Mycobacterial porins – new channel proteins in unique outer membranes

Abstract

SummaryMycobacteria protect themselves with an outer lipid bilayer, which is the thickest biological membrane hitherto known and has an exceptionally low permeability rendering mycobacteria intrinsically resistant to many antibiotics. Pore proteins spanning the outer membrane mediate the diffusion of hydrophilic nutrients. Mycobacterium tuberculosis possesses at least two porins in addition to the low activity channel protein OmpATb. OmpATb is essential for adaptation of M. tuberculosis to low pH and survival in macrophages and mice. The channel activity of OmpATb is likely to play a major role in the defence of M. tuberculosis against acidification within the phagosome of macrophages. MspA is the main porin of Mycobacterium smegmatis . It forms a tetrameric complex with a single central pore of 10 nm length and a cone-like structure. This structure differs clearly from that of the trimeric porins of Gram-negative bacteria, which form one 4 nm long pore per monomer. The 45-fold lower number of porins compared to Gram-negative bacteria and the exceptional length of the pores are two major determinants of the low permeability of the outer membrane of M. smegmatis for hydrophilic solutes. The importance of the synergism between slow transport through the porins and drug efflux or inactivation for the development of drugs against M. tuberculosis is discussed.

Porins limit the intracellular persistence of Mycobacterium smegmatis

“…Whereas members of the M. tuberculosis complex are known to persist intracellularly within macrophages (Deretic & Fratti, 1999) and protozoa (Taylor et al, 2003), M. smegmatis has been shown to be killed by human monocytes and A. castellanii (Barker et al, 1996;Cirillo et al, 1997). The complement of porins is a further attribute that differs in slowly growing mycobacteria and rapidly growing mycobacteria such as M. smegmatis (Niederweis, 2003). To detect the potential impact of porins on survival in eukaryotic cells, we investigated the intracellular persistence of the wild-type SMR5 and two porin deletion mutant strains of M. smegmatis MN01 (DmspA) and ML10 (DmspADmspC) in different models.…”

Disclosure of the mycobacterial outer membrane: Cryo-electron tomography and vitreous sections reveal the lipid bilayer structure

“…M ycobacteria have evolved a complex cell wall, comprising a peptidoglycan-arabinogalactan polymer with covalently bound mycolic acids of considerable size (up to 90 carbon atoms), a variety of extractable lipids, and pore-forming proteins (1)(2)(3). The cell wall provides an extraordinarily efficient permeability barrier to noxious compounds and contributes to the high intrinsic resistance of mycobacteria to many drugs (4).…”

Characterization of the MSMEG_2631 Gene ( mmp ) Encoding a Multidrug and Toxic Compound Extrusion (MATE) Family Protein in Mycobacterium smegmatis and Exploration of Its Polyspecific Nature Using Biolog Phenotype MicroArray

“…Initially, intrinsic drug resistance was characterized as a passive mechanism associated with the absence of the drug target or lack of bacterial permeability (15,16), but recent studies have established that it can depend on drug efflux (46)(47)(48). Efflux pumps can contribute to acquired drug resistance due to overexpression (24,49) and to MDR (24,50,51).…”

“…For Mycobacterium intrinsic drug resistance, strong support exists for a major role for low cell wall permeability (15,16) and an array of efficient multidrug efflux pumps (17). LfrA, an MFS efflux pump that confers resistance to fluoroquinolones, EtBr, acridine, and quaternary ammonium compounds, was the first such pump characterized in mycobacteria (53).…”

“…These strategies may be an intrinsic feature of an organism or acquired via mutations or acquisition of exogenous genes (13,14). Low mycobacterial cell wall permeability due to its lipid-rich composition, limiting drug entry, is one reason for intrinsic resistance (15,16), but Mycobacterium also has several multidrug efflux pumps (17) that enhance drug resistance by drug extrusion; however, their roles in intrinsic resistance are not well explored.…”