Glycobiology Advance Access originally published online on March 12, 2008
Glycobiology 2008 18(6):447-455; doi:10.1093/glycob/cwn021
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A unique glycosyltransferase involved in the initial assembly of Moraxella catarrhalis lipooligosaccharides
2 Department of Microbiology and Immunology, State University of New York at Buffalo, Buffalo, NY, 14214, USA
3 Witebsky Center for Microbial Pathogenesis and Immunology, State University of New York at Buffalo, Buffalo, NY, 14214, USA
4 Department of Medicine, State University of New York at Buffalo, Buffalo, NY, 14214, USA
5 New York State Center of Excellence in Bioinformatics & Life Sciences, State University of New York at Buffalo, Buffalo, NY, 14214, USA
6 Institute for Biological Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada
1 To whom correspondence should be addressed: Tel: +1-716-829-2673; Fax: +1-716-829-3889; e-mail: aac{at}buffalo.edu
Received on October 9, 2007; revised on February 29, 2008; accepted on March 2, 2008
Moraxella catarrhalis express three predominant forms of lipooligosaccharide (LOS) molecules on the bacterial surface. These major glycolipids contain specific carbohydrate epitopes that distinguish each glycoform into serotype A, B, or C LOS. All three serotypes, however, share a common glucose containing inner-core structure, consisting of an 
-glucose attached to 2-keto-3-deoxyoctulosonic acid (KDO), which is unique among Gram-negative bacteria. Many of the LOS glycosyltransferase genes (lgt) responsible for assembly of the extended M. catarrhalis LOS structure have been identified. In this report, we now describe the identification and characterization of Lgt6, a unique glycosyltransferase that is responsible for the addition of the first glucose to the inner core thus initiating the assembly of full length LOS. Isogenic mutants defective in the expression of lgt6 were constructed in all three M. catarrhalis LOS serotypes and the resulting LOS glycoforms consisted of KDO2-lipid A-OH as analyzed by urea sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE) and mass spectrometry. In addition, the expression of lgt6 in trans in a heptose-deficient Neisseria meningitidis NMB gmhX mutant resulted in the addition of a hexose to the LOS of this strain. These studies demonstrate that Lgt6 functions as an -(1-5)-glucosyltransferase in M. catarrhalis adding the primary glucose to the KDO2-lipid A-OH in LOS biosynthesis. The function of Lgt6 is required for the completion of both the major and minor oligosaccharide chains in M. catarrhalis.
Key words: glycosyltransferase / lipooligosaccharide (LOS) / Moraxella catarrhalis