Cleavage furrows formed between centrosomes lacking an intervening spindleand chromosomes contain microtubule bundles, INCENP, and CHO1 but not CENP-E

PtK1 cells containing two independent mitotic spindles can cleave between n eighboring centrosomes, in the absence of an intervening spindle, as well a s at the spindle equators. We used same-cell video, immunofluorescence, and electron microscopy to compare the structure and composition of normal equ atorial furrows with that of ectopic furrows formed between spindles. As in controls, ectopic furrows contained midbodies composed of microtubule bund les and an electron-opaque matrix. Despite the absence of an intervening sp indle and chromosomes, the midbodies associated with ectopic furrows also c ontained the microtubule-bundling protein CHO1 and the chromosomal passenge r protein INCENP. However, CENP-E, another passenger protein, was not found in ectopic furrows but was always present in controls. We also examined ce lls in which the ectopic furrow initiated but relaxed. Although relaxing fu rrows contained overlapping microtubules from opposing centrosomes, they la cked microtubule bundles as well as INCENP and CHO1. Together these data su ggest that the mechanism defining the site of furrow formation during mitos is in vertebrates does not depend on the presence of underlying microtubule bundles and chromosomes or on the stable association of INCENP or CHO1. Th e data also suggest that the completion of cytokinesis requires the presenc e of microtubule bundles and specific proteins (e.g., INCENP, CHO1, etc.) t hat do not include CENP-E.