PagP Activation in the Outer Membrane Triggers R3 Core Oligosaccharide Truncation in the Cytoplasm of Escherichia coli O157:H7*

The Escherichia coli outer membrane phospholipid:lipid A palmitoyltransferase PagP is normally a latent enzyme, but it can be directly activated in outer membranes by lipid redistribution associated with a breach in the permeability barrier. We now demonstrate that a lipid A myristate deficiency in an E. coli O157:H7 msbB mutant constitutively activates PagP in outer membranes. The lipid A myristate deficiency is associated with hydrophobic antibiotic sensitivity and, unexpectedly, with serum sensitivity, which resulted from O-antigen polysaccharide absence due to a cytoplasmically determined truncation at the first outer core glucose unit of the R3 core oligosaccharide. Mutational inactivation of pagP in the myristate-deficient lipid A background aggravated the hydrophobic antibiotic sensitivity as a result of losing a partially compensatory increase in lipid A palmitoylation while simultaneously restoring serum resistance and O-antigen attachment to intact lipopolysaccharide. Complementation with either wild-type pagP or catalytically inactive pagPSer77Ala alleles restored the R3 core truncation. However, the intact lipopolysaccharide was preserved after complementation with an internal deletion pagPDelta5-14 allele, which mostly eliminates a periplasmic amphipathic alpha-helical domain but fully supports cell surface lipid A palmitoylation. Our findings indicate that activation of PagP not only triggers lipid A palmitoylation in the outer membrane but also separately truncates the R3 core oligosaccharide in the cytoplasm. We discuss the implication that PagP might function as an apical sensory transducer, which can be activated by a breach in the outer membrane permeability barrier.