Structural and functional studies of the secreted metalloprotease PrtV from Vibrio cholerae

Abstract: Cholera, an acute diarrheal diseases caused by the intestinal infection of the pathogenic bacterium Vibrio cholerae, continues to be a global killer in the world today. PrtV, a secreted zinc metalloprotease, is a potent cytotoxic virulence factor of V. cholerae. The 102 kDa full length multi-domain PrtV protein undergoes several N and C terminal modifications before being secreted as a 81 kDa pro-protein. The activation of the pro-protein is calcium dependent. The removal of calcium triggers auto-proteolysis to give a stable active protease with the catalytic zinc binding domain. The aim of the thesis was to study the structure and function of the PrtV protein. The results from paper I, identified the end product of the maturation of PrtV as the stable 37 kDa M6 active domain, and not a 55 kDa complex as reported earlier. Results also showed the this 37 kDa active M6 domain alone was sufficient for catalytic activity. A revised model for the maturation of PrtV was proposed. Individual domains were isolated from the PrtV protein by domain phasing methods. This included the N-terminal domain (residues 23-103), the PKD1 domain (residues 755-839), and a 25 kDa fragment (residues 589-839). The isolated domains were recombinantly over expressed as fusion proteins to increase expression and solubility. The PKD1 domain was purified to homogeneity and crystallized. The structure of the PKD1 domain reported in paper II, was solved by X-ray crystallography at an atomic resolution of 1.1 Å. From the structure, a previously unknown calcium binding site was identified at the N-terminal of the PKD1 domain. The structure also revealed two conformations for the PKD1 domain depending on free or bound calcium. From the structure, a function of the PKD1 domain as a protector of the cleavage site in the linker region between the M6 domain and the PKD1 domain in the presence of calcium was elucidated. A new model for the activation of PrtV was given. In paper III, the structure of the N-terminal domain solved by NMR spectroscopy was reported. The structure revealed two well defined helices but a third predicted helix was found to be unstructured.