In humans perturbations of centriole number are associated with tumorigenesis and microcephaly, therefore appropriate regulation of centriole duplication is critical. The C. elegans homolog of Plk4, ZYG-1, is required for centriole duplication, but our understanding of how ZYG-1 levels are regulated remains incomplete. We have identified the two PP1 orthologs, GSP-1 and GSP-2, and their regulators I-2SZY-2 and SDS-22 as key regulators of ZYG-1 protein levels. We find that down-regulation of PP1 activity either directly, or by mutation of szy-2 or sds-22 can rescue the loss of centriole duplication associated with a zyg-1 hypomorphic allele. Suppression is achieved through an increase in ZYG-1 levels, and our data indicate that PP1 normally regulates ZYG-1 through a post-translational mechanism. While moderate inhibition of PP1 activity can restore centriole duplication to a zyg-1 mutant, strong inhibition of PP1 in a wild-type background leads to centriole amplification via the production of more than one daughter centriole. Our results thus define a new pathway that limits the number of daughter centrioles produced each cycle.

Centrosomes are major determinants of mitotic spindle structure, but the mechanisms regulating their behavior remain poorly understood. The spd-2 gene of C. elegans is required for centrosome assembly or “maturation.” Here we show that spd-2 encodes a coiled-coil protein that localizes within pericentriolar material (PCM) and in the immediate vicinity of centrioles. During maturation, SPD-2 gradually accumulates at the centrosome in a manner that is partially dependent on Aurora-A kinase and cytoplasmic dynein. Interestingly, SPD-2 interacts genetically with dynein heavy chain and SPD-5, another coiled-coil protein required for centrosome maturation. SPD-2 and SPD-5 are codependent for localization to the PCM, but SPD-2 localizes to centrioles independently of SPD-5. Surprisingly, we also find that SPD-2 is required for centrosome duplication and genetically interacts with ZYG-1, a kinase required for duplication. Thus, we have identified SPD-2 as a factor critical for the two basic functions of the centrosome—microtubule organization and duplication.

Accurate cell division is crucial to cellular and organismal viability and mistakes can have serious consequences such as developmental defects or disease. Successful cell division results in the production of two daughter cells, each possessing a complete and accurately replicated copy of the parental genome. This is achieved through segregation of the replicated chromosomes on the bipolar mitotic spindle and subsequent cellular fission to produce two daughter cells. Here, we review how use of the model organism Caenorhabditis elegans has contributed to our understanding of cell division.

Centrosome duplication is a critical step in assembly of the bipolar mitotic spindle, but the molecular mechanisms regulating this process during the cell cycle and during animal development are poorly understood. Here, we report that the zyg-1 gene of Caenorhabditis elegans is an essential regulator of centrosome duplication. ZYG-1 is a protein kinase specifically required for daughter centriole formation that localizes transiently to centrosomes and acts at least one cell cycle prior to each spindle assembly event. In the embryo, ZYG-1 participates in a unique regulatory scheme whereby paternal ZYG-1 regulates duplication and bipolar spindle assembly during the first cell cycle, and maternal ZYG-1 regulates these processes thereafter. ZYG-1 is therefore a key molecular component of the centrosome/centriole duplication process.