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

Many crucial functions of the plasma membrane (PM) rely on the minor inner leaflet lipid, phosphatidylinositol 4,5‐bisphosphate [PI(4,5)P 2 ]. The major intermediate in PI(4,5)P 2 synthesis, PI4P, seems paradoxically to be superfluous for maintenance or function of PI(4,5)P 2 . Inhibiting the enzyme that generates PM PI4P, PI 4‐kinase (PI4K) type III α (PI4KA), leads to depletion of PI(4,5)P 2 levels only when phospholipase C (PLC) is activated, suggesting that other PI4K activities synthesize PI(4,5)P 2 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)P 2 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)P 2 . Use of radiolabelled tracers demonstrated that PI4KA inhibitor caused reduced absolute levels of PI4P with little change in PI(4,5)P 2 , despite a dramatic reduction in acute PI4P and PI(4,5)P 2 synthesis. Therefore, PI(4,5)P 2 levels are maintained when PI4KA is blocked not because there is an alternative route of synthesis, but because PI(4,5)P 2 turnover simply stops. Consistently, in cells depleted of PM PI4P by a PI4KA inhibitor, forced degradation of PI(4,5)P 2 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)P 2 pool. This work was supported by the intramural research program of the Eunice Kennedy Shriver National Institute for Child Health and Human Development, NIH.