Anti-microbials through protein structure

Led by Profs. Ted Baker and John Fraser

One of the most dramatic consequences of the genome revolution has been the completion of genomes of many of man's deadliest pathogens. Within these genomes lies a wealth of information about how the organism survives, what defences they present to the the immune system, and what makes them pathogenic to humans. The MWC has a large programme of research dedicated to understanding the mechanisms of pathogenesis of two of man's commonest bacterial pathogenes, Mycobacterium tuberculosis and Staphylococcus aureus. M. tuberculosis of course causes tuberculosis, a disease that is on the rise in many countries around the world as drug resistant strains evolve. S. aureus is the archetypal superbug, able to develop resistance to all known antibiotics.

Structural genomics of Mycobacterium Tuberculosis

Mycobacterium tuberculosis is one of man's oldest pathogens and is again becoming a major global problem as strains develop resistance to existing antibiotics. Prof. Baker's group is part of an international consortium who's goal is to determine the structure of every single Mtb protein, so that the genomic information can be directly related to the proteins and to function. This is a major undertaking and can only be achieved through high throughput cloning, expressing and crystallography. There are many genes in the Mtb genome that still have no know function so it is hoped that their structures will reveal much more about what they do than the traditional approach of simply comparing primary sequences.

Virulence and pathogenicity of Staphylococcus aureus

Produced by all strains of S. aureus, the 14 ssl genes are located in contiguous alignment within a pathogenicity island of the staphylococcal genome. The ssls are retained to provide resistance and virulence for the bacteria. The SSL proteins block critical elements of the innate immune response and the Fraser and Baker groups have undertaken a structure/function analysis that has revealing some startling activities and important roles in survival.
SSL7 is the best understood SSL. It blocks the function of both IgA and complement C5 and the co-crystal structure of SSL7 (blue) bound to the IgA Fc receptor (orange) has been solved (Figure 1) (7).
In a more recent study (11) conducted with colleagues at the University of Aarhus, Denmark, the structure of SSL7 bound to the complement component C5 was resolved. This immediately suggests and evidence supports, that SSL7 forms a large pentameric complex with IgA and complement C5 in order to block access by C5 convertase and to inhibit formation of the membrane attack complex (MAC).

The SSL reactivity toward molecules of the innate and adaptive immune system suggest that this simple structural domain has served as an important protein scaffold for evolution of surfaces that bind a range of different cell surface receptors regulating immunity and the inflammatory response. SSL11 is another in the staphylococcal arsenal which is a potent inhibitor of neutrophil binding to endothelial cells. It targets cell surface glycoproteins which express the blood group antigen Sialyl Lewis X.

The crystal structure of SSL11 bound by sLeX is shown in the figure to the left (see Chung et al 2007).

References

Proft T, Fraser J. 2008 The bacterial superantigen and superantigen-like proteins. Immunol Rev. 225:226-43.
Baker, H.M., Basu, I., Chung, M.C., Caradoc-Davies, T., Fraser, J.D., and Baker, E.N. (2007) Crystal structures of a staphylococcal toxin in complex with sialyl-Lewis X reveal a conserved binding site that shares common features with viral sialic acid-binding proteins Journal Of Biological Chemistry 374: 1298-1308 Read
Chung, M.C., Wines, B.D., Baker, H., Langley, R.J., Baker, E.N., and Fraser, J.D. (2007) The crystal structure of Staphylococcal Superantigen-Like Toxin 11 (SSL11) in complex with Sialyl Lewis X reveals the mechanism for cell binding and immune inhibition. Molecular Microbiology 66:6 1342-1354. Read
Ramsland, P., Willoughby, N., Trist, H., Farrugia, W., Hogarth, P., Fraser, J., and Wines, B. (2007). Structural basis for evasion of IgA immunity by Staphylococcus aureus revealed in the complex of SSL7 with Fc of human IgA1. Proc. Natl. Acad. Sci. (USA) 104, 15051-15056. Read
Fraser, J., and Proft, T. (2007) The Streptococcal Superantigens. In Superantigens: Molecular Basis for Their Role in Human Diseases. Vol. 1. Kotb, M. and Fraser, J. (eds). New York: ASM Press. ISBN 978-1-55581-3.
Fraser, J.D., and Kotb, M (2007) Superantigens: Molecular Basis and Role in Human Disease. New York: ASM Press. ISBN 978-1-55581-3
Langley R, Wines B, Willoughby N, Basu I, Proft T, Fraser JD (2005) The staphylococcal superantigen-like protein 7 (SSL7) binds IgA and complement C5 and inhibits IgA-FcaRI binding and serum killing of bacteria.. JOURNAL OF IMMUNOLOGY. 174(5) : 2926-2933 Read
Wines BD, Willoughby N, Fraser JD, Hogarth PM (2006) A competitive mechanism for Staphylococcal toxin SSL7 inhibiting the leukocyte IgA receptor, Fcalpha RI, is revealed by SSL7 binding at the Calpha 2/Calpha 3 interface of IgA.. J BIOL CHEM.. 281(3) : 1389-1393 Read
Wines, B. D., N. Willoughby, J. D. Fraser, and P. M. Hogarth. 2006. A competitive mechanism for staphylococcal toxin SSL7 inhibiting the leukocyte IgA receptor, Fc alphaRI, is revealed by SSL7 binding at the C alpha2/C alpha3 interface of IgA. J Biol Chem 281:1389-1393. Read
Ramsland, P. A., N. Willoughby, H. M. Trist, W. Farrugia, P. M. Hogarth, J. D. Fraser, and B. D. Wines. 2007. Structural basis for evasion of IgA immunity by Staphylococcus aureus revealed in the complex of SSL7 with Fc of human IgA1. Proc. Natl. Acad. Sci. USA 104:15051-15056. Read
Laursen, N.S., N. Gordon, S. Hermans, N. Lorenz, N. Jackson, B. Wines, E. Spillner, J.B. Christensen, M. Jensen, F. Fredslund, M. Bjerre, L. Sottrup-Jensen, J.D. Fraser, and G.R. Andersen. 2010. Structural basis for inhibition of complement C5 by the SSL7 protein from Staphylococcus aureus. Proc Natl. Acad. Sci. (USA)107: 3681-3686. Read