13 ESAT-6 and the Mycobacterial ESX Secretion Systems

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The search for important antigens among secreted proteins from present in culture filtrates led to the identification of ESAT-6 (early secretory antigenic target of 6 kDa), a small molecule that migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with an apparent molecular mass of 6 kDa. When administered as a subunit vaccine in mice, the mixture of culture filtrate proteins induced protection at the same level as the live vaccine BCG (BCG). This chapter talks about ESAT-6 and related proteins in the past few years since their discovery. The original ESAT-6 protein was purified from culture filtrates by ammonium sulfate precipitation, hydrophobic interaction chromatography, followed by anion exchange chromatography. CFP-10 is another important T-cell antigen present in culture filtrates and identified as a member of the ESX protein family. The active secretion of ESAT-6 and CFP-10 and the surface features of the CFP-10-ESAT-6 complex suggested a function based on specific binding to host proteins rather than a pore-forming role of the complex to mediate cell lysis activity. The ESAT-6 family proteins are also found to be important in the development of vaccines against tuberculosis. The infectious cycle of ranges from lifelong asymptomatic parasitism to the rapid development of tuberculous cavities in the lungs of patients that have become susceptible to develop the disease.

Citation: Rosenkrands I, Bottai D, Andersen P, Brosch R. 2008. 13 ESAT-6 and the Mycobacterial ESX Secretion Systems, p 211-221. In Daffé M, Reyrat J, Avenir G (ed), The Mycobacterial Cell Envelope. ASM Press, Washington, DC. doi: 10.1128/9781555815783.ch13
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Image of Figure 1.
Figure 1.

Genomic organization of the ESX operons in H37Rv, after Tekaia et al. ( ) and Gey van Pittius et al. ( ). The upper part provides information about the proteins encoded by the various genes and their corresponding partitions shown in abridged form. The figure is centered around the ESAT-6- and CFP-10-encoding genes and and is not to scale. Operon structures are not shown.

Citation: Rosenkrands I, Bottai D, Andersen P, Brosch R. 2008. 13 ESAT-6 and the Mycobacterial ESX Secretion Systems, p 211-221. In Daffé M, Reyrat J, Avenir G (ed), The Mycobacterial Cell Envelope. ASM Press, Washington, DC. doi: 10.1128/9781555815783.ch13
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Image of Figure 2.
Figure 2.

A ribbon representation of the backbone topology of the CFP-10·ESAT-6 complex based on the solution structure ( ) (A) and the model of the ESAT-6 CFP-10 complex ( ) (B), showing experimentally introduced mutations into ESAT-6 mutations and their effect in the SCID mouse model.

Citation: Rosenkrands I, Bottai D, Andersen P, Brosch R. 2008. 13 ESAT-6 and the Mycobacterial ESX Secretion Systems, p 211-221. In Daffé M, Reyrat J, Avenir G (ed), The Mycobacterial Cell Envelope. ASM Press, Washington, DC. doi: 10.1128/9781555815783.ch13
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Image of Figure 3.
Figure 3.

Working model for biogenesis and export of ESAT-6 proteins in H37Rv plus the positions of various genes and deletions ( ). The upper part presents a possible functional model indicating predicted subcellular localization and known or potential protein-protein interactions. Rosetta stone analysis indicates direct interaction between proteins Rv3870 and Rv3871; Rv3868 is an AAA-ATPase that is likely to act as a chaperone. ESAT-6 and CFP-10 are predicted to be exported through a transmembrane channel, consisting of at least Rv3870, Rv3871, and Rv3877 and possibly Rv3869 in a process catalyzed by ATP hydrolysis. The lower part shows key genes, the various proteins from the RD1 region, their sizes (number of amino acid residues), and the protein families.

Citation: Rosenkrands I, Bottai D, Andersen P, Brosch R. 2008. 13 ESAT-6 and the Mycobacterial ESX Secretion Systems, p 211-221. In Daffé M, Reyrat J, Avenir G (ed), The Mycobacterial Cell Envelope. ASM Press, Washington, DC. doi: 10.1128/9781555815783.ch13
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