ZYG-9, A Caenorhabditis elegans Protein Required for Microtubule Organization and Function, Is a Component of Meiotic and Mitotic Spindle Poles

We describe the molecular characterization of zyg-9, a maternally acting gene essential for microtubule organization and function in early Caenorhabditis elegans embryos. Defects in zyg-9 mutants suggest that the zyg-9 product functions in the organization of the meiotic spindle and the formation of long microtubules. One-cell zyg-9 embryos exhibit both meiotic and mitotic spindle defects. Meiotic spindles are disorganized, pronuclear migration fails, and the mitotic apparatus forms at the posterior, orients incorrectly, and contains unusually short microtubules. We find that zyg-9 encodes a component of the meiotic and mitotic spindle poles. In addition to the strong staining of spindle poles, we consistently detect staining in the region of the kinetochore microtubules at metaphase and early anaphase in mitotic spindles. The ZYG-9 signal at the mitotic centrosomes is not reduced by nocodazole treatment, indicating that ZYG-9 localization to the mitotic centrosomes is not dependent upon long astral microtubules. Interestingly, in embryos lacking an organized meiotic spindle, produced either by nocodazole treatment or mutations in the mei-1 gene, ZYG-9 forms a halo around the meiotic chromosomes. The protein sequence shows partial similarity to a small set of proteins that also localize to spindle poles, suggesting a common activity of the proteins.