| Description | The E. coli O9 monomer is a linear pentasaccharide consisting solely of mannose units [->3)aDMan(1->3)aDMan(1->2)aDMan(1->2)aDMan(1->2)aDMan(1->] [1]. This structure is identical to the Citrobacter braakii O7a,3b,1c, Klebsiella pneumoniae O3, and Hafnia alvei PCM 1223 antigens [2]. It is believed that the MtfC mannosyltransferase is solely involved with the initiating transfer onto the LPS core, while MtfB and MtfA handle all the subsequent mannose 1->3 and 1->2 transfers respectively [3]. The O9 antigen is very similar to the E. coli O8 antigen (GenProp1018), differing only in the number of consecutive 1->3 and 1->2 linkages. While the MtfC and MtfB are extremely similar to the corresponding O8 enzymes WbdC and WbdB, MtfA and WbdA are quite distinct, exhibiting only 35% amino acid identity and 2 and 3 glycosyltransferase domains, respectively. Thus, these enzymes are likely responsible for the length and patterning of the monomer. Indeed, it has been shown that the O9a variant (which is one mannose shorter than O9) found in some E. coli strains is entirely due to a mutation in the MtfA gene [4]. These monomer units are handled in an ABC-transporter dependent process in which polymerization occurs on the cytoplasmic face of the plasma membrane followed by transport (flipping) of the undecaprenyl-linked full-length polymer. It has been shown that the overall antigen chain length is determined by the addition of a methylphosphate moiety on the O3 position of the terminal mannose residue [5], and that this modification is carried out by the MtfD (WbdD(O9)) enzyme, which carries out the sequential kinase and methyltransferase functions [6]. This is in contrast to the mechanism in the O8 antigen where only a methylase reaction is involved, and where the corresponding WbdD(O8) enzyme lacks a kinase domain. The ABC transporter appears to require the presence of the
modified final residue to move the polymer to the periplasmic side of the membrane in both systems. |
| Literature References | [ 1 ]Prehm P, Jann B, Jann K The O9 antigen of Escherichia coli. Structure of the polysaccharide chain. Eur J Biochem. 1976 Aug 1;67(1):53-6. PMID 786626
[ 2 ]Kocharova NA, Zatonsky GV, Bystrova OV, Shashkov AS, Knirel YA, Kholodkova EV, Stanislavsky ES Structure of the O-specific polysaccharide of Citrobacter braakii O7a,3b,1c. Carbohydr Res. 2001 Jul 19;333(4):335-8. PMID 11454340
[ 3 ]Kido N, Torgov VI, Sugiyama T, Uchiya K, Sugihara H, Komatsu T, Kato N, Jann K Expression of the O9 polysaccharide of Escherichia coli: sequencing of the E. coli O9 rfb gene cluster, characterization of mannosyl transferases, and evidence for an ATP-binding cassette transport system. J Bacteriol. 1995 Apr;177(8):2178-87. PMID 7536735
[ 4 ]Kido N, Kobayashi H A single amino acid substitution in a mannosyltransferase, WbdA, converts the Escherichia coli O9 polysaccharide into O9a: generation of a new O-serotype group. J Bacteriol. 2000 May;182(9):2567-73. PMID 10762260
[ 5 ]Kubler-Kielb J, Whitfield C, Katzenellenbogen E, Vinogradov E Identification of the methyl phosphate substituent at the non-reducing terminal mannose residue of the O-specific polysaccharides of Klebsiella pneumoniae O3, Hafnia alvei PCM 1223 and Escherichia coli O9/O9a LPS. AB - O-specific polysaccharides of Gram-n Carbohydr Res. 2011 Dec 3. PMID 22169179
[ 6 ]Clarke BR, Cuthbertson L, Whitfield C Nonreducing terminal modifications determine the chain length of polymannose O antigens of Escherichia coli and couple chain termination to polymer export via an ATP-binding cassette transporter. J Biol Chem. 2004 Aug 20;279(34):35709-18. Epub 2004 Jun 7. PMID 15184370
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