[Show abstract][Hide abstract] ABSTRACT: The Escherichia coli O9a and O8 polymannose O-polysaccharides (O-PSs) serve as model systems for the biosynthesis of bacterial polysaccharides
by ATP-binding cassette transporter-dependent pathways. Both O-PSs contain a conserved primer-adaptor domain at the reducing
terminus and a serotype-specific repeat unit domain. The repeat unit domain is polymerized by the serotype-specific WbdA mannosyltransferase.
In serotype O9a, WbdA is a bifunctional α-(1→2)-, α-(1→3)-mannosyltransferase, and its counterpart in serotype O8 is trifunctional
(α-(1→2), α-(1→3), and β-(1→2)). Little is known about the detailed structures or mechanisms of action of the WbdA polymerases,
and here we establish that they are multidomain enzymes. WbdAO9a contains two separable and functionally active domains, whereas WbdAO8 possesses three. In WbdCO9a and WbdBO9a, substitution of the first Glu of the EX7E motif had detrimental effects on the enzyme activity, whereas substitution of the second had no significant effect on activity
in vivo. Mutation of the Glu residues in the EX7E motif of the N-terminal WbdAO9a domain resulted in WbdA variants unable to synthesize O-PS. In contrast, mutation of the Glu residues in the motif of the
C-terminal WbdAO9a domain generated an enzyme capable of synthesizing an altered O-PS repeat unit consisting of only α-(1→2) linkages. In vitro assays with synthetic acceptors unequivocally confirmed that the N-terminal domain of WbdAO9a possesses α-(1→2)-mannosyltransferase activity. Together, these studies form a framework for detailed structure-function
studies on individual domains and a strategy applicable for dissection and analysis of other multidomain glycosyltransferases.
[Show abstract][Hide abstract] ABSTRACT: The Escherichia coli O9a and O8 O-antigen serotypes represent model systems for the ABC transporter-dependent synthesis of bacterial polysaccharides. The O9a and O8 antigens are linear mannose homopolymers containing conserved reducing termini (the primer-adaptor), a serotype-specific repeat unit domain, and a terminator. Synthesis of these glycans occurs on the polyisoprenoid lipid-linked primer, undecaprenol pyrophosphoryl-GlcpNAc, by two conserved mannosyltransferases, WbdC and WbdB, and a serotype-specific mannosyltransferase, WbdA. The glycan structure and pattern of conservation in the O9a and O8 mannosyltransferases are not consistent with the existing model of O9a biosynthesis. Here we establish a revised pathway using a combination of in vivo (mutant complementation) experiments and in vitro strategies with purified enzymes and synthetic acceptors. WbdC and WbdB synthesize the adaptor region, where they transfer one and two α-(1→3)-linked mannose residues, respectively. The WbdA enzymes are solely responsible for forming the repeat unit domains of these O-antigens. WbdA(O9a) has two predicted active sites and polymerizes a tetrasaccharide repeat unit containing two α-(1→3)- and two α-(1→2)-linked mannopyranose residues. In contrast, WbdA(O8) polymerizes trisaccharide repeat units containing single α-(1→3)-, α-(1→2)-, and β-(1→2)-mannopyranoses. These studies illustrate assembly systems exploiting several mannosyltransferases with flexible active sites, arranged in single- and multiple-domain formats.
Preview · Article · Aug 2012 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The O-polysaccharide (O-PS; O-antigen) of bacterial lipopolysaccharides is made up of repeating units of one or more sugar residues and displays remarkable structural diversity. Despite the structural variations, there are only three strategies for O-PS assembly. The ATP-binding cassette (ABC)-transporter-dependent mechanism of O-PS biosynthesis is widespread. The Escherichia coli O9a and Klebsiella pneumoniae O2a antigens provide prototypes, which are distinguished by the fine details that link glycan polymerization and chain termination at the cytoplasmic face of the inner membrane to its export via the ABC transporter. Here, we describe the current understanding of these processes. Since glycoconjugate assembly complexes that utilize an ABC transporter-dependent pathway are widespread among the bacterial kingdom, the models described here are expected to extend beyond O-PS biosynthesis systems.
No preview · Article · Mar 2012 · Carbohydrate research
[Show abstract][Hide abstract] ABSTRACT: The Escherichia coli O9a O-polysaccharide (O-PS) represents a model system for glycan biosynthesis and export by the ATP-binding cassette (ABC) transporter-dependent pathway. The polymannose O9a O-PS is synthesized using an undecaprenol-diphosphate-linked acceptor by mannosyltransferases located at the cytoplasmic membrane. An ABC-transporter subsequently exports the polymer to the periplasm where it is assembled onto lipopolysaccharide prior to translocation to the cell surface. The chain length of the O9a O-PS is regulated by the dual kinase/methyltransferase activity of the WbdD enzyme and modification of the polymer is crucial for binding and export by the ABC-transporter. Previous biochemical data provided evidence for phosphorylation/methylation at the non-reducing end of the O9a O-PS but the structure of the terminus has not been determined. Here, we describe the exploitation of a synthetic O9a O-PS repeating unit carrying a fluorescent tag as an acceptor for in vitro phosphorylation and methylation by a purified soluble form of WbdD. Phosphorylation of the acceptor was evident by both a mobility shift in thin layer chromatography and radiolabeling of the acceptor using [γ-(33)P]ATP. Methylation of the acceptor was dependent on phosphorylation and was demonstrated by radiolabeling using S-[methyl-(3)H]adenosyl-methionine as a substrate, in the presence of ATP. NMR spectroscopic and mass spectrometric methods were used to determine the precise structure of the terminal modification, leading to the conclusion that WbdD catalyzes the addition of a novel methyl phosphate group to the 3-position of the non-reducing terminal mannose of the O9a O-PS repeating unit.
No preview · Article · Dec 2011 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The Escherichia coli O9a O-polysaccharide (O-PS) is a prototype for O-PS synthesis and export by the ATP-binding cassette transporter-dependent pathway. Comparable systems are widespread in Gram-negative bacteria. The polymannose O9a O-PS is assembled on a polyisoprenoid lipid intermediate by mannosyltransferases located at the cytoplasmic membrane, and the final polysaccharide chain length is determined by the chain terminating dual kinase/methyltransferase, WbdD. The WbdD protein is tethered to the membrane via a C-terminal region containing amphipathic helices located between residues 601 and 669. Here, we establish that the C-terminal domain of WbdD plays an additional pivotal role in assembly of the O-PS by forming a complex with the chain-extending mannosyltransferase, WbdA. Membrane preparations from a DeltawbdD mutant had severely diminished mannosyltransferase activity in vitro, and no significant amounts of the WbdA protein are targeted to the membrane fraction. Expression of a polypeptide comprising the WbdD C-terminal region was sufficient to restore both proper localization of WbdA and mannosyltransferase activity. In contrast to WbdA, the other required mannosyltransferases (WbdBC) are targeted to the membrane independent of WbdD. A bacterial two-hybrid system confirmed the interaction of WbdD and WbdA and identified two regions in the C terminus of WbdD that contributed to the interaction. Therefore, in the O9a assembly export system, the WbdD protein orchestrates the critical localization and coordination of activities involved in O-PS chain extension and termination at the cytoplasmic membrane.
No preview · Article · Oct 2009 · Journal of Biological Chemistry