RNA-binding proteins FBF-1 and FBF-2 are required for stem cell maintenance in C. elegans, although the mechanisms by which FBF protein levels are regulated remain unknown. Using a yeast two-hybrid screen, we identified an interaction between both FBFs and CSN-5, a component of the COP9 (constitutive photomorphogenesis 9) signalosome. This highly conserved COP9 complex can affect protein stability through a range of mechanisms including deneddylation, deubiquitination, and phosphorylation (Wolf et al., 2003). We discovered that CSN-5 promotes the accumulation of FBF-1 and FBF-2 proteins in C. elegans stem and progenitor cells. Phenotypic analysis results were consistent with
csn-5 contributing to FBF function since
csn-5 germlines are masculinized (produce only sperm similar to
fbf-1/2 loss of function) and show reduced numbers of stem cells. Similar phenotypes were observed in worms mutant for another COP9 holoenzyme component,
csn-6. Curiously, phenotypes of the
csn-2 mutant were clearly distinct, where oocytes were still forming and stem cell numbers were not as affected. Additionally, FBF protein levels were not as affected in the
csn-2 mutant as observed in the
csn-5 mutant. This suggests that
csn-5's effect on FBFs might be independent of the COP9 holoenzyme. Mapping protein-protein interactions between FBFs and CSN-5 suggested that the MPN (Mpr1/Pad1 N-terminal) metalloprotease domain of CSN-5 interacts with the RNA-binding domain of FBFs at physiologically relevant (micromolar) concentrations. Furthermore, these conserved domains of the human homologs PUM1 and CSN5 interact as well, thus identifying a protein complex conserved in evolution. Investigating CSN-5 contribution to FBF protein activity and stem cell maintenance will have implications for human stem cell biology and improve our understanding of diseases like cancer.