Two defining characteristics of muscle cells are the many precisely positioned nuclei and the linearly arranged sarcomeres, yet the relationship between these two features is not known. We show that nuclear positioning precedes sarcomere formation. Furthermore, ZASP-GFP, a Z-line protein, colocalizes with F-actin in puncta at the cytoplasmic face of nuclei before sarcomere assembly. In embryos with mispositioned nuclei, ZASP-GFP is still recruited to the nuclei before its incorporation into sarcomeres. Furthermore, the first sarcomeres appear in positions close to the nuclei, regardless of nuclear position. These data suggest that the interaction between sarcomere proteins and nuclei is not dependent on properly positioned nuclei. Mechanistically, ZASP-GFP localization to the cytoplasmic face of the nucleus did require the linker of nucleoskeleton and cytoskeleton (LINC) complex. Muscle-specific depletion of klarsicht (nesprin) or klariod (SUN) blocked the recruitment of ZASP-GFP to the nucleus during the early stages of sarcomere assembly. As a result, sarcomeres were poorly formed and the general myofibril network was less stable, incomplete, and/or torn. These data suggest that the nucleus, through the LINC complex, is crucial for the proper assembly and stability of the sarcomere network.
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