Trehalose-recycling ABC transporter LpqY-SugA-SugB-SugC is essential for virulence of Mycobacterium tuberculosis

Abstract

Mycobacterium tuberculosis (Mtb) is an exclusively human pathogen that proliferates within phagosomes of host phagocytes. Host lipids are believed to provide the major carbon and energy sources for Mtb, with only limited availability of carbohydrates. There is an apparent paradox because five putative carbohydrate uptake permeases are present in Mtb, but there are essentially no host carbohydrates inside phagosomes. Nevertheless, carbohydrate transporters have been implicated in Mtb pathogenesis, suggesting that acquisition of host sugars is important during some stages of infection. Here we show, however, that the LpqY-SugA-SugB-SugC ATP-binding cassette transporter is highly specific for uptake of the disaccharide trehalose, a sugar not present in mammals, thus refuting a role in nutrient acquisition from the host. Trehalose release is known to occur as a byproduct of the biosynthesis of the mycolic acid cell envelope by Mtb's antigen 85 complex. The antigen 85 complex constitutes a group of extracellular mycolyl transferases, which transfer the lipid moiety of the glycolipid trehalose monomycolate (TMM) to arabinogalactan or another molecule of TMM, yielding trehalose dimycolate. These reactions also lead to the concomitant extracellular release of the trehalose moiety of TMM. We found that the LpqY-SugA-SugB-SugC ATP-binding cassette transporter is a recycling system mediating the retrograde transport of released trehalose. Perturbations in trehalose recycling strongly impaired virulence of Mtb. This study reveals an unexpected accessory component involved in the formation of the mycolic acid cell envelope in mycobacteria and provides a previously unknown role for sugar transporters in bacterial pathogenesis. microbial pathogenesis | mycolic acid biosynthesis | cell wall formation | carbon metabolism T uberculosis, caused by the bacterium Mycobacterium tuberculosis (Mtb), remains a major threat to global health, claiming the life of two million individuals each year (1). Mtb is an obligate human pathogen predominantly growing intracellularly within phagosomes of host phagocytes, although other cell types and niches might also be occupied during different phases of infection. Notwithstanding, there is strong evidence that host lipids provide the main carbon and energy sources for Mtb during infection, with carbohydrates being largely inaccessible for the bacilli (2-5). Support for this, among further findings, comes from the observed up-regulation of lipid catabolism genes of Mtb during intracellular replication in macrophages (4) and from the joint essentiality of the two isocitrate lyase isoforms, icl1 and icl2, for growth of Mtb in mice (6). It has to be mentioned that the importance of some lipid catabolic pathways for in vivo carbon metabolism of Mtb may be somewhat overestimated, as attenuation of mutants might be caused by accumulation of toxic intermediates of incomplete metabolism rather than by blocked utilization of a substrate (7). Nevertheless, the published literature strongly suggests that Mtb ...

Genetics of Mycobacterial Trehalose Metabolism

“…Thus, it is unlikely to be involved in nutrient acquisition from the host. In contrast, it was demonstrated that this ABC transporter instead plays a role in recycling of trehalose that is extracellularly released from mycobacteria themselves as a by-product during cell wall biosynthesis (30). As outlined previously in this article, it has been established that trehalose functions as a carrier molecule in buildup of the mycolic acid cell wall layer (Fig.…”

“…Inhibition of this recycling function in M. tuberculosis ΔlpqY or ΔsugC mutants led to secretion of substantial amounts of free trehalose into the culture medium in vitro (30). The importance of the LpqY-SugABC transporter for virulence of M. tuberculosis is highlighted by the strong attenuation of the ΔlpqY or ΔsugC mutants during the acute infection phase in the mouse model (30). The detailed molecular mechanisms causing attenuation during infection in vivo have not been elucidated yet.…”

“…Recently, an ABC transporter was identified in M. tuberculosis that mediates the active transport of trehalose across the cytoplasm membrane (30). This transporter, which is highly conserved among mycobacteria, is composed of the two transmembrane proteins SugA and SugB, the ATP-hydrolyzing protein SugC, and the periplasmic solute-binding lipoprotein LpqY, which is tethered by a lipid anchor to the cytoplasm membrane.…”

“…The substrate specificity is so pronounced that this transporter can discriminate between trehalose [α-D-glucopyranosyl-(1→1)-α-D-glucopyranoside] and maltose [α-D-glucopyranosyl-(1→4)-α-Dglucopyranoside] and even between the α,α-trehalose and α,β-trehalose stereoisomers. According to its transport function, gene deletion mutants with inactivated lpqY or sugC genes in M. tuberculosis and M. smegmatis were unable to grow in vitro with trehalose as the sole carbon source, whereas growth on other substrates was not impaired (30). Since the mycomembrane is supposed to constitute a relatively selective permeability barrier, the presence of some sort of transport mechanism in this cell wall layer facilitating trehalose uptake must be postulated, e.g., maybe a pore-forming membrane channel such as a porin specific for low-molecular-weight hydrophilic molecules (Fig.…”

Mycobacterium tuberculosis Proteome Response to Antituberculosis Compounds Reveals Metabolic “Escape” Pathways That Prolong Bacterial Survival

“…M. tuberculosis is capable of bypassing glucose phosphorylation and utilizing trehalose as a sole source of carbon and energy (23). Recent work indicates that a functional loss of the LpqY-SugA-SugB-SugC transporter gradually causes carbon starvation in nutrient-limited microenvironments in vivo (24).…”