV groove, with key side chains from three CMG2 loops (β1–α1, blue; β2–β3, red; α2–α3, green) shown in ball-and-stick representation.
Figure 5: Hypothetical model of the receptor-bound, membrane-inserted PA pore.

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Acknowledgements

We thank the NIH and the DOD for financial support, and the DOE and staff at the SSRL for synchrotron access and support.

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Correspondence to Robert C. Liddington.

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The authors declare that they have no competing financial interests.

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Santelli, E., Bankston, L., Leppla, S. et al. Crystal structure of a complex between anthrax toxin and its host cell receptor. Nature 430, 905–908 (2004). https://doi.org/10.1038/nature02763

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