The augmin complex generates new microtubules in anaphase to build the central spindle and complete cytokinesis, Uehara and Goshima report.
The central spindle forms between segregating chromosomes in anaphase and is required for the subsequent cleavage of cells into two daughters. Whether this structure is formed purely by microtubules recycled from the metaphase spindle or whether new microtubules are also needed is unclear. Uehara and Goshima depolymerized existing microtubules in anaphase cells with nocodazole and a low temperature, and saw that new filaments formed between chromosomes after the drug was removed, generating a functional central spindle from scratch.
Fewer microtubules formed in nocodazole-treated cells lacking augmin, a protein complex required for central spindle assembly and cytokinesis. In metaphase cells, augmin amplifies the number of spindle microtubules by recruiting γ-tubulin to the mitotic spindle to initiate the assembly of new filaments. In anaphase, however, the researchers found that augmin builds upon a combination of preexisting filaments, centrosome-generated microtubules, and/or new filaments nucleated from chromosomes by the protein HURP. HURP was essential for central spindle formation in anaphase cells treated with nocodazole, but was less important in untreated cells that could reuse their metaphase microtubules as templates for augmin amplification.
Without augmin, anaphase cells still assemble some microtubules in their central region, but they can't complete cytokinesis. Senior author Gohta Goshima now wants to investigate why cell cleavage requires augmin-dependent expansion of the central spindle. Of note, one of augmin's subunits is frequently mutated in breast cancer, potentially causing cytokinesis failure and polyploidy.
