Transfer of palmitate from phospholipids to lipid A in outer membranes of Gram-negative bacteria

Regulated covalent modifications of lipid A are implicated in virulence of pathogenic Gram-negative bacteria. The Salmonella typhimurium PhoP/PhoQ-act ivated gene pagP is required both for biosynthesis of hepta-acylated lipid A species containing palmitate and for resistance to cationic anti-microbia l peptides. Palmitoylated lipid A can also function as an endotoxin antagon ist. We now show that pagP and its Escherichia coli homolog (crcA) encode a n unusual enzyme of lipid A biosynthesis localized in the outer membrane. P agP transfers a palmitate residue from the sn-1 position of a phospholipid to the N-linked hydroxymyristate on the proximal unit of lipid A (or its pr ecursors). PagP bearing a C-terminal His(6)-tag accumulated in outer membra nes during overproduction, was purified with full activity and was shown by cross-linking to behave as a homodimer. PagP is the first example of an ou ter membrane enzyme involved in lipid A biosynthesis. Additional pagP homol ogs are encoded in the genomes of Yersinia and Bordetella species. PagP may provide an adaptive response toward both Mg2+ limitation and host innate i mmune defenses.