A new role for plasma membrane phosphatidylinositol 4‐phosphate (PI4P)?

Abstract

Many crucial functions of the plasma membrane (PM) rely on the minor inner leaflet lipid, phosphatidylinositol 4,5‐bisphosphate [PI(4,5)P2]. The major intermediate in PI(4,5)P2 synthesis, PI4P, seems paradoxically to be superfluous for maintenance or function of PI(4,5)P2. Inhibiting the enzyme that generates PM PI4P, PI 4‐kinase (PI4K) type III α (PI4KA), leads to depletion of PI(4,5)P2 levels only when phospholipase C (PLC) is activated, suggesting that other PI4K activities synthesize PI(4,5)P2 in resting cells. Here, we set out to test this hypothesis. Using a combination of live cell imaging with fluorescent PI4P and PI(4,5)P2 biosensors, we show that type II or type III β PI4K inhibitors lead to reductions in endosomal and Golgi‐localized PI4P pools, whereas only an inhibitor of PI4KA produces a reduction in PM PI4P. However, no combination of these agents led to substantial depletion of PM PI(4,5)P2. Use of radiolabelled tracers demonstrated that PI4KA inhibitor caused reduced absolute levels of PI4P with little change in PI(4,5)P2, despite a dramatic reduction in acute PI4P and PI(4,5)P2 synthesis. Therefore, PI(4,5)P2 levels are maintained when PI4KA is blocked not because there is an alternative route of synthesis, but because PI(4,5)P2 turnover simply stops. Consistently, in cells depleted of PM PI4P by a PI4KA inhibitor, forced degradation of PI(4,5)P2 with a 5‐phosphatase yields a pool of PI4P, which rapidly runs down due to the absence of PI4KA activity to maintain it. These results suggest a novel function for PM PI4P, which is as a metabolic “buffer” whose levels and turnover can be rapidly sensed and adjusted in order to maintain the functionally crucial PI(4,5)P2 pool. This work was supported by the intramural research program of the Eunice Kennedy Shriver National Institute for Child Health and Human Development, NIH.