Biosynthesis and Modification of Helicobacter pylori Lipid A

Abstract

The secondary acylation steps of Helicobacter pylori lipid A biosynthesis are poorly understood because H. pylori only has one homolog (Jhp0265) to the Escherichia coli secondary acyl transferases LpxL and LpxM. Jhp0265 was shown to be responsible for the transfer of a secondary C18 acyl chain to the 2′-linked acyl chain of lipid A, making Jhp0265 homologous to LpxL. An activity was also demonstrated for the addition of a secondary acyl chain to the 3′-linked acyl chain of H. pylori lipid A, although the enzyme responsible for the transfer remains unknown. After synthesis, H. pylori lipid A is modified by the action of five enzymes. Mutation of the candidate modification enzyme Jhp0634 demonstrated that the enzyme catalyzes the removal of the 3′-linked acyl chains of H. pylori lipid A, producing a tetra-acylated lipid A species. Continuing with the characterization of H. pylori lipid A modification enzymes, we were also able to demonstrate an activity for a Kdo trimming enzyme in vitro. Requirement for a Kdo hydrolase in vivo was confirmed after the Kdo transferase of H. pylori was shown to be bifunctional despite the presence of only one Kdo sugar in H. pylori lipopolysaccharide. Attempted identification of the Kdo hydrolase revealed that both Hp0579 and Hp0580 were required for the removal of the Kdo sugar, which occurred in the periplasm. A Kdo hydrolase mutant revealed two unexpected phenotypes related to interaction with the innate immune system. The first was an increased sensitivity to cationic antimicrobial peptides, which was explained by a downstream effect on modification to the 4′- phosphate group of lipid A. The second phenotype related to the expression of Oantigen on the bacterial cell surface. The Kdo hydrolase mutants produced a reduced amount of fully extended lipopolysaccharide and conversely, an increased amount of core-lipid A. The type of O-antigen epitope displayed was also affected by a Kdo hydrolase mutation, in a strain specific manner.