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Infect. Immun., 05 1995, 1710-1717, Vol 63, No. 5
AL Sorensen, S Nagai, G Houen, P Andersen and AB Andersen
A novel immunogenic antigen, the 6-kDa early secretory antigenic target
(ESAT-6), from short-term culture filtrates of Mycobacterium tuberculosis
was purified by hydrophobic interaction chromatography and anion-exchange
chromatography by use of fast protein liquid chromatography. The antigen
focused at two different pIs of 4.0 and 4.5 during isoelectric focusing,
and each of these components separated into three spots ranging from 4 to 6
kDa during two-dimensional sodium dodecyl sulfate-polyacrylamide gel
electrophoresis. The apparent differences in molecular masses or pIs of
these isoforms were not due to posttranslational glycosylation. The
molecular weight of the purified native protein was determined by applying
gel filtration and nondenaturing polyacrylamide gel electrophoresis and
found to be 24 kDa. ESAT-6 is recognized by the murine monoclonal antibody
HYB 76-8, which was used to screen a recombinant lambda gt11 M.
tuberculosis DNA library. A phage expressing a gene product recognized by
HYB 76-8 was isolated, and a 1.7-kbp fragment of the mycobacterial DNA
insert was sequenced. The structural gene of ESAT-6 was identified as the
sequence encoding a polypeptide of 95 amino acids. The N terminus of the
deduced sequence could be aligned with the 10 amino-terminal amino acids
derived from sequence analyses of the native protein. N-terminal sequence
analysis showed that the purified antigen was essentially free from
contaminants, and the amino acid analysis of the antigen was in good
agreement with the DNA sequence-deduced amino acid composition. Thus, the
heterogeneities observed in the pI and molecular weight of the purified
antigen do not derive from contaminating proteins but are most likely due
to heterogeneity of the antigen itself. Native and recombinant ESAT-6 are
immunologically active in that both elicited a high release of gamma
interferon from T cells isolated from memory- immune mice challenged with
M. tuberculosis. Analyses of subcellular fractions of M. tuberculosis
showed the presence of ESAT-6 in cytosol- and cell wall-containing
fractions. Interspecies analyses showed the presence of ESAT-6 in filtrates
from M. tuberculosis complex species. Among filtrates from mycobacteria not
belonging to the M. tuberculosis complex, reactivity was observed in
Mycobacterium kansasii, Mycobacterium szulgai, and Mycobacterium marinum.
Copyright © 1995, American Society for Microbiology
Purification and characterization of a low-molecular-mass T-cell antigen secreted by Mycobacterium tuberculosis
Mycobacteria Department, Statens Seruminstitut, Copenhagen, Denmark.
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