A novel calcium-independent phospholipase A2 and its physiological roles in development and immunity of a lepidopteran insect, Spodoptera exigua

Phospholipase A2 (PLA2) catalyzes hydrolysis of ester linkage at sn-2 position of phospholipids. At least 15 groups (I-XV) of PLA2 gene superfamily are associated with various physiological processes such as digestion, secretion, immunity, and maintenance of membrane integrity. This study suggests that various insects encode putative Group VI PLA2s representing intracellular and calcium-independent PLA2s (iPLA2). These insect iPLA2s are separated into at least two subgroups: iPLA2A (Group VIA-like) and iPLA2B (non-Group VIA). Most insects encode genes of iPLA2B type, although their biological functions are currently unknown. This study predicted a novel iPLA2 from Spodoptera exigua (a lepidopteran insect) (SeiPLA2B) and analyzed its physiological functions by RNA interference (RNAi). SeiPLA2B encodes 336 amino acid sequence with a predicted size of about 36.6 kDa and an isoelectric point at pH 8.61. It possesses a lipase catalytic site, but does not have ankyrin repeats in the amino terminal region. Phylogenetic analysis indicated that SeiPLA2B was clustered with other Group VI iPLA2s, in which SeiPLA2B was closely associated with Group VIF gene while SeiPLA2A was closely related to Group VIA gene. SeiPLA2B was expressed in all developmental stages of S. exigua. In larval stage, SeiPLA2B was expressed in fat body, hemocyte, and epidermis, but not in digestive tract. SeiPLA2B RNAi significantly reduced PLA2 enzyme activities and resulted in developmental retardation and immunosuppression. Though RNAi treatment did not significantly change fatty acid composition in fat body lipids, it significantly increased lipid peroxidation. Taken together, our results suggest that SeiPLA2B plays important roles in the development and immunity of S. exigua.