Unique membrane interaction mode of group IIF phospholipase A2.

The mechanisms by which secretory phospholipases A(2) (PLA(2)s) exert cellular effects are not fully understood. Group IIF PLA(2) (gIIFPLA(2)) is a structurally unique secretory PLA(2) with a long C-terminal extension. Homology modeling suggests that the membrane-binding surface of this acidic PLA(2) contains hydrophobic residues clustered near the C-terminal extension. Vesicle leakage and monolayer penetration measurements showed that gIIFPLA(2) had a unique ability to penetrate and disrupt compactly packed monolayers and bilayers whose lipid composition recapitulates that of the outer plasma membrane of mammalian cells. Fluorescence imaging showed that gIIFPLA(2) could also readily enter and deform plasma membrane-mimicking giant unilamellar vesicles. Mutation analysis indicates that hydrophobic residues (Tyr(115), Phe(116), Val(118), and Tyr(119)) near the C-terminal extension are responsible for these activities. When gIIFPLA(2) was exogenously added to HEK293 cells, it initially bound to the plasma membrane and then rapidly entered the cells in an endocytosis-independent manner, but the cell entry did not lead to a significant degree of phospholipid hydrolysis. GIIFPLA(2) mRNA was detected endogenously in human CD4(+) helper T cells after in vitro stimulation and exogenously added gIIFPLA(2) inhibited the proliferation of a T cell line, which was not seen with group IIA PLA(2). Collectively, these data suggest that unique membrane-binding properties of gIIFPLA(2) may confer special functionality on this secretory PLA(2) under certain physiological conditions.