SACCHAROMYCES-CEREVISIAE CELLS WITH DEFECTIVE SPINDLE POLE BODY OUTERPLAQUES ACCOMPLISH NUCLEAR MIGRATION VIA HALF-BRIDGE-ORGANIZED MICROTUBULES

Cnm67p, a novel yeast protein, localizes to the microtubule organizing center, the spindle pole body (SPB). Deletion of CNM67 (YNL225c) freq uently results in spindle misorientation and impaired nuclear migratio n, leading to the generation of bi-and multinucleated cells (40%). Ele ctron microscopy indicated that CNM67 is required for proper formation of the SPB outer plaque, a structure that nucleates cytoplasmic (astr al) microtubules. Interestingly, cytoplasmic microtubules that are ess ential for spindle orientation and nuclear migration are still present in cnm67 Delta 1 cells that lack a detectable outer plaque. These mic rotubules are attached to the SPB half-bridge throughout the cell cycl e. This interaction presumably allows for low-efficiency nuclear migra tion and thus provides a rescue mechanism in the absence of a function al outer plaque. Although CNM67 is not strictly required for mitosis, it is essential for sporulation. Time-lapse microscopy of cnm67 Delta 1 cells with green fluorescent protein (GFP)-labeled nuclei indicated that CNM67 is dispensable for nuclear migration (congression) and nucl ear fusion during conjugation. This is in agreement with previous data , indicating that cytoplasmic microtubules are organized by the half-b ridge during mating.