Trends in Microbiology
ReviewMycobacterial outer membranes: in search of proteins
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.
References (104)
Interpreting cell wall ‘virulence factors’ of Mycobacterium tuberculosis
Trends Microbiol.
(2001)The methyl-branched fortifications of Mycobacterium tuberculosis
Chem. Biol.
(2002)The identification and location of succinyl residues and the characterization of the interior arabinan region allows for a model of the complete primary structure of Mycobacterium tuberculosis mycolyl arabinogalactan
J. Biol. Chem.
(2008)Topology of the porin MspA in the outer membrane of Mycobacterium smegmatis
J. Biol. Chem.
(2006)The mycobacterial cell wall: structure, biosynthesis and sites of drug action
Curr. Opin. Chem. Biol.
(1997)Comparative cell wall core biosynthesis in the mycolated pathogens, Mycobacterium tuberculosis and Corynebacterium diphtheriae
FEMS Microbiol. Rev.
(2004)Mycolic acids: structure, biosynthesis and physiological functions
Prog. Lipid Res.
(1998)Temperature dependence of the Langmuir monolayer packing of mycolic acids from Mycobacterium tuberculosis
Biochim. Biophys. Acta
(2005)Conformational behavior of oxygenated mycobacterial mycolic acids from Mycobacterium bovis BCG
Biochim. Biophys. Acta
(2007)A novel mycolic acid cyclopropane synthetase is required for cording, persistence, and virulence of Mycobacterium tuberculosis
Mol. Cell
(2000)
Loss of a mycobacterial gene encoding a reductase leads to an altered cell wall containing beta-oxo-mycolic acid analogs and accumulation of ketones
Chem. Biol.
Cell wall structure of a mutant of Mycobacterium smegmatis defective in the biosynthesis of mycolic acids
J. Biol. Chem.
Compartmentalization of lipid biosynthesis in mycobacteria
J. Biol. Chem.
Identification of outer membrane proteins of Mycobacterium tuberculosis
Tuberculosis
The porin MspA from Mycobacterium smegmatis improves growth of Mycobacterium bovis BCG
Int. J. Med. Microbiol.
Rv1698 of Mycobacterium tuberculosis represents a new class of channel-forming outer membrane proteins
J. Biol. Chem.
Functions of the periplasmic loop of the porin MspA from Mycobacterium smegmatis
J. Biol. Chem.
Transmembrane beta-barrel proteins
Adv. Protein Chem.
Bacterial solutions to the iron-supply problem
Trends Biochem. Sci.
Transport of heptafluorostearate across model membranes. Membrane transport of long-chain fatty acid anions I
J. Lipid. Res.
Different mechanisms of free fatty acid flip–flop and dissociation revealed by temperature and molecular species dependence of transport across lipid vesicles
J. Biol. Chem.
The FadL family: unusual transporters for unusual substrates
Curr. Opin. Struct. Biol.
Preventing drug access to targets: cell surface permeability barriers and active efflux in bacteria
Semin. Cell. Dev. Biol.
Loss of outer membrane proteins without inhibition of lipid export in an Escherichia coli YaeT mutant
J. Biol. Chem.
GroEL1: a dedicated chaperone involved in mycolic acid biosynthesis during biofilm formation in mycobacteria
Cell
Wza: a new structural paradigm for outer membrane secretory proteins?
Trends Microbiol.
ESX/type VII secretion systems and their role in host–pathogen interaction
Curr. Opin. Microbiol.
Characterization of a novel cell wall-anchored protein with carboxylesterase activity required for virulence in Mycobacterium tuberculosis
J. Biol. Chem.
The envelope of mycobacteria
Annu. Rev. Biochem.
Lipids: complex lipids, their chemistry, biosynthesis and roles
The mycobacteria: an introduction to nomenclature and pathogenesis
Rev. Sci. Tech.
Physical organization of lipids in the cell wall of Mycobacterium chelonae
Mol. Microbiol.
Porins in the cell wall of mycobacteria
Science
Cloning of the mspA gene encoding a porin from Mycobacterium smegmatis
Mol. Microbiol.
The structure of a mycobacterial outer-membrane channel
Science
Mycobacterial porins – new channel proteins in unique outer membranes
Mol. Microbiol.
Structure of the cell envelope of corynebacteria: importance of the non-covalently bound lipids in the formation of the cell wall permeability barrier and fracture plane
Microbiology
The cell envelope structure and properties of Mycobacterium smegmatis mc(2)155: is there a clue for the unique transformability of the strain?
Microbiology
Reevaluation of envelope profiles and cytoplasmic ultrastructure of mycobacteria processed by conventional embedding and freeze-substitution protocols
J. Bacteriol.
Disclosure of the mycobacterial outer membrane: cryo-electron tomography and vitreous sections reveal the lipid bilayer structure
Proc. Natl. Acad. Sci. U. S. A.
Direct visualization of the outer membrane of native mycobacteria and corynebacteria
J. Bacteriol.
Cryo-transmission electron microscopy of frozen-hydrated sections of Escherichia coli and Pseudomonas aeruginosa
J. Bacteriol.
Construction, molecular modeling, and simulation of Mycobacterium tuberculosis cell walls
Biomacromolecules
Trans-cyclopropanation of mycolic acids on trehalose dimycolate suppresses Mycobacterium tuberculosis-induced inflammation and virulence
J. Clin. Invest.
A mutant of Mycobacterium smegmatis defective in the biosynthesis of mycolic acids accumulates meromycolates
Proc. Natl. Acad. Sci. U. S. A.
A polyketide synthase catalyzes the last condensation step of mycolic acid biosynthesis in mycobacteria and related organisms
Proc. Natl. Acad. Sci. U. S. A.
The key role of the mycolic acid content in the functionality of the cell wall permeability barrier in Corynebacterineae
Microbiology
Principles of macromolecular organization and cell function in bacteria and archaea
Cell Biochem. Biophys.
Proteomic analysis of the Escherichia coli outer membrane
Eur. J. Biochem.
Molecular basis of bacterial outer membrane permeability revisited
Microbiol. Mol. Biol. Rev.
Cited by (174)
Bacterial, Viral, and Prion Infectious Diseases of the Brain
2023, Magnetic Resonance Imaging Clinics of North AmericaSpatiotemporal localization of proteins in mycobacteria
2021, Cell ReportsChemical probes for tagging mycobacterial lipids
2021, Current Opinion in Chemical BiologyTeleological cooption of Mycobacterium tuberculosis PE/PPE proteins as porins: Role in molecular immigration and emigration
2021, International Journal of Medical MicrobiologyCitation Excerpt :The recently suggested delipidation strategy to selectively attenuate the BCG for mucosal delivery as a vaccine has been an interesting turning point to effectively synergize and boost its efficacy (Sheikh et al., 2020). The high content of lipids in the outer membrane increases the hydrophobicity and acts as a transport barrier for molecular uptake (Niederweis et al., 2010). The evolution of the restrictive membrane and the absence of specialized porins ensures the compensatory evolution of transport families for the uptake of nutrients and other molecules.
Features of the biochemistry of Mycobacterium smegmatis, as a possible model for Mycobacterium tuberculosis
2020, Journal of Infection and Public Health