Microtubule organization in the final stages of cytokinesis as revealed by cryo-electron tomography.

The completion of cytokinesis is dominated by the midbody, a tightly-packed microtubule (MT)-based bridge that transiently connects the two daughter cells. Assembled from condensed, spindle-MTs and numerous associated proteins, the midbody gradually narrows down until daughter cell partitioning occurs at this site. Although described many years ago, detailed understanding of the abscission process remains lacking. Applying cryo-electron tomography to purified midbodies, in combination with fluorescence microscopy, we present here new insight into MT organization within the midbody. We find that the midbody is spatially divided into a core bundle of MTs that traverses the electron-dense overlap region (continuous MTs), surrounded by MTs that terminate within the overlap region (polar MTs). Residual continuous MTs remained intact up to the verge of abscission, whereas the residual polar MTs lost their organization and retreated from the overlap region at late cytokinesis stages. A detailed localization of the microtubule-bundling protein PRC1 supports the above notion. Our study thus provides a detailed account of the abscission process and suggests that the midbody, having acquired a distinct MT architecture as compared to the preceding central spindle, actively facilitates the final stage of cytokinesis.