Monomeric actin is abundant in the nucleus and has been shown to have roles in many nuclear processes, whereas stable nuclear actin filaments are found in pathological contexts and in oocytes where they confer structural stability to the large nuclei. It thus remains unclear what is the effect of nuclear actin polymerisation, for instance on transcription. In this issue (p. 3412), Primal de Lanerolle and colleagues now ask how stabilising actin filaments in the nucleus affects transcription by RNA polymerase II (RNAPII). By expressing a fragment of the actin bundler supervillin in COS7 cells to facilitate actin polymerisation or by use of an actin mutant that causes the human disease intranuclear rod myopathy, they obtain stable nuclear actin filaments that sequester nucleoplasmic actin and result in a decrease of the monomeric nuclear actin pool. Importantly, this coincides with reduced levels of global transcription. The authors use structured illumination microscopy to investigate how exactly these nuclear actin filaments affect RNAPII localisation and dynamics. They show that formation of nuclear actin filaments reduces the interaction of actin with RNAPII and inhibits the recruitment of RNAPII to gene promoters. These effects can be recapitulated in vitro by crosslinking or polymerising actin, lending further support to the notion that maintaining nuclear actin in a monomeric form is crucial for the proper function of the nucleus. These findings might therefore also explain the absence of classical actin filaments in the normal somatic nucleus.
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