The centrosome serves as the primary microtubule organizing center (MTOC) of the cell and plays an important role in establishing a bipolar mitotic spindle. To ensure spindle bipolarity, centrosome duplication must be precisely regulated. Previously the kinase ZYG-1 was identified as a regulator of centrosome duplication in C. elegans (O"Connell et al., 2000). In
zyg-1 mutants, centriole replication fails and results in the formation of monopolar spindles. To further our knowledge of this process, we sought to identify additional factors that interact with ZYG-1 to regulate centrosome duplication. Toward this end, we carried out a screen for mutations that suppress the temperature sensitive lethality of the
zyg-1 (
it25) mutation. Among the suppressors identified is
bs52, a mutation that defines a gene called
szy-20 (for suppressor of
zyg-1). The
szy-20 encodes a novel coiled-coil protein that localizes to the nucleus, nuclear periphery, and cytoplasm. The
szy-20 (
bs52) mutation itself exhibits a strong temperature-sensitive embryonic lethality, marked by defects in a variety of processes including polar body formation, pronuclear migration and rotation, polarity, cytokinesis, and cell cycle progression. We further found that
zyg-1 (
it25) mutation also suppresses the embryonic lethality of the
szy-20 (
bs52). Thus, there is mutual suppression between these two genes. That is while the
szy-20 (
bs52) mutation restores centriole assembly to the
zyg-1 (
it25) mutant, the
zyg-1 (
it25) mutation rescues the pronuclear rotation and cytokinesis defects seen in
szy-20 (
bs52) mutants. To understand how
szy-20 interacts with
zyg-1 as a suppressor, we have analyzed centrosomes in the
szy-20 (
bs52) mutant and found that the amount of pericentriolar material (PCM) is significantly increased. Our data indicate that SZY-20 functions to limit centrosome size by acting as a negative regulator of PCM recruitment. Thus, loss of SZY-20 activity likely restores centriole assembly in
zyg-1 mutants by allowing increased amounts of ZYG-1 to accumulate at centrosomes.