Molecular mechanisms of altered membrane microdomain sensitivity leading to initiation of the arachidonic acid metabolism

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In this study, we have investigated the molecular mechanisms of the perturbation of cellular membrane microdomains leading to the initiation of arachidonic acid (AA) release by various mammalian secretory phospholipase A2(sPLA2) enzymes. We have subdivided sPLA2s into three classes in terms of their different membrane microdomain sensisivities: (i) the sPLA2-IIA type, which acts on elicited membranes in activated cells;(ii) the sPLA2-X type, which acts on unmodified membranes in quiescent cells;and (iii) the sPLA-V type, which displays both the sPLA2-IIA and sPLA2-X type properties. sPLA2-IIA and related enzymes (IID and IIE) bind to the heparan sulfate proteoglycan (HSPG) glypican that is enriched in the rafts/caveolae,and are internalized into the perinuclear membtane compartments where they exhibit the AA-releasing function(the HSPG-shuttling pathway). sPLA2-X is unable to utilize the HSPG-shuttling pathway because of its inability to bind HSPG,yet it has a high affinity for phosphatidylcholine(PC) and can act on the PC-rich outer leaflet of the plasma membrane (the external plasma membrane (EPM) pathway). sPLA2-V has high affinity for both HSPG and PC, thereby being able to utilize both pathways, Although sPLA2-IIF weakly elicits the EPM pathway-based AA release, its function is greatly faciliated in activated cells, where this enzyme may interacts with the perturbed plasma membrame microdomain through its unique C-terminal extension. sPLA2III,which is composed of the central sPLA2 domain flanked with unique C-terminal domains, is capable of utilizing the EPM pathway. In addition, the highly cationic N-and C-terminal domains allow sPLA2-III to bind anionic HSPG and thus to enter the HSPG-shuttiling route. Collectively, our present analyses have revealed that the cellular actions of various sPLA2 enzymes are crucially affected by their anzymatic properties and subcellular localization, the latter of which is tightly linked with the dymanics of cellular membranes.