cyk-1 encodes an FH protein required for cytokinesis in C. elegans embryos. Embryos homozygous for weak alleles of
cyk-1 form cleavage furrows which frequently ingress extensively through the cytoplasm before failing. CYK-1 protein localizes cortically in cleavage furrows, but is detectable only after substantial ingression. We have proposed a model in which CYK-1 functions late in cytokinesis to stabilize cleavage furrows through interactions with the contractile ring and mitotic spindle, and that CYK-1 requires these interactions for its localization. We have tested this model in three ways: analysis of a stronger
cyk-1 allele, double mutant analysis, and immunolocalization experiments. Worms homozygous for
cyk-1(
s2833) , our strongest allele, are sterile, so the cytokinesis defect observed in weak alleles may not be the null phenotype. Embryos from
cyk-1(
s2833)/cyk-1
(t1568) mutant worms fail to furrow. Thus,
cyk-1 may also be required for an early step in cytokinesis. Further evidence that
cyk-1 functions early in cytokinesis comes from analysis of
cyk-1;
zen-4 double mutant embryos. ZEN-4 is a mitotic kinesin-like protein that localizes to the spindle interzone.
zen-4(
or153ts) mutant embryos produce extensive furrows that ultimately regress, similar to the weak
cyk-1(
or36) phenotype. In contrast to either single mutant,
cyk-1(
or36);
zen-4(
or153ts) double mutant embryos fail to form cleavage furrows. We conclude that
cyk-1 interacts with the ZEN-4 kinesin-like protein, and these genes function early in cytokinesis. To further test our model, we examined CYK-1 localization in
zen-4 mutant embryos. CYK-1 accumulates normally in
zen-4 mutant embryos; thus, function of this spindle-associated protein is not required for CYK-1 localization. Additionally, inactivation of
mlc-4 , a non-muscle myosin II regulatory light chain required for cleavage furrow ingression, does not prevent cortical CYK-1 localization. Thus, proximity between the cleavage furrow and the spindle are not required for CYK-1 localization. We are currently performing experiments to identify requirements for CYK-1 localization and to understand the genetic interaction between
cyk-1 and
zen-4 .