Isolation and partial characterization of major protein antigens in the culture fluid of Mycobacterium tuberculosis.

Five actively secreted proteins (MPT32, MPT45, MPT51, MPT53, and MPT63) and the MPT46 protein were purified to homogeneity from Mycobacterium tuberculosis culture fluid and compared with proteins previously purified by ourselves and other investigators. Antisera were obtained by immunization of rabbits with all of the newly isolated proteins identified to be immunogenic. Two-dimensional electrophoresis of culture fluids obtained each week for 2 to 10 weeks of culturing of M. tuberculosis revealed characteristic changes, permitting identification of two distinct groups of proteins being actively secreted from the mycobacterial cells or appearing later in the culture fluids as a result of the release of soluble proteins from the cytosol after lysis of bacteria. The N-terminal amino acid sequences of five MPTs were shown to be identical to those of proteins previously isolated by other investigators and given different designations, and five new sequences are given. These sequences and the use of the antisera may serve to identify these proteins with mycobacterial constituents isolated by other investigators. The previously identified but not isolated MPT45 protein was shown to correspond to the C component of the antigen 85 complex. The 27-kDa MPT51 protein was demonstrated to cross-react with the three components of the antigen 85 complex, and the N-terminal amino acid sequences of MPT51 and MPT59 showed 60% homology. This finding and the extensive cross-reactivity between the components of the antigen 85 complex may indicate that there is a family of closely related secreted proteins in mycobacteria.