The S. cerevisiae cell cycle regulator CDC53 functions to degrade G1 cyclins and cyclin-dependent kinase inhibitors (CKIs) through ubiquitin-mediated proteolysis. The ancestral CDC53 evolved into the higher eukaryotic
cul-1 and
cul-2 genes. Loss-of-function
cdc53 mutants undergo cell cycle arrest in the G1 phase of the cell cycle, as the CKI Sic1p is not degraded and inhibits cell cycle progression. Unlike its yeast homolog, C. elegans
cul-1 null mutations cause hyperplasia rather than cell cycle arrest. We speculate that
cul-1 is involved in the degradation of G1 cyclins, however, the hyperplastic phenotype makes it unlikely that
cul-1 targets CKIs for degradation (as CDC53 does). We hypothesize that
cul-2 might function to degrade CKIs, thereby acting as a positive cell cycle regulator. To test this, antisense
cul-2 RNA was injected into the gonads of young adult worms. We observed that 100% of the progeny of the injected animals arrested at an early embryonic stage (ca 24 cells). To test whether the loss of
cul-2 resulted in cell cycle arrest because of an inability to degrade CKIs, we cloned two CKI cDNAs (
cki-1 and
cki-2 corresponding to T05A6.1 and T05A6.2). When antisense
cul-2 RNA was injected with antisense RNA for either
cki-1 or
cki-2, early embryonic arrest was still observed. However, when antisense
cul-2 RNA was injected along with both
cki-1 and
cki-2 antisense RNAs, the early embryonic arrest was rescued. We conclude that
cul-2 functions to negatively regulate both
cki-1 and
cki-2, which apparently have redundant functions in the early embryo. Interestingly, injection of
cki-1 antisense RNA alone produces postembryonic hyperplasia (similar to the
cul-1 mutant phenotype), suggesting that
cki-1 is required non-redundantly to negatively regulate cell cycle progression in certain tissues.