Trends in Microbiology
Volume 18, Issue 3, March 2010, Pages 109-116
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Review
Mycobacterial outer membranes: in search of proteins

https://doi.org/10.1016/j.tim.2009.12.005Get rights and content

The cell wall is a major virulence factor of Mycobacterium tuberculosis and contributes to its intrinsic drug resistance. Recently, cryo-electron microscopy showed that mycobacterial cell wall lipids form an unusual outer membrane. Identification of the components of the uptake and secretion machinery across this membrane will be crucial for understanding the physiology and pathogenicity of M. tuberculosis and for the development of better anti-tuberculosis drugs. Although the genome of M. tuberculosis appears to encode over 100 putative outer membrane proteins, only a few have been identified and characterized. Here, we summarize the current knowledge on the structure of the mycobacterial outer membrane and its known proteins. Through comparison to transport processes in Gram-negative bacteria, we highlight several hypothetical outer membrane proteins of M. tuberculosis that await discovery.

Section snippets

Mycobacteria have a complex cell envelope

Scientific interest in mycobacteria has been sparked by the medical importance of Mycobacterium tuberculosis and by the properties that distinguish mycobacteria from other microorganisms. In particular, mycobacteria possess a remarkably complex cell envelope consisting of a cytoplasmic membrane and a cell wall; these constitute an efficient permeability barrier that plays a crucial role in intrinsic drug resistance and in survival under harsh conditions [1].

Mycobacteria produce a fascinating

The mycobacterial outer membrane

In 1982, Minnikin proposed that mycobacteria have a second lipid bilayer formed by an inner leaflet of mycolic acids (covalently bound to the peptidoglycan) and an outer leaflet of free lipids [2]. This proposal was the basis for a variety of models that suggested an asymmetric outer membrane-like lipid layer of exceptional thickness (≥10 nm) 1, 13, 14. Although freeze-fracture experiments supported the existence of this second membrane [15], electron microscopy of ultrathin sections failed to

The porin pathway across mycobacterial outer membranes

Whereas hydrophobic compounds can penetrate membranes by temporarily dissolving in the lipid bilayer, direct diffusion of water-soluble compounds across any lipid bilayer is too slow to support bacterial growth. Thus, uptake of most if not all hydrophilic solutes across the mycobacterial outer membrane probably requires proteins. A strong argument in favor of this hypothesis is provided by the existence of porins such as MspA in mycobacteria. Porins are defined as non-specific protein channels

Structure of mycobacterial outer membrane proteins

MspA is the only mycobacterial outer membrane protein whose crystal structure has been solved [10]. The structure has proved to be of immense value not only as a paradigm for a new class of proteins, but also for understanding the function of MspA [50], for elucidating its membrane topology [12], and for applications in nanotechnology 51, 52, 53. The porin has an octameric goblet-like conformation with a single central channel 10 nm in length (Figure 3). This structure is different from that of

Energy-dependent uptake of nutrients across outer membranes

Despite its important role in the uptake of some hydrophilic nutrients, the porin pathway is not efficient enough for (i) solutes of very low abundance (below 1 μM), such as iron, because small concentration gradients result in very low diffusion rates; and (ii) large solutes such as vitamin B12 that exceed the size exclusion limit of most porin channels. Hence, uptake of these solutes across the outer membrane of Gram-negative bacteria requires active transport [56]. Substrates of

Uptake of hydrophobic compounds across outer membranes

Nikaido and co-workers have shown that diffusion rates through the water-filled channels of porins drop drastically with increasing solute hydrophobicity [35]. Both direct diffusion of anionic fatty acids through lipid membranes [65] and an alternative ‘flip–flop’ movement of protonated fatty acids through membranes [66] are slow. These findings explain why bacteria and eukaryotes have evolved proteins for fatty acid uptake across membranes [67]. For example, the outer membrane protein FadL

Efflux processes

M. tuberculosis is intrinsically resistant to many antibiotics due to the formidable permeability barrier established by the outer membrane, in synergy with other resistance mechanisms such as multi-drug efflux [76]. Considering that its genome encodes 69 putative drug efflux pumps [77], it is not surprising that all current tuberculosis drugs are substrates for efflux. In Gram-negative bacteria, only efflux across both membranes is an effective resistance mechanism [78], and we expect a

Other putative outer membrane proteins

In the previous sections, we have highlighted a few transport processes requiring outer membrane proteins. Yet, many other functions in Gram-negative bacteria are performed by proteins embedded in the outer membrane [34]. We propose that functionally equivalent proteins exist in mycobacteria.

For example, YaeT is required by E. coli to insert proteins correctly into the outer membrane 81, 82. Conditional depletion of the homologous Omp85 in Neisseria gonorrhoeae results in periplasmic

Concluding remarks and future directions

Outer membrane proteins of M. tuberculosis are intriguing for multiple reasons. First, considering that many nutrient molecules are hydrophilic and thus have inherently slow diffusion rates across lipid membranes, it is likely that proteins in the outer membrane are required for their uptake [100]. Hence, their identification is essential for understanding the physiology and pathogenicity of this microorganism. Second, outer membrane proteins of M. tuberculosis reside in an unusual lipid

Acknowledgements

We thank Dr. Cesar Sanchez for editorial improvements to the manuscript and the anonymous reviewers and the members of our labs for helpful suggestions. This work was supported by the Network of Excellence for 3D-Electron Microscopy of the 6th framework of the European Union and by grants AI063432 and AI083632 of the National Institutes of Health to M.N.

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