Actin moving in and out of the nucleus: in what form?

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With the role of actin in chromatin remodeling, transcription and RNA processing now being widely accepted, the quest for its oligomeric and polymeric form(s) in the nucleus has become a key issue [ Jockusch et al. (2006) Trends Cell Biol 22,]. To this end, we have shown by means of monoclonal antibodies (mAb) that different forms of actin exist in different cellular locations [ Schoenenberger et al. (2005) J Struct Biol 152, ]. Moreover, uncommon forms of actin like the lower dimer, bipolar filaments, ribbons, tubes and sheets have been observed in vitro [ Steinmetz et al. (1997) J Struct Biol 119, ]. To unravel the mystery of less common forms of actin in vivo, in particular those in the nucleus, we chose to tailor mAb to recognize distinct forms of actin by using peptide nanoparticles that are designed to mimic a virus-like capsid and allow the repetitive display of a specific epitope on their surface [ Raman et al. (2006) Nanomed 2, ]. Immunofluorescence studies on cells revealed that several of these actin-form specific mAb labeled actin-containing structures predominantly residing in either the cytoplasm or the nucleus. Taken together, our results suggest that oligomeric and/or polymeric actin forms exist in the nucleus that clearly differ from their cytoplasmic counterparts.