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9 Mycobacterial Porins, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815783/9781555814687_Chap09-1.gif /docserver/preview/fulltext/10.1128/9781555815783/9781555814687_Chap09-2.gifAbstract:
Mycobacteria devote a large part of the coding capacity of its genome to fatty acid biosynthesis. Porins of Neisseria meningitidis induce actin nucleation in the host cell, suggesting a role in cell actin reorganization, which might influence the invasive ability of the bacteria. The growth and nutritional requirements of mycobacteria have been intensely studied since the discovery of Mycobacterium tuberculosis. Nutrient uptake mechanisms obviously depend on the permeability barriers imposed by the cell envelope. Mycobacteria 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. The unique mycolic acid bilayer is an extremely efficient permeability barrier protecting the cell from toxic compounds and is generally thought to be the major determinant of the intrinsic resistance of mycobacteria to most common antibiotics, chemotherapeutic agents and chemical disinfectants. The susceptibility of both Mycobacterium bovis BCG and M. tuberculosis to zwitterionic β-lactam antibiotics was substantially enhanced by MspA, decreasing the minimal inhibitory concentration up to 16-fold. The accelerated growth of M. bovis BCG on expression of MspA identified slow nutrient uptake as one of the determinants of slow growth in mycobacteria.
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Transport processes across the mycobacterial cell envelope. A simplified schematic representation of the mycobacterial cell envelope is shown. Adapted from Niederweis, ( 2003 ). This representation is based on the model proposed by Minnikin ( 1982 ). According to this model, the inner leaflet of the asymmetric outer membrane (OM) is composed of mycolic acids (MA), which are covalently linked to the arabinogalactan (AG)-peptidoglycan (PG) copolymer. A variety of extractable lipids presumably form the outer leaflet of the outer membrane. Surface layers such as a capsule ( Daffé and Draper, 1998 ; Daffé and Etienne, 1999 ; Lemassu et al., 1996 ) were omitted from this figure for clarity.
Functions of porins of gram-negative bacteria in pathogenesis. See the text for references.
MspA, a general porin of M. smegmatis. (A) Side view of MspA integrated into a lipid bilayer. (B) Electrostatic potential of MspA in top view. The electrostatic potential is represented by the Gasteiger charges for the atoms in the surface of MspA. These figures are based on the crystal structure of MspA ( Faller et al., 2004 ). Panels A and B were created with the visualization software PyMol (DeLano Scientific LLC) and ViewerLight (Accelrys), respectively. (See color insert for the color version of this figure.)
Mycobacterial porins of the MspA family a
Kinetic parameters and permeability coefficients P of mycobacteria and E. coli for glucose