The dynactin complex is required for cleavage plane specification in early Caenorhabditis elegans embryos.

During metazoan development, cell diversity arises primarily from asymmetric cell divisions which are executed in two phases: segregation of cytoplasmic factors and positioning of the mitotic spindle - and hence the cleavage plane -relative to the axis of segregation. When polarized cells divide, spindle alignment probably occurs through the capture and subsequent shortening of astral microtubules by a site in the cortex. Here, we report that dynactin, the dynein-activator complex, is localized at cortical microtubule attachment sites and is necessary for mitotic spindle alignment in early Caenorhabditis elegans embryos. Using RNA interference techniques, we eliminated expression in early embryos of dnc-1 (the ortholog of the vertebrate gene for p150(Glued)) and dnc-2 (the ortholog of the vertebrate gene for p50/Dynamitin). In both cases, misalignment of mitotic spindles occurred, demonstrating that two components of the dynactin complex, DNC-1 and DNC-2, are necessary to align the spindle. Dynactin complexes may serve as a tether for dynein at the cortex and allow dynein to produce forces on the astral microtubules required for mitotic spindle alignment.