In C. elegans, inactivation of the
cul-1 gene causes hyperplasia of all tissues.
cul-1 is required for cells to transit from the G1 phase of the cell cycle to the G0 phase or the apoptotic pathway. Structural analysis of the CUL-1 protein reveals that CUL-1 is a homolog of yeast ubiquitin ligase Cdc53, which is required for the ubiquitin-mediated degradation of G1 phase cyclins and cyclin-dependent kinase inhibitors. We propose that CUL-1 targets the degradation of G1 phase cyclins, and that the
cul-1 mutant phenotype is due to an increase in the level of G1 phase cyclins. The Cul-1 phenotype, with abnormally small cells and an inability to respond to differentiation signals, is reminiscent of yeast cyclin mutants with elevated G1 cyclin levels due to truncation of the C-terminal cyclin PEST regions that are required for their proteolysis. There are two classes of G1 phase cyclins in vertebrates, cyclin D and cyclin E. Homology searches revealed that C. elegans contains a homolog of vertebrate cyclin D located on YAC Y38F1, and a homolog of vertebrate cyclin E on cosmid C37A2.4. There are no other known genes in C. elegans that have specific homology to vertebrate cyclin D and cyclin E. We propose that these two C. elegans genes are orthologs of cyclin D and cyclin E. To study the role these two cyclins play in the organism, we injected cyclin E dsRNA into wild type C. elegans to inhibit zygotic gene expression and maternal mRNA function. We observed embryonic arrest with several hundred cells after the injection. Although there was some differentiation in the embryos as indicated by muscle cell twisting, there was no overt morphogenesis. These results indicate that the C. elegans cyclin E homolog is required for embryonic development. Injection of cyclin D dsRNA will be done in the near future. We are currently screening for cyclin D and cyclin E deletion mutants in C. elegans. Once we isolate the mutants, we intend to construct
cul-1, cyclin D and
cul-1, cyclin E double mutants to test for genetic interaction. In particular, we will determine whether reduced levels of cyclins can rescue
cul-1 mutants, as might be expected if an increase in cyclin level is responsible for the Cul-1 hyperplasia. We also plan to measure the level of Cyclin protein in
cul-1 mutants.