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

• Published in: Nature (London) Vol. 501; no. 7468; pp. 547 - 550
• Main Authors: Busby, Jason N., Panjikar, Santosh, Landsberg, Michael J., Hurst, Mark R. H., Lott, J. Shaun
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 26.09.2013; Nature Publishing Group
• Subjects: 631/326/421; 631/535/1258; 631/535/1261; 631/535/1266; Amino Acid Motifs; Amino Acid Sequence; Bacterial infections; Bacterial Toxins - chemistry; Bacterial Toxins - metabolism; Chromatography; Consensus Sequence; Conserved Sequence; Crystallography; Crystallography, X-Ray; Cytotoxicity; Humanities and Social Sciences; Insecticides - chemistry; letter; Models, Molecular; Molecular Sequence Data; multidisciplinary; Physiological aspects; Polypeptides; Protein Subunits - chemistry; Protein Subunits - metabolism; Proteins; Proteolysis; Repetitive Sequences, Amino Acid; Science; Toxins; Translocation; Yersinia - chemistry
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature12465

The crystal structure of the complex formed by the B and C toxin complex proteins is reported, revealing how toxin complexes are processed and protected; the proteins assemble to form a large hollow structure that sequesters the cytotoxic portion of the C protein, and a β-propeller domain mediates attachment to the A protein in the native ABC complex. The BC of ABC toxins Shaun Lott and colleagues have determined the crystal structure of the complex formed by the B and C proteins of the ABC toxin from the insect pathogen Yersinia entomophaga . Toxin complexes have previously been visualized by single-particle electron microscopy, but no high-resolution structures for the component proteins have been available. The new data reveal a large hollow structure that encapsulates and sequesters the cytotoxic portion of the C protein and a β-propeller domain that mediates attachment to the A protein in the native ABC complex. This is the first demonstration of a mechanism for toxin delivery, complementing the recently published cryo-electron microscopy structure of an A component. The ABC toxin complexes produced by certain bacteria are of interest owing to their potent insecticidal activity 1 , 2 and potential role in human disease 3 . These complexes comprise at least three proteins (A, B and C), which must assemble to be fully toxic 4 . The carboxy-terminal region of the C protein is the main cytotoxic component 5 , 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 cells 5 . Toxin complexes have been visualized using single-particle electron microscopy 6 , 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) repeats 8 , 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 teneurins 9 . The structure provides the first clues about the function of these protein repeat families, and suggests a generic mechanism for protein encapsulation and delivery.