Free mycolic acid accumulation in the cell wall of the mce1 operon mutant strain of Mycobacterium tuberculosis

Abstract

The lipid-rich cell wall of Mycobacterium tuberculosis, the agent of tuberculosis, serves as an effective barrier against many chemotherapeutic agents and toxic host cell effector molecules, and it may contribute to the mechanism of persistence. Mycobacterium tuberculosis strains mutated in a 13-gene operon called mce1, which encodes a putative ABC lipid transporter, induce aberrant granulomatous response in mouse lungs. Because of the postulated role of the mce1 operon in lipid importation, we compared the cell wall lipid composition of wild type and mce1 operon mutant M. tuberculosis H37Rv strains. High resolution mass spectrometric analyses of the mce1 mutant lipid extracts showed unbound mycolic acids to accumulate in the cell wall. Quantitative analysis revealed a 10.7 fold greater amount of free mycolates in the mutant compared to that of the wild type strain. The free mycolates were comprised of alpha, methoxy and keto mycolates in the ratio 1:0.9:0.6, respectively. Since the mce1 operon is regulated in vivo, the free mycolates that accumulate during infection may serve as a barrier for M. tuberculosis against toxic products and contribute to the pathogen's persistence.

Architectures of Lipid Transport Systems for the Bacterial Outer Membrane

“…Strikingly, most triple mutant Δ mlaE Δ pqiA Δ yebS cells exhibited cell surface ruffling, suggestive of gross disorganization of the OM, whereas the IM and OM of wild-type cells were smooth with a well-defined periplasm in between (Figures 1D, S1I, and S1J). This is consistent with results from Streptomyces coelicolor in which mutations in the single MCE gene cluster led to global changes in the cell surface (Clark et al, 2013), as well as other work suggesting that MCE mutants have altered OM lipid composition (Cantrell et al, 2013; Malinverni and Silhavy, 2009). Thus, MCE proteins are important for maintaining the morphology of the OM and the uniform spacing between the IM and OM.…”

Bacterial immunostat: Mycobacterium tuberculosis lipids and their role in the host immune response

“…Recently, Cantrell et al 19 and Ojha et al 20 reported that M. tuberculosis cell wall contains unattached MA (free MA or FM) (Figure 1). One potential source of FM is the hydrolysis of TDM.…”

“…M. tuberculosis has four homologs of mce operon (mce1-4) encoding ATP-binding cassette transporters (ABC transporters) possibly involved in lipid import 7,37,38 . Cantrell et al 19 and Forrellad et al 39 reported that disruption of mce1 operon results in a M. tuberculosis mutant that accumulates several-fold more FM in its cell wall than wild type bacteria. Both groups suggested that the operon might encode an importer system involved in recycling of MA.…”

“…It has been long recognized that the thickness of the cell wall of M. tuberculosis cells changes during stationary phase of growth 92 . Cantrell et al 19 and Forrellad et al 39 independently reported increased abundance of FM in the cell wall of M. tuberculosis mce1 mutants, suggesting that the increased thickness of the cell wall may be attributed to such lipids. Such cells may become impermeable to the host's effector molecules, albeit they may also experience the stress of nutrient starvation.…”

Label-free Quantitative Proteomics Reveals a Role for the Mycobacterium tuberculosis SecA2 Pathway in Exporting Solute Binding Proteins and Mce Transporters to the Cell Wall*

“…Although there is one conflicting report, multiple studies show Mce1 (like SecA2) is important for growth in macrophages and during the early phase of murine infection (5-7, 67, 80 -83). The specific function of Mce1 during infection is not clear, but a potential role of Mce1 in importing free mycolic acids was recently proposed (57,84). Mce4 is the best studied Mce transporter, with a shown role in cholesterol import (85,86) and a role in virulence (85,87).…”