Crystal Structures of a CTXφ pIII Domain Unbound and in Complex with a Vibrio cholerae TolA Domain Reveal Novel Interaction Interfaces

• Published in: The Journal of biological chemistry 2012-10, Vol.287 (43), p.36258-36272
• Main Authors: Ford, Christopher G., Kolappan, Subramaniapillai, Phan, Hanh T.H., Waldor, Matthew K., Winther-Larsen, Hanne C., Craig, Lisa
• Format: Article
• Language: English
• Subjects: Bacterial Pathogenesis; Bacteriophage; Membrane Proteins; Microbiology; TolA; Type IV Pili; Vibrio cholerae; X-ray Crystallography
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M112.403386

Vibrio cholerae colonize the small intestine where they secrete cholera toxin, an ADP-ribosylating enzyme that is responsible for the voluminous diarrhea characteristic of cholera disease. The genes encoding cholera toxin are located on the genome of the filamentous bacteriophage, CTXφ, that integrates as a prophage into the V. cholerae chromosome. CTXφ infection of V. cholerae requires the toxin-coregulated pilus and the periplasmic protein TolA. This infection process parallels that of Escherichia coli infection by the Ff family of filamentous coliphage. Here we demonstrate a direct interaction between the N-terminal domain of the CTXφ minor coat protein pIII (pIII-N1) and the C-terminal domain of TolA (TolA-C) and present x-ray crystal structures of pIII-N1 alone and in complex with TolA-C. The structures of CTXφ pIII-N1 and V. cholerae TolA-C are similar to coliphage pIII-N1 and E. coli TolA-C, respectively, yet these proteins bind via a distinct interface that in E. coli TolA corresponds to a colicin binding site. Our data suggest that the TolA binding site on pIII-N1 of CTXφ is accessible in the native pIII protein. This contrasts with the Ff family phage, where the TolA binding site on pIII is blocked and requires a pilus-induced unfolding event to become exposed. We propose that CTXφ pIII accesses the periplasmic TolA through retraction of toxin-coregulated pilus, which brings the phage through the outer membrane pilus secretin channel. These data help to explain the process by which CTXφ converts a harmless marine microbe into a deadly human pathogen. Background: CTXφ infection of Vibrio cholerae confers toxigenicity. Results: We report crystal structures of CTXφ-pIII domain N1 alone and bound to V. cholerae TolA domain C. Conclusion: CTXφ and coliphage pIII use distinct mechanisms to bind to TolA. Significance: These structures contribute to our understanding of a critical step in the evolution of pandemic V. cholerae with implications for emerging V. cholerae pathogens.