Actin, which is best known as a cytoskeletal component, also participates in the control of gene expression. We report a function of nuclear actin in the regulation of MAL, a coactivator of the transcription factor serum response factor (SRF). MAL, which binds monomelic actin, is cytoplasmic in many cells but accumulates in the nucleus upon serum-induced actin polymerization. MAL rapidly shuttles between cytoplasm and nucleus in unstimulated cells. Serum stimulation effectively blocks MAL nuclear export, which requires MAL-actin interaction. Nuclear MAL binds SRF target genes but remains inactive unless actin binding is disrupted. Fluorescence resonance energy transfer analysis demonstrates that the MAL-actin interaction responds to extracellular signals. Serum-induced signaling is thus communicated to nuclear actin to control a transcriptional regulator.
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