The functions of the G-actin-binding proteins ADF/cofilin and profilin in actin dynamics are relatively well understood: ADF/cofilin promotes filament depolymerization and severing, whereas profilin promotes filament assembly and suppresses nucleation. But there is a third ubiquitous G-actin-binding protein: twinfilin. What is its role? Pekka Lappalainen and co-workers discuss studies that are shedding light on the biology of this novel protein. Twinfilin possess two ADF/cofilin-like domains but, in contrast to ADF/cofilin, binds only to actin monomers and does not promote filament depolymerization. The protein has a high affinity for actin monomers in the inactive, ADP-bound form, and its absence in yeast and Drosophilaproduces phenotypes attributable to uncontrolled actin polymerization. Biochemical experiments show that twinfilin localizes to cortical actin patches through direct interactions with capping protein. Lappalainen and co-workers speculate that it functions as a link between filament depolymerization and assembly. They propose that twinfilin receives actin monomers from ADF/cofilin, blocks...

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