Evidence for the nature of the link between the arabinogalactan and peptidoglycan of mycobacterial cell walls

The long-posed question of the nature of the link between the mycolylarabinogalactan and the underlying peptidoglycan of the cell walls of Mycobacterium sp. has been addressed. The insoluble cell wall matrix of Mycobacterium leprae, Mycobacterium tuberculosis, and Mycobacterium bovis was partially hydrolyzed with acid either before or after per-O-methylation and the resulting oligosaccharides further derivatized and analyzed by gas chromatography/mass spectrometry. The structures of fragments arising from the reducing end of arabinogalactan demonstrated the existence of the terminal sequence----5)-D-Galf-(1----4)-L-Rhap-(1---3)-D-GlcNAc. Other analyses confirmed the presence of muramyl-6-P within the peptidoglycan of these mycobacteria. Based on the acid lability of the 3-linked GlcNAc unit, the presence of about equimolar amounts of Rhap-(1----3)-D-GlcNAc and muramyl-6-P in an isolated cell wall fragment, and 31P NMR analysis, it was concluded that the GlcNAc residue of the terminal triglycosyl unit of arabinogalactan is joined by 1-O-phosphoryl linkage to the 6-position of some muramyl residues within the peptidoglycan. Thus, it is reasoned that the massive mycolylarabinogalactan of mycobacteria, responsible for aspects of disease pathogenesis and much of the antibody response in infections, is attached to the peptidoglycan framework by the actinomycete-specific diglycosylphosphoryl bridge, L-Rhap-(1----3)-D-GlcNAc-(1----P, perhaps thereby providing a unique target for site-directed chemotherapy of mycobacterial infections.