Architecture of the outer membrane of Escherichia coli K12. I. Action of phospholipases A2 and C on wild type strains and outer membrane mutants.

Phospholipids in whole cells of wild type Escherichia coli K12 are not degraded by exogenous phospholipases, whereas those of isolated outer membranes are completely degraded. It is concluded that the resistance of phospholipids in whole cells is due to shielding by one or more other outer membrane components. The nature of the shielding component(s) was investigated by testing the sensitivity of whole cells of a number of outer membrane mutants. Mutants lacking both major outer membrane proteins b and d or the heptose-bound glucose of their lipopolysaccharide, are sensitive to exogenous exogenous phospholipases. Moreover, cells of a mutant which lacks protein d can be sensitized by pretreatment of the cells with EDTA. From these results and from data on the chemical composition of the outer membranes, it is concluded that proteins b and d, the heptose-bound glucose of lipopolysaccharide and divalent cations are responsible for the inaccessibility of phospholipids to to exogenous phospholipases.