Molecular mechanism of CTXΦ phage uptake into Vibrio cholerae

Vibrio cholerae colonize the intestinal lumen and secrete cholera toxin, inducing a massive efflux of water and electrolytes, causing the diarrheal disease cholera. Non-pathogenic serogroups of V. cholerae are made pathogenic by infection with the filamentous cholera toxin bacteriophage Φ (CTXΦ). CTXΦ binding and uptake may occur via a similar mechanism to M13 phage infection of E. coli. By this model the minor coat protein of CTX, pIII, first binds to the toxin coregulated pilus (TCP) on the surface of V. cholerae followed by TolA in the periplasm of V. cholerae. To understand CTXΦ uptake we expressed and purified the N-terminal domain of CTXΦ pIII (N-pIII) and the C-terminal domain of TolA (TolA-C) and demonstrated an interaction between these protein domains using pull down assays. We solved the de novo crystal structure of N-pIII to 2.9 Å resolution and the structure of N-pIII in complex with TolA-C to 1.44 Å. CTXΦ N-pIII has a structure similar to its corresponding domain in M13 pIII proteins despite only 15% sequence identity, but surprisingly binds to TolA at a site distinct from that of M13 N-pIII. These crystal structures provide valuable insights for understanding the mechanism by which CTXΦ infects V. cholerae.