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We recently demonstrated that incorporation of 4-amino-4- deoxy - l -arabinose ( l -Ara4N) to the lipid A moiety of lipopolysaccharide (LPS) is required for transport of LPS to the outer membrane and viability of the Gram-negative bacterium  Burkholderia cenocepacia . ArnT is a membrane protein catalyzing the transfer of  l -Ara4N to the LPS molecule at the periplasmic face of the inner membrane, but its topology and mechanism of action are not well characterized. Here, we elucidate the topology of ArnT and identify key amino acids that likely contribute to its enzymatic function. PEGylation assays using a cysteineless version of ArnT support a model of 13 transmembrane helices and a large C-terminal region exposed to the periplasm. The same topological configuration is proposed for the  Salmonella enterica  serovar Typhimurium ArnT. Four highly conserved periplasmic residues in  B. cenocepacia  ArnT, tyrosine-43, lysine-69, arginine-254 and glutamic acid-493, were required for activity. Tyrosine-43 and lysine-69 span two highly conserved motifs,  42 RYA 44  and  66 YFEKP 70 , that are found in ArnT homologues from other species. The same residues in  S. enterica  ArnT are also needed for function. We propose these aromatic and charged amino acids participate in either undecaprenyl phosphate- l -Ara4N substrate recognition or transfer of  l -Ara4N to the LPS.

scientific reports

Burkholderia cenocepacia and Salmonella enterica ArnT proteins that transfer 4-amino-4-deoxy-l-arabinose to lipopolysaccharide share membrane topology and functional amino acids