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Nuclear Actin in Action
Actin polymerization is essential for structures in mammalian cells. Although actin filament network structures are observed in the cytoplasm and at the plasma membrane, monomeric actin is also seen in the nucleus. Baarlink et al. (p. 864, published online 4 April) directly visualized a distinct and dynamic actin network within the nucleus in living cells. The network spanned the entire nucleus and appeared to be enriched along the nuclear cortex. Transient formation of a nuclear actin network may be induced by the transcriptional serum response.
Formins are potent activators of actin filament assembly in the cytoplasm. In turn, cytoplasmic actin polymerization can promote release of actin from megakaryocytic acute leukemia (MAL) protein for serum response factor (SRF) transcriptional activity. We found that formins polymerized actin inside the mammalian nucleus to drive serum-dependent MAL-SRF activity. Serum stimulated rapid assembly of actin filaments within the nucleus in a formin-dependent manner. The endogenous formin mDia was regulated with an optogenetic tool, which allowed for photoreactive release of nuclear formin autoinhibition. Activated mDia promoted rapid and reversible nuclear actin network assembly, subsequent MAL nuclear accumulation, and SRF activity. Thus, a dynamic polymeric actin structure within the nucleus is part of the serum response.