Nuclear functions of actin

Abstract

Actin participates in several essential processes in the cell nucleus. Even though the presence of actin in the nucleus was proposed more than 30 years ago, nuclear processes that require actin have been only recently identified. Actin is part of chromatin remodeling complexes; it is associated with the transcription machineries; it becomes incorporated into newly synthesized ribonucleoproteins; and it influences long-range chromatin organization. As in the cytoplasm, nuclear actin works in conjunction with different types of actin-binding proteins that regulate actin function and bridge interactions between actin and other nuclear components.

Figure 1.

Actin and NM1 localize to mammalian nucleoli. Homogeneous preparations of nucleoli from HeLa cells were analyzed by immunofluorescence labeling and confocal microscopy using rabbit polyclonal antibodies to actin (scale bar, 2 µm) and NM1 (scale bar, 2 µm) or corresponding preimmune sera (scale bar, 1 µm). All the specimens were coimmunostained with a mouse monoclonal antibody to fibrillarin as marker for nucleoli.

Figure 2.

Two possible ways by which actin contributes to transcription elongation. (A) An actomyosin motor helps the transcription machinery slide along the DNA. The architecture of the motor is not well understood but we envisage that the actin-NM1 complex makes contact with both the transcription machinery and the DNA/chromatin. Two possible configurations are depicted in the figure. (B) Actin participates in the recruitment of histone modifiers to protein-coding genes. Actin binds to hnRNP proteins and becomes incorporated into the nascent pre-mRNPs. Actin forms a complex with specific hnRNP adaptor proteins such as Hrp65 in Chironomus tentans and hnRNP U in mammals, and recruits HATs that acetylate histones and facilitate transcription elongation. The HATs might be components of larger chromatin remodeling complexes that might also establish direct contacts with actin.