Biosynthesis and Fatty Acid- and Cation-Binding Properties of Mycobacterium tuberculosis Methylglucose Lipopolysaccharides

Methylglucose lipopolysaccharides (MGLPs) are small cytoplasmic polysaccharides produced by mycobacteria and a few Nocardia species. Although their ability to form 1:1 complexes with long-chain fatty acids and acyl-CoAs in vitro has long suggested that they play a role in fatty acid and lipid metabolism, direct evidence for this assumption has been waiting for the availability of mutants with well-defined MGLP contents and fatty acyl-binding properties. Here, we report on the generation of eight mycobacterial mutants with various defects in the biosynthesis of MGLPs. The characterization of the MGLP content of these mutants provides the most comprehensive picture of the MGLP biosynthetic machinery to date. Importantly, the successful generation of three mutants in which MGLP synthesis is abolished indicates that, despite their conservation in mycobacteria, MGLPs are not essential for growth under standard laboratory conditions. Analysis of the fatty acyl-binding properties of truncated forms of MGLPs produced by some mutants provided unexpected new insight into the structural determinants governing the ability of MGLPs to form complexes with long acyl-CoAs calling for a revision of the structural model of these lipopolysaccharides. The characterization of the MGLP produced by the different mutants further unveiled previously unknown cation-binding properties associated with acylated forms of these polysaccharides. The availability of well-defined Mycobacterium tuberculosis MGLP mutants opens the way to a precise assessment of the role of these unique cytoplasmic lipopolysaccharides in the physiology and virulence of this vital bacterial pathogen.