Nuclear roles for actin.

Actin's presence in the nucleus is a subject that has ignited a lot of controversy in the past. With our review, we attempt to reach out not only to the specialists but also to a broader audience that might be skeptical in light of the controversies. We take a rather conservative approach to build an argument that recent studies provide multiple independent lines of evidence substantiating actin's diverse nuclear functions, especially in its monomeric state. We then particularly focus on how the concentration of monomeric actin, and potentially of specific polymerized forms of actin, can be used by the cell as indicators of cellular state and how this information can be transduced into the nucleus by transcriptional regulators, eliciting a response. We also provide examples that in specific cell types and specific physiological conditions, actin is functional in the nucleus in its polymeric form. However, we also discuss that in many instances, the presence of actin regulators in the nucleus, which is often seen as proof of their function within this compartment, may simply reflect an additional means of their regulation by compartmentalization.