A BAFfling actin function

New results from Oliver Rando, Jerry Crabtree, and colleagues (Stanford University, Stanford, CA) suggest an unusual function for nuclear actin. Rather than restricting itself to the cytoskeleton, actin may help anchor a chromatin-remodeling complex in the nucleus.

Crabtree's study of antigen-stimulated lymphocyte activation has previously shown that phosphatidylinositol 4,5-bisphosphate (PIP₂) signaling helps retain a greater proportion of a mammalian chromatin remodeling complex called BAF in the nucleus. Rando and Crabtree have now determined that BAF binds PIP₂ directly, but only if the complex retains two particular subunits: actin and the actin-related protein BAF53.

PIP₂ does not trigger exchange of BAF's actin subunit, but PIP₂ plus BAF does increase the extent of in vitro actin polymerization. In vitro, BAF binds PIP₂ vesicles and the ends and junctions of actin filaments....