Membrane phosphoinositides control organization and dynamics of the actin cytoskeleton by regulating the activities of several key actin-binding proteins. Twinfilin is an evolutionarily conserved protein, which contributes to cytoskeletal dynamics by interacting with actin monomers, filaments, and the heterodimeric capping protein. Twinfilin also binds phosphoinositides, which inhibit its interactions with actin, but the underlying mechanism has remained unknown. Here we show that the highaffinity binding site of twinfilin for phosphoinositides is located at the carboxy-terminal tail-region, while the two ADF/cofilin like ADF-H domains of twinfilin bind phosphoinositides only with low affinity. Mutagenesis and biochemical experiments combined with atomistic molecular dynamics simulations reveal that the carboxy-terminal tail of twinfilin interacts with membranes through a multivalent electrostatic interaction with a preference towards PI(3,5)P 2 , PI(4,5)P 2 , and PI(3,4,5)P 3 . This initial interaction places the actin-binding ADF-H domains of twinfilin to a close proximity of the membrane and subsequently promotes their association with the membrane, thus leading to inhibition of the actininteractions. In support to this model, a twinfilin mutant lacking the carboxy-terminal tail inhibits actin filament assembly in a phosphoinositide-insensitive manner. Our mutagenesis data also reveal that the phosphoinositide-and capping proteinbinding sites overlap in the carboxy-terminal tail of twinfilin, suggesting that phosphoinositide-binding additionally inhibits the interactions of twinfilin with the heterodimeric capping protein. The results demonstrate that the conserved carboxy-terminal tail of twinfilin is a multi-functional binding motif, which is crucial for interaction with the heterodimeric capping protein and for tethering twinfilin to phosphoinositiderich membranes.The dynamic interplay between the actin cytoskeleton and plasma membrane is critical for several cellular processes, such as migration, morphogenesis, endocytosis, and phagocytosis. Coordinated polymerization of actin filaments provides a force for generation of membrane invaginations in endocytic processes (1, 2). In migrating cells, polymerization of actin filaments against the plasma membrane at the leading edge pushes the membrane forward to generate plasma membrane protrusions, such as lamellipodia and filopodia (3-6).While actin polymerization controls the geometry of cellular membranes, membrane phospholipids, especially PI(4,5)P 2 , reciprocally regulate the organization and the dynamics of the actin cytoskeleton. Typically an increase in the plasma membrane PI(4,5)P 2 density leads to actin filament assembly beneath the membrane, whereas a decrease in PI(4,5)P 2 concentration results in diminished actin Mechanism of twinfilin-PI(4,5)P 2 interaction filament assembly (7-10). Phosphoinositides regulate actin filament assembly and disassembly by directly interacting with several actin-binding proteins (11). The activities and the pl...