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Structural basis for the activation of anthrax adenylyl cyclase exotoxin by calmodulin

Abstract

Oedema factor, a calmodulin-activated adenylyl cyclase, is important in the pathogenesis of anthrax. Here we report the X-ray structures of oedema factor with and without bound calmodulin. Oedema factor shares no significant structural homology with mammalian adenylyl cyclases or other proteins. In the active site, 3′-deoxy-ATP and a single metal ion are well positioned for catalysis with histidine 351 as the catalytic base. This mechanism differs from the mechanism of two-metal-ion catalysis proposed for mammalian adenylyl cyclases. Four discrete regions of oedema factor form a surface that recognizes an extended conformation of calmodulin, which is very different from the collapsed conformation observed in other structures of calmodulin bound to effector peptides. On calmodulin binding, an oedema factor helical domain of relative molecular mass 15,000 undergoes a 15 Å translation and a 30° rotation away from the oedema factor catalytic core, which stabilizes a disordered loop and leads to enzyme activation. These allosteric changes provide the first molecular details of how calmodulin modulates one of its targets.

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Figure 1: Secondary structures of EF.
Figure 2: Sequence alignment of EF and CyaA.
Figure 3: CPK representation of EF structures.
Figure 4: Switch movement results in catalytic activation.
Figure 5: The active site of EF and its comparison with mAC.
Figure 6: Calmodulin binding of EF.
Figure 7: Comparison of CaM conformations and of CaM/effector contacts.

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Acknowledgements

We thank Y. Mabuchi and A. Higbie for technical assistance; X. Yang for computer maintenance; B. Bernat for help with mass spectrometry; P. Gardner for DNA sequencing; J. Geiger, C. Harrison, G. Meinke, K. Moffat, J. Piccirilli, P. Rice, M. Rosner and C.-R. Wang for discussions; and the beamline personnel at NSLS beamline X25 and APS, especially those at BioCars. This work was supported by grants from the NIH NIGMS and American Heart Association to W.-J.T., by an NIH NIDA fellowship to C.L.D., by AHA fellowship to S.-Z.Y., and a grant from the NIH NIAMSD to Z.G. A.B. also acknowledges partial support from the American Cancer Society.

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Correspondence to Wei-Jen Tang.

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Drum, C., Yan, SZ., Bard, J. et al. Structural basis for the activation of anthrax adenylyl cyclase exotoxin by calmodulin. Nature 415, 396–402 (2002). https://doi.org/10.1038/415396a

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