Regulation and role of phospholipases in host-bacteria interaction

Abstract

Deacylating phospholipases play essential roles in numerous biological events, requiring tight control of hydrolytic activity. Most cells, unless stimulated or perturbed, exhibit little phospholipid turnover. Activation of phospholipases A (PLA) is usually triggered by membrane perturbing conditions or agents. Some activators indiscriminately activate any PLA, others are highly specific. Our studies concern an activator that is a potent bactericidal protein with membrane-perturbing properties, isolated from polymorphonuclear leukocytes (PMN), that is only cytotoxic for gram-negative bacteria and primarily responsible for the fate of several gram-negative bacterial species, ingested and killed by the PMN. It is this protein that activates the hydrolysis of the phospholipids of the killed bacteria (E. coli) by three PLA: 1) an E. coli PLA, the pldA gene product; 2) a PLA2 of PMN; 3) a soluble PLA2 in the extracellular fluid of an inflammatory exudate. However, this activator protein does not trigger the action of many other PLA2, all members of a highly conserved class of PLA. Our structural studies (including genetic engineering) of both responsive and non-responsive PLA2 have revealed that the amino acid composition and sequence of the NH2-terminal alpha-helix of the PLA2 molecule are major determinants of the ability of the PMN protein to activate a given PLA2. Our results provide another demonstration that these important enzymes have diverged during evolution to perform different biological functions.