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The BC component of ABC toxins is an RHS-repeat-containing protein encapsulation device

Abstract

The ABC toxin complexes produced by certain bacteria are of interest owing to their potent insecticidal activity1,2 and potential role in human disease3. These complexes comprise at least three proteins (A, B and C), which must assemble to be fully toxic4. The carboxy-terminal region of the C protein is the main cytotoxic component5, and is poorly conserved between different toxin complexes. A general model of action has been proposed, in which the toxin complex binds to the cell surface via the A protein, is endocytosed, and subsequently forms a pH-triggered channel, allowing the translocation of C into the cytoplasm, where it can cause cytoskeletal disruption in both insect and mammalian cells5. Toxin complexes have been visualized using single-particle electron microscopy6,7, but no high-resolution structures of the components are available, and the role of the B protein in the mechanism of toxicity remains unknown. Here we report the three-dimensional structure of the complex formed between the B and C proteins, determined to 2.5 Å by X-ray crystallography. These proteins assemble to form an unprecedented, large hollow structure that encapsulates and sequesters the cytotoxic, C-terminal region of the C protein like the shell of an egg. The shell is decorated on one end by a β-propeller domain, which mediates attachment of the B–C heterodimer to the A protein in the native complex. The structure reveals how C auto-proteolyses when folded in complex with B. The C protein is the first example, to our knowledge, of a structure that contains rearrangement hotspot (RHS) repeats8, and illustrates a marked structural architecture that is probably conserved across both this widely distributed bacterial protein family and the related eukaryotic tyrosine-aspartate (YD)-repeat-containing protein family, which includes the teneurins9. The structure provides the first clues about the function of these protein repeat families, and suggests a generic mechanism for protein encapsulation and delivery.

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Figure 1: Structure of the B–CNTR complex.
Figure 2: Auto-proteolysis of the C protein.
Figure 3: RHS repeat structure.
Figure 4: Position of the B–CNTR complex in the complete Yen-Tc particle.

Accession codes

Accessions

Protein Data Bank

Data deposits

The atomic coordinates of the B–CNTR complex and the Chi2 chitinase have been deposited in the Protein Data Bank (http://www.pdb.org) under the accession codes 4IGL and 4DWS, respectively.

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Acknowledgements

We are grateful to M. Sullivan, R. Kingston, T. Baker and many other members of the Structural Biology section at the University of Auckland for discussions, to V. Arcus for initial enthusiasm, and to B. Hankamer for provision of laboratory space at the University of Queensland and for mentorship. This work was supported by the New Zealand Foundation for Research, Science and Technology contract C10X0804, awarded to M.R.H.H. We would like to thank all beamline staff at the MX1, MX2 and SAXS/WAXS beamlines at the Australian Synchrotron for their support, and to the New Zealand Synchrotron Group Ltd for synchrotron access arrangements.

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J.N.B. cloned constructs, expressed, purified and crystallized proteins, collected and processed X-ray crystallography and SAXS data, and refined and analysed the protein structure; S.P. determined the protein structure; M.J.L. processed, refined and analysed negative-stain electron microscopy data; M.R.H.H. and J.S.L. designed the study; J.N.B. and J.S.L. wrote the paper and all authors discussed the results and contributed to the manuscript.

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Correspondence to Mark R. H. Hurst or J. Shaun Lott.

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Busby, J., Panjikar, S., Landsberg, M. et al. The BC component of ABC toxins is an RHS-repeat-containing protein encapsulation device. Nature 501, 547–550 (2013). https://doi.org/10.1038/nature12465

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