A maternally expressed Caenorhabditis elegans gene called
cyk-1 is required for polar body extrusion during meiosis and for a late step in cytokinesis during embryonic mitosis. Other microfilament- and microtubule-dependent processes appear normal in
cyk-1 mutant embryos, indicating that
cyk-1 regulates a specific subset of cytoskeletal functions. Because cytokinesis initiates normally and cleavage furrows ingress extensively in
cyk-1 mutant embryos, we propose that the wild-type
cyk-1 gene is required for a late step in cytokinesis. Cleavage furrows regress after completion of mitosis in
cyk-1 mutants, leaving multiple nuclei in a single cell. Positional cloning and sequence analysis of the
cyk-1 gene reveal that it encodes an FH protein, a newly defined family of proteins that appear to interact with the cytoskeleton during cytokinesis and in the regulation of cell polarity. Consistent with
cyk-1 function being required for a late step in embryonic cytokinesis, we show that the CYK-1 protein co-localizes with actin microfilaments as a ring at the leading edge of the cleavage furrow, but only after extensive furrow ingression. We discuss our findings in the context of other studies suggesting that FH genes in yeast and insects function early in cytokinesis to assemble a cleavage furrow.