Dissecting the role of PtdIns(4,5)P2 in endocytosis and recycling of the transferrin receptor.

Endocytosis and recycling of membrane proteins are key processes for nutrient uptake, receptor signaling and synaptic transmission. Different steps in these fission and fusion cycles have been proposed to be regulated by physiological changes in plasma membrane (PM) phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] concentration. Here, we use a chemical enzyme-translocation strategy to rapidly reduce PM PtdIns(4,5)P(2) levels while monitoring clathrin-mediated endocytosis and recycling. PtdIns(4,5)P(2) hydrolysis blocked transferrin receptor endocytosis and led to a marked increase in the concentration of transferrin receptors in the PM, suggesting that endocytosis is more sensitive to changes in PtdIns(4,5)P(2) than recycling. Reduction of PM PtdIns(4,5)P(2) levels led to a near complete dissociation of Adaptor protein 2 (AP-2) from the PM but had only a small effect on clathrin assembly. This argues that receptor-mediated PtdIns(4,5)P(2) reduction preferentially suppresses AP-2-mediated targeting of cargo to endocytic sites rather than the assembly of clathrin coats or recycling of endocytic vesicles.