Abstract
Phosphatidylinositol (PI) is an abundant phospholipid in the cytoplasmic membrane of mycobacteria and the precursor for more complex glycolipids, such as the PI mannosides (PIMs) and lipoarabinomannan (LAM). To investigate whether the large steady-state pools of PI and apolar PIMs are required for mycobacterial growth, we have generated a Mycobacterium smegmatis inositol auxotroph by disruption of the ino1 gene. The ino1 mutant displayed wild-type growth rates and steady-state levels of PI, PIM, and LAM when grown in the presence of 1 mM inositol. The non-dividing ino1 mutant was highly resistant to inositol starvation, reflecting the slow turnover of inositol lipids in this stage. In contrast, dilution of growing or stationary-phase ino1 mutant in inositol-free medium resulted in the rapid depletion of PI and apolar PIMs. Whereas depletion of these lipids was not associated with loss of viability, subsequent depletion of polar PIMs coincided with loss of major cell wall components and cell viability. Metabolic labeling experiments confirmed that the large pools of PI and apolar PIMs were used to sustain polar PIM and LAM biosynthesis during inositol limitation. They also showed that under non-limiting conditions, PI is catabolized via lyso-PI. These data suggest that large pools of PI and apolar PIMs are not essential for membrane integrity but are required to sustain polar PIM biosynthesis, which is essential for mycobacterial growth.
Highlights
Mycobacterium tuberculosis, the causative agent of tuberculosis, infects nearly one third of the world population and causes active disease in an estimated 16 million people worldwide [1]
Characterization of the M. smegmatis ino1 Mutant—A gene with 87% amino acid identity to the M. tuberculosis ino1 gene was isolated from M. smegmatis
A major function of the large cellular pool of PI is to provide a dynamic pool of precursors for polar PI mannosides (PIMs) and LM/LAM biosynthesis
Summary
To investigate whether the large steady-state pools of PI and apolar PIMs are required for mycobacterial growth, we have generated a Mycobacterium smegmatis inositol auxotroph by disruption of the ino gene. Whereas there is accumulating evidence that the PIMs and LM/LAM have potent immuno-modulatory activities that may be important for the pathogenesis of M. tuberculosis [7,8,9,10], the presence of structurally related PIMs and LAMs in saprophytic mycobacterial species suggests that these lipoglycans have a more fundamental role(s) in mycobacterial physiology This conclusion is supported by the finding that both phosphatidylinositol synthase and PimA, the first mannosyltransferase in the PIM/LM/LAM pathway (Fig. 1), are essential for growth and viability of the saprophytic mycobacteria species M. smegmatis [11, 12].
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