Nuclear actin. Examples of the three general types of nuclear actin function. Top: Actin monomer as a tightly bound component of multiprotein complexes. Shown here is the INO80 chromatin remodeling complex. A complex of actin and two Arps (Arp4 and Arp8) forms a module that is connected to the main complex by the helical HSA domain (yellow), and serves to specify the position of INO80 on the nucleosome. Left: Actin monomer as a reversible regulator of nuclear proteins. Shown here is actin bound to MRTF, both in the cytoplasm and in the nucleus. In both compartments, actin monomer inhibits MRTF’s ability to activate transcription through SRF. Actin polymerization in both the cytosol and the nucleus releases MRTF for SRF activation. Here, the nuclear actin polymerization is formin-mediated. Right: Actin filaments as force-producing structures for nuclear dynamics. Shown here is myosin-mediated movement of a heterochromatin double-strand break (bound to MRN) along an Arp2/3 complex–polymerized actin filament to the nuclear periphery for repair. MRTF, myocardin-related transcription factor; NPC, nuclear pore complex; MRN, MRE11-Rad50-NBS1 complex.