In searching for RING finger proteins in C. elegans that resemble mammalian RINGs known to exhibit E3 ligase activities, we characterized
rnf-5 (C16C10.7), the nematode homolog of RNF5, an unknown protein that shares features within the RING domain with BRCA1, Cbl and Mdm2. Three different spliced transcripts of
rnf-5 were expressed during development, with the highest expression in young adults. Interestingly, L2d pre-dauer larvae preferentially expressed an
rnf-5 mRNA that lacks the RING domain. We used RNAi to determine the function of
rnf-5 during C. elegans development. We found that
rnf-5(RNAi)-treated worm populations showed an increased fraction of dauer and dauer-like larvae. Adult progeny of animals treated with
rnf-5 dsRNA had an overcrowed germ line containing a higher density of germ cell nuclei. We also noticed a weak egg-laying defect in these animals. Over-expression of
rnf-5 resulted in slower growth. In vitro ubiquitination assays revealed that RNF-5 exhibits E3 ligase activity, as reflected by efficient self-ubiquitination upon the addition of E1, E2 and ubiquitin. The E3 ligase activity of RNF-5 was confirmed in 293T cells, which exhibit a high basal level of ubiquitination under normal growth conditions. A marked decrease in RNF-5 ubiquitination was noted following UV-irradiation, suggesting that stress inhibits the E3 ligase activity of the protein. PI3K
p110 or AKT efficiently inhibit RNF-5 E3 ligase activity, suggesting that RNF-5 might be regulated by the insulin pathway. Taken together, our results identify a new RING finger protein that exhibits E3 ligase activity and is likely important to the regulation of cell proliferation and development in C. elegans.