The microtubule-organizing center (MTOC) is reoriented between the nucleus and the leading edge in many migrating cells and contributes to directional migration. Models suggest that the MTOC is moved to its position during reorientation. By direct imaging of wound-edge fibroblasts after triggering MTOC reorientation with soluble factors, we found instead that the nucleus moved away from the leading edge to reorient the MTOC, while the MTOC remained stationary. Rearward nuclear movement was coupled with actin retrograde flow and was regulated by a pathway involving Cdc42, MRCK, myosin, and actin. Nuclear movement was unaffected by the inhibition of dynein, Par6, or PKCzeta, yet these components were essential for MTOC reorientation, as they maintained the MTOC at the cell centroid. These results show that nuclear repositioning is an initial polarizing event in migrating cells and that the positions of the nucleus and the MTOC are established by separate regulatory pathways.

Nuclei move to specific locations to polarize migrating and differentiating cells. Many nuclear movements are microtubule-dependent. However, nuclear movement to reorient the centrosome in migrating fibroblasts occurs through an unknown actin-dependent mechanism. Here, we found that linear arrays of outer (nesprin2G) and inner (SUN2) nuclear membrane proteins assembled on and moved with retrogradely moving dorsal actin cables during nuclear movement in polarizing fibroblasts. Inhibition of nesprin2G, SUN2 or actin prevented nuclear movement and centrosome reorientation. The coupling of actin cables to the nuclear membrane for nuclear movement via specific membrane proteins indicates that like plasma membrane integrins, nuclear membrane proteins assemble into actin-dependent arrays for force transduction.