Saccharomyces cerevisiae Cells with Defective Spindle Pole Body Outer Plaques 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) frequently results in spindle misorientation and impaired nuclear migration, leading to the generation of bi- and multinucleated cells (40%). Electron microscopy indicated that CNM67 is required for proper formation of the SPB outer plaque, a structure that nucleates cytoplasmic (astral) microtubules. Interestingly, cytoplasmic microtubules that are essential for spindle orientation and nuclear migration are still present in cnm67Δ1 cells that lack a detectable outer plaque. These microtubules are attached to the SPB half- bridge throughout the cell cycle. This interaction presumably allows for low-efficiency nuclear migration and thus provides a rescue mechanism in the absence of a functional outer plaque. Although CNM67 is not strictly required for mitosis, it is essential for sporulation. Time-lapse microscopy of cnm67Δ1 cells with green fluorescent protein (GFP)-labeled nuclei indicated that CNM67 is dispensable for nuclear migration (congression) and nuclear fusion during conjugation. This is in agreement with previous data, indicating that cytoplasmic microtubules are organized by the half-bridge during mating.

Cnm67p是一种新型酵母蛋白，定位于微管组织中心——纺锤体极体（SPB）。CNM67（YNL225c）基因的敲除常导致纺锤体定向异常与核迁移受损，进而使40%的细胞形成双核或多核细胞。电子显微镜观测结果显示，CNM67是纺锤体极体外板（SPB outer plaque）正确组装所必需的，该结构负责胞质（星体）微管的成核。值得注意的是，在无法检测到外板的cnm67Δ1细胞中，纺锤体定向与核迁移所必需的胞质微管依然存在。这些微管在整个细胞周期中均附着于SPB半桥（SPB half-bridge）。此种相互作用或可介导低效率的核迁移，从而在功能性外板缺失时提供一种补救机制。尽管CNM67并非有丝分裂的必需基因，但其对于孢子形成至关重要。对带有绿色荧光蛋白（GFP）标记细胞核的cnm67Δ1细胞进行延时显微成像后发现，CNM67对于接合过程中的核迁移（核列队，congression）与核融合并非必需。这与此前的研究数据一致，即交配过程中胞质微管由半桥介导组装。