Detection of mycoloylglycerol by thin-layer chromatography as a tool for the rapid inclusion of corynebacteria of clinical origin in the genus Corynebacterium

Current knowledge on mycolic acids in Corynebacterium glutamicum and their relevance for biotechnological processes

Corynebacterium glutamicum is the world's largest producer of glutamate and lysine. Industrial glutamate overproduction is induced by empirical processes, such as biotin limitation, supplementation with specific surfactants or addition of sublethal concentration of certain antibiotics to the culture media. Although Gram-positive bacteria, C. glutamicum and related bacterial species and genera contain, in addition to the plasma membrane, an outer permeability membrane similar to that of Gram-negative microorganisms. As the amino acids have to cross both membranes, their integrity, composition and fluidity influence the export process. While the precise mechanism of the export of the amino acids by C. glutamicum is not fully understood, the excretion of amino acids through the inner membrane involved at least a major export system mechanosensitive channel MscS family (MscCG) encoded by NCgl1221. As the various industrial treatments have been shown to affect the lipid content of the bacterial cell, it is strongly believed that defects in the hallmark of the outer membrane, 2-alkyl, 3-hydroxylated long-chain fatty acids (mycolic acids), could be key factors in the glutamate overproduction. This review aims at giving an overview of the current knowledge on mycolic acids structure, biosynthesis and transfer in C. glutamicum and their relevance for amino acid biotechnological production.

A putative alpha-helical porin from Corynebacterium glutamicum

The cell wall of Corynebacterium glutamicum contains a mycolic acid layer, which is a protective nonpolar barrier similar to the outer membrane of Gram-negative bacteria. The exchange of material across this barrier requires porins. Porin B (PorB) is one of them. Recombinant PorB has been produced in Escherichia coli, purified, crystallized and analyzed by X-ray diffraction, yielding 16 independent molecular structures in four different crystal forms at resolutions up to 1.8 A. All 16 molecules have the same globular core, which consists of 70 residues forming four alpha-helices tied together by a disulfide bridge. The 16 structures vary greatly with respect to the 29 residues in the N- and C-terminal extensions. Since corynebacteria belong to the group of mycolata that includes some prominent human pathogens, the observed structure may be of medical relevance. Due to the clearly established solid structure of the core, the native porin has to be oligomeric, and the reported structure is one of the subunits. An alpha-helical porin in a bacterial outer envelope is surprising because all presently known structures of such porins consist of beta-barrels. Since none of the four crystal packing arrangements was compatible with an oligomeric membrane channel, we constructed a model of such an oligomer that was consistent with all available data of native PorB. The proposed model is based on the required polar interior and nonpolar exterior of the porin, on a recurring crystal packing contact around a 2-fold axis, on the assumption of a simple C(n) symmetry (a symmetric arrangement around an n-fold axis), on the experimentally established electric conductivity and anion selectivity and on the generally observed shape of porin channels.

Identification and characterization of PorH, a new cell wall channel of Corynebacterium glutamicum

The cell wall of Corynebacterium glutamicum contains the cation-selective channel (porin) PorA(C.glut) and the anion-selective channel PorB(C.glut) for the passage of hydrophilic solutes. Lipid bilayer experiments with organic solvent extracts of whole C. glutamicum cells cultivated in minimal medium suggested that also another cation-selective channel-forming protein, named PorH(C.glut), is present in C. glutamicum. The protein was purified to homogeneity by fast-protein liquid chromatography across a HiTrap-Q column. The pure protein had an apparent molecular mass of about 12 kDa on SDS-PAGE. Western blot analysis suggested that the cell wall channel is presumably formed by protein oligomers. The purified protein forms cation-selective channels with an average single-channel conductance of about 2.5 nS in 1 M KCl in the lipid bilayer assay. The PorH(C.glut) protein was partially sequenced, and based on the resulting amino acid sequence, the corresponding gene, designated as porH(C.glut), was identified in the published genome sequence of C. glutamicum ATCC13032. PorH(C.glut) contains only the inducer methionine but no N-terminal extension, which suggests that the export and assembly of the protein follow a yet unknown pathway. PorH(C.glut) is coded in the bacterial chromosome by a gene that is localized in the vicinity of porA(C.glut), within a putative operon of 13 genes. RT-PCR revealed that both porins are cotranscribed. They coexist according to immunological detection experiments in the cell wall of C. glutamicum together with PorB(C.glut) and PorC(C.glut).