Figure 5.

Model for a salvage mechanism for spindle orientation and nuclear migration in cnm67Δ1 cells. (A) Wild-type: SPB half-bridges and bridges bind cytoplasmic microtubules only when the satellite forms and SPB duplication takes place. After SPB separation, cytoplasmic microtubules are no longer found at the half-bridge (Byers and Goetsch, 1975). During all cell cycle stages, the outer plaque acts as an attachment site. The first phase of nuclear migration begins in S when cytoplasmic microtubules originate from the bridge of side-by-side SPBs. The second phase of nuclear migration in G2/M coincides with cytoplasmic microtubule attachment at the outer plaque. (B) In cnm67Δ1 cells cytoplasmic microtubules originate from the half-bridge throughout the cell cycle while the outer plaque is much reduced or absent and thus incapable of microtubule binding. Consequently, the second phase of nuclear migration is impaired, frequently leading to nuclear division within the mother cell. However, half-bridge–organized cytoplasmic microtubules still have a limited capability to mediate spindle positioning and nuclear migration forces that allow many cells to finally migrate the daughter nucleus into the bud. Cells in which nuclear migration failed double in ploidy and often undergo another round of nuclear division.