Mycolate outer membrane

Abstract: Mycobacterial 6-kDa early secreted antigenic target (ESAT-6) system (ESX) exporters transport proteins across the cytoplasmic membrane. Many proteins transported by ESX systems are then translocated across the mycobacterial cell envelope and secreted from the cell. Although the mechanism underlying protein transport across the mycolate outer membrane remains elusive, the ESX systems are closely connected with and localize to the cell envelope. Links between ESX-associated proteins, cell wall synthesis, and the maintenance of cell envelope integrity have been reported. Genes encoding the ESX systems and those required for biosynthesis of the mycobacterial envelope are coregulated. Here, we review the interplay between ESX systems and the mycobacterial cell envelope.

Abstract: Corynebacterineae is a specific suborder of Gram-positive bacteria that includes Mycobacterium tuberculosis and Corynebacterium glutamicum. The ultrastructure of the cell envelope is very atypical. It is composed of a heteropolymer of peptidoglycan and arabinogalactan (AG) covalently associated to an outer membrane. Five arabinosyltransferases are involved in the biosynthesis of AG in C. glutamicum. AftB catalyzes the transfer of Araf (arabinofuranosyl) onto the arabinan domain of the arabinogalactan to form terminal β(1 → 2)-linked Araf residues. Here we show that ΔaftB cells lack half of the arabinogalactan mycoloylation sites but are still able to assemble an outer membrane. In addition, we show that a ΔaftB mutant grown on a rich medium has a perturbed cell envelope and sheds a significant amount of membrane fragments in the external culture medium. These fragments contain mono- and dimycolate of trehalose and PorA/H, the major porin of C. glutamicum, but lack conventional phospholipids that typify the plasma membrane, suggesting that they are derived from the atypical mycolate outer membrane of the cell envelope. This is the first report of outer membrane destabilization in the Corynebacterineae, and it suggests that a strong interaction between the mycolate outer membrane and the underlying polymer is essential for cell envelope integrity. The presence of outer membrane-derived fragments (OMFs) in the external medium of the ΔaftB mutant is also a very promising tool for outer membrane characterization. Indeed, fingerprint analysis of major OMF-associated proteins has already led to the identification of 3 associated mycoloyltransferases and an unknown protein with a C-terminal hydrophobic anchoring domain reminiscent of that found for the S-layer protein PS2 of C. glutamicum.

Abstract: Corynebacterineae is a specific suborder of Gram-positive bacteria that includes Mycobacterium tuberculosis and Corynebacterium glutamicum. The cell wall of these bacteria is composed of a heteropolymer of peptidoglycan (PG) linked to arabinogalactan (AG), which in turn is covalently associated with an atypical outer membrane, here called mycomembrane (M). The latter structure has been visualized by cryo-electron microscopy of vitreous sections, but its biochemical composition is still poorly defined, thereby hampering the elucidation of its physiological function. In this report, we show for the first time that the mycomembrane-linked heteropolymer of PG and AG (M-AG-PG) of C. glutamicum can be physically separated from the inner membrane on a flotation density gradient. Analysis of purified M-AG-PG showed that the lipids that composed the mycomembrane consisted almost exclusively of mycolic acid derivatives, with only a tiny amount, if any, of phospholipids and lipomannans, which were found with the characteristic lipoarabinomannans in the plasma membrane. Proteins associated with or inserted in the mycomembrane were extracted from M-AG-PG with lauryl-dimethylamine-oxide (LDAO), loaded on an SDS-PAGE gel, and analyzed by tandem mass spectrometry or by Western blotting. Sixty-eight different proteins were identified, 19 of which were also found in mycomembrane fragments released by the terminal-arabinosyl-transferase-defective ΔAftB strain. Almost all of them are predicted to contain a signal sequence and to adopt the characteristic β-barrel structure of Gram-negative outer membrane proteins. These presumed mycomembrane proteins include the already-known pore-forming proteins (PorA and PorB), 5 mycoloyltransferases (cMytA, cMytB, cMytC, cMytD, and cMytF), several lipoproteins, and unknown proteins typified by a putative C-terminal hydrophobic anchor.