Mycobacteria construct and extend a diderm envelope asymmetrically from the poles, with the old pole elongating more robustly than the new one. Among the evolutionarily distinct components of this envelope are the phosphatidylinositol-anchored lipoglycans, lipomannan (LM) and lipoarabinomannan (LAM). While LM and LAM modulate host immunity during infection, their physiological role in mycobacterial cell remains poorly understood. We recently found that LAM-deficient mutants, but not LM-deficient ones, were unable to maintain cell wall integrity, with envelope deforming primarily at the septa and new poles. On the other hand, mutants producing abnormally large LAM developed multiseptated cells, distinct from the defects seen in septal hydrolase mutants. These findings highlight the critical and specific roles LAM plays at subcellular sites associated with mycobacterial division, including maintaining cell envelope integrity and ensuring proper septal placement. Interestingly, M. smegmatis ΔmptA, lacking both LM and LAM (see figure), exhibited morphological defects when grown on tryptic soy or LB agar, but not on Middlebrook 7H10 agar. By leveraging this medium-dependent growth defect, we isolated suppressor transposon mutants that restored wildtype growth and morphology on tryptic soy agar. One such suppressor mutant contained a transposon insertion in the pstB gene, which is involved in phosphate uptake. We further demonstrated that adding phosphate to a permissive medium specifically impaired the growth of ΔmptA, but not the wild type, suggesting a toxic effect of phosphate on the mutant. This observation highlights the unique vulnerability of ΔmptA to phosphate, which is not present in the wildtype strain. Currently, we are investigating how phosphate metabolism influences LM/LAM biosynthesis in ΔmptA. Understanding the relationship between phosphate and these biosynthetic pathways may provide insights into the broader metabolic network affected in the mutant.

Biosynthesis of lipomannan (LM) and lipoarabinomannan (LAM) in mycobacteria.

Ian Sparks

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