Nuclear actin and myosins at a glance

Actin and myosin II form the archetypical molecular motor complex. Myosin II, like all members of the myosin superfamily, is an actin-activated ATPase that uses the energy released when ATP is hydrolyzed to do work. Although we typically associate work with muscle contraction, cell motility and cell division, many nuclear processes require energy. Historically, the notion that the nucleus contains actin or myosin has been highly controversial. Because filaments are central to the biological functions of actin, this controversy was fueled by the absence of actin or myosin filaments in the nucleus. Nevertheless, myosins I, II, V, VI, X, XVI and XVIII have been described in the nucleus, and the presence of actin in the nucleus is now indisputable. However, it has been difficult to recognize structures formed by nuclear myosins and actin that are comparable to actin–myosin structures found in the cytoplasm. Because structure (or ‘form’) is normally a guide to function in biology, the absence of nuclear structures that are obviously similar to those in the cytoplasm has raised the possibility that nuclear motors work in unique ways. This Cell Science at a Glance article will review what is known about actin and myosin in the nucleus and how they differ in form and function from their cytoplasmic counterparts.