Mycobacterium tuberculosis and Mycobacterium bovis, the causative agents of human and bovine tuberculosis, have been reported to express a range of surface and secreted glycoproteins, although only one of these has been subjected to detailed structural analysis. We describe the use of a genetic system, in conjunction with lectin binding, to characterize the points of attachment of carbohydrate moieties to the polypeptide backbone of a second mycobacterial glycoprotein, antigen MPB83 from M. bovis. Biochemical and structural analysis of the native MPB83 protein and derived peptides demonstrated the presence of 3 mannose units attached to two threonine residues. Mannose residues were joined by a (1 --> 3) linkage, in contrast to the (1 --> 2) linkage previously observed in antigen MPT32 from M. tuberculosis and the (1 --> 2) and (1 --> 6) linkages in other mycobacterial glycolipids and polysaccharides. The identification of glycosylated antigens within the M. tuberculosis complex raises the possibility that the carbohydrate moiety of these glycoproteins might be involved in pathogenesis, either by interaction with mannose receptors on host cells, or as targets or modulators of the cell-mediated immune response. Given such a possibility characterization of mycobacterial glycoproteins is a step toward understanding their functional role and elucidating the mechanisms of mycobacterial glycosylation.
