Molecular architecture of KedS8, a sugar N-methyltransferase from Streptoalloteichus sp. ATCC 53650.

Kedarcidin, produced by Streptoalloteichus sp. ATCC 53650, is a fascinating chromoprotein of 114 amino acid residues that displays both antibiotic and anticancer activity. The chromophore responsible for its chemotherapeutic activity is an ansa-bridged enediyne with two attached sugars, l-mycarose, and l-kedarosamine. The biosynthesis of l-kedarosamine, a highly unusual trideoxysugar, is beginning to be revealed through bioinformatics approaches. One of the enzymes putatively involved in the production of this carbohydrate is referred to as KedS8. It has been proposed that KedS8 is an N-methyltransferase that utilizes S-adenosylmethionine as the methyl donor and a dTDP-linked C-4' amino sugar as the substrate. Here we describe the three-dimensional architecture of KedS8 in complex with S-adenosylhomocysteine. The structure was solved to 2.0 Å resolution and refined to an overall R-factor of 17.1%. Unlike that observed for other sugar N-methyltransferases, KedS8 adopts a novel tetrameric quaternary structure due to the swapping of β-strands at the N-termini of its subunits. The structure presented here represents the first example of an N-methyltransferase that functions on C-4' rather than C-3' amino sugars.