In polarized motile cells, stress fibers display specific three-dimensional organization. Ventral stress fibers, attached to focal adhesions at both ends, are restricted to the basal side of the cell and nonprotruding cell sides. Dorsal fibers, transverse actin arcs, and perinuclear actin fibers emanate from protruding cell front toward the nucleus and toward apical side of the cell. Perinuclear cap fibers further extend above the nucleus, associate with nuclear envelope through LINC (linker of nucleoskeleton and cytoskeleton) complex and terminate in focal adhesions at cell rear. How are perinuclear actin fibers formed is poorly understood. We show that the formation of perinuclear actin fibers requires dorsal stress fibers that polymerize from focal adhesions at leading edge, and transverse actin arcs that are interconnected with dorsal fibers in spots rich in α-actinin-1. During cell polarization, the interconnected dorsal fibers and transverse arcs move from leading edge toward dorsal side of the cell. As they move, transverse arcs associate with one end of stress fibers present at nonprotruding cell sides, move them above the nucleus thus forming perinuclear actin fibers. Furthermore, the formation of perinuclear actin fibers induces temporal rotational movement of the nucleus resulting in nuclear reorientation to the direction of migration. These results suggest that the network of dorsal fibers, transverse arcs, and perinuclear fibers transfers mechanical signal between the focal adhesions and nuclear envelope that regulates the nuclear reorientation in polarizing cells.
In polarized motile cells, stress fibers are the major mediators of cell contraction. Perinuclear actin cap fibers are arranged above the nucleus, in a pole-to-pole manner, and physically associate with the nuclear envelope. How these actin structures are formed and function in the regulation of cell polarization, however, is poorly understood. Maninová and Vomastek now show that dorsal fibers and transverse arcs, crosslinked by α-actinin-1, recruit ventral stress fibers and move them to the dorsal side to form the perinuclear actin cap. This actomyosin network mechanically links the nucleus with adhesions at the cell front and drives nuclear reorientation.