Three-dimensional structure of a sugar N-formyltransferase from Francisella tularensis.

N-formylated sugars have been observed on the O-antigens of such pathogenic Gram-negative bacteria as Campylobacter jejuni and Francisella tularensis. Until recently, however, little was known regarding the overall molecular architectures of the N-formyltransferases that are required for the biosynthesis of these unusual sugars. Here we demonstrate that the protein encoded by the wbtj gene from F. tularensis is an N-formyltransferase that functions on dTDP-4-amino-4,6-dideoxy-d-glucose as its substrate. The enzyme, hereafter referred to as WbtJ, demonstrates a strict requirement for N(10) -formyltetrahydrofolate as its carbon source. In addition to the kinetic analysis, the three-dimensional structure of the enzyme was solved in the presence of dTDP-sugar ligands to a nominal resolution of 2.1 Å. Each subunit of the dimeric enzyme is dominated by a "core" domain defined by Met 1 to Ser 185. This core motif harbors the active site residues. Following the core domain, the last 56 residues fold into two α-helices and a β-hairpin motif. The hairpin motif is responsible primarily for the subunit:subunit interface, which is characterized by a rather hydrophobic pocket. From the study presented here, it is now known that WbtJ functions on C-4' amino sugars. Another enzyme recently investigated in the laboratory, WlaRD, formylates only C-3' amino sugars. Strikingly, the quaternary structures of WbtJ and WlaRD are remarkably different. In addition, there are several significant variations in the side chains that line their active site pockets, which may be important for substrate specificity. Details concerning the kinetic and structural properties of WbtJ are presented.