Active positioning of the nucleus is integral to division, migration and differentiation of mammalian cells. Fibroblasts polarizing for migration orient their centrosomes by actin-dependent nuclear movement. This nuclear movement depends on nesprin-2 giant (N2G), a large, actin-binding outer nuclear membrane component of transmembrane actin-associated (TAN) lines that couple nuclei to moving actin cables. Here, we identify the diaphanous formin FHOD1 as an interaction partner of N2G. Silencing FHOD1 expression or expression of fragments containing binding sites for N2G or FHOD1 disrupted nuclear movement and centrosome orientation in polarizing fibroblasts. Unexpectedly, silencing of FHOD1 expression did not affect the formation or rearward flow of dorsal actin cables required for nuclear positioning. Rather, N2G-FHOD1 interaction provided a second connection to actin cables essential for TAN line formation and thus nuclear movement. These results reveal a unique function for a formin in coupling an organelle to actin filaments for translocation, and suggest that TAN lines require multi-point attachments to actin cables to resist the large forces necessary to move the nucleus.
Bibliographical noteFunding Information:
We thank D. Discher and M. Geyer for helpful discussion. This work was financially supported in part by the Deutsche Forschungsgemeinschaft (GRK1188 to S.K., grant FA 378/6-2 to O.T.F.) and NIH (grant GM099481 to G.G.G.). O.T.F. is a member of the CellNetworks Cluster of Excellence EXC81.
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