Functional interaction of bacterial virulence factors of Xenorhabdus nematophila with a calcium-independent cytosolic PLA2 of Spodoptera exigua.

Phospholipase A2 (PLA2) hydrolyzes the ester bond of phospholipids (PLs) at sn-2 and releases free fatty acids and lysophospholipids that are subsequently changed into various signal molecules to mediate various physiological processes. Numerous PLA2s are known in various biological systems and can be divided into at least 16 groups. Although different PLA2s recently have been annotated from several insect species, physiological roles are known for only a few genes. Two calcium-independent PLA2s (Se-iPLA2A and Se-iPLA2B) are known in the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae). We generated and purified a recombinant Se-iPLA2B (rSe-iPLA2B) using a bacterial expression system and analyzed the enzyme kinetics. rSe-iPLA2B exhibited catalytic activities against both arachidonyl (AA)-PL and non-AA-PL substrates. It was highly susceptible to iPLA2-specific inhibitor, but insensitive to inhibitors specific to secretory PLA2s or calcium-dependent cytosolic PLA2s. Increasing calcium concentrations prevented enzyme activity, and culture medium of an entomopathogenic bacterium, Xenorhabdus nematophila, or its organic extracts significantly inhibited enzyme activity. Binding assays of rSe-iPLA2B with known secondary metabolites identified from X. nematophila indicated that benzylideneacetone was the most potent inhibitor with a high binding affinity at 0.2 μM against rSe-iPLA2B. Furthermore, rSe-iPLA2B catalyzed the release of fatty acids from PLs extracted from S. exigua fat body, suggesting its physiological role in maintaining PL integrity. All these catalytic activities indicate that Se-iPLA2B has the typical biochemical properties of other iPLA2s. Its high binding affinity to secondary metabolites of X. nematophila suggests that it is a molecular target of X. nematophila, an entomopathogen.