Function of Phosphatidylinositol in Mycobacteria

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.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). The distinctive cell wall of M. tuberculosis and other pathogenic mycobacteria (M. leprae, M. avium-M. intracellulare complex, and M. ulcerans) confers protection against a range of microbicidal processes and many classes of antibiotics and undoubtedly contributes to the success of these organisms as pathogens. The mycobacterial cell wall contains a number of unusual features, including the presence of a highly structured peptidoglycan-arabinogalactan (AG) 1 -mycolic acid macromolecule and a diverse array of glycolipids that form an asymmetric outer bilayer with the mycolic acids (2-4). Mycobacteria and other members of the Actinomycetales also differ from other eubacteria in synthesizing phosphatidylinositol (PI) and the biosynthetically related lipoglycans, PI mannosides (PIMs), lipomannan (LM), and lipoarabinomannan (LAM) (5-7). 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 (7-10), 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...

Mycobacterial outer membrane is a lipid bilayer and the inner membrane is unusually rich in diacyl phosphatidylinositol dimannosides

“…2B). Although Ac 2 PIM 2 was reported in the cell envelope of mycobacteria, it was not known to be the major lipid in its IM (8)(9)(10)(11). Even in this study, the CMW extract of intact cells contained a much lower amount of this lipid, almost four times lower (∼1.6% of dcm) than that obtained in the CMW extract of RMS-pretreated cells (∼5% of dcm), as seen in Figs.…”

Role of Phosphatidylinositol Mannosides in the Interaction between Mycobacteria and DC-SIGN

“…Although polar PIMs have been reported to be essential for cell wall integrity and mycobacterial growth (31), no differences in growth between the mutant and parent strains were observed (see Fig. S6 in the supplemental material), except for a higher tendency of the mutant strains to clump in older cultures (Ͼ1.5 weeks) at high optical densities at 600 nm (data not shown).…”

Variation in lipid and fatty acid uptake among nematode and cestode parasites and their host, domestic fowl: host–parasite interaction

“…PI is essential for more complex GLs and growth (Haites et al 2005). It is the main source of diacylglycerols that serve as signaling molecules in membranes (Gardocki et al 2005).…”