Stem cells support tissue maintenance, but the mechanisms that coordinate the rate of stem cell self-renewal with differentiation at a population level remain uncharacterized. We find that two PUF family RNA-binding proteins FBF-1 and FBF-2 have opposite effects on C. elegans germline stem cell dynamics: FBF-1 restricts the rate of meiotic entry, while FBF-2 promotes both cell division and meiotic entry rates. Antagonistic effects of FBFs are mediated by their distinct activities towards the shared set of target mRNAs, where FBF-1-mediated post-transcriptional control requires the activity of CCR4-NOT deadenylase, while FBF-2 is deadenylase-independent and might protect the targets from deadenylation. These regulatory differences depend on protein sequences outside of the conserved PUF family RNA-binding domain. We propose that the opposing FBF-1 and FBF-2 activities serve to modulate stem cell division rate simultaneously with the rate of meiotic entry.

Developmental transitions of germ cells are often regulated at the level of post-transcriptional control of gene expression. In the Caenorhabditis elegans germline, stem and progenitor cells exit the proliferative phase and enter meiotic differentiation to form gametes essential for fertility. The RNA binding protein GLD-1 is a cell fate regulator that promotes meiosis and germ cell differentiation during development by binding to and repressing translation of target messenger RNAs. Here, we discovered that some GLD-1 functions are promoted by binding to DLC-1, a small protein that functions as an allosteric regulator of multisubunit protein complexes. We found that DLC-1 is required to regulate a subset of GLD-1 target messenger RNAs and that DLC-1 binding GLD-1 prevents ectopic germ cell proliferation and facilitates gametogenesis in vivo. Additionally, our results reveal a new requirement for GLD-1 in the events of oogenesis leading to ovulation. DLC-1 contributes to GLD-1 function independent of its role as a light chain component of the dynein motor. Instead, we propose that DLC-1 promotes assembly of GLD-1 with other binding partners, which facilitates formation of regulatory ribonucleoprotein complexes and may direct GLD-1 target messenger RNA selectivity.

RNA-binding proteins FBF-1 and FBF-2 (FBFs) are required for germline stem cell maintenance in Caenorhabditis elegans and regulate the dynamics of progenitor cell proliferation and differentiation. The mechanisms controlling FBF protein levels remain unknown. We identified an interaction between both FBFs and CSN-5, a component of the COP9 (constitutive photomorphogenesis 9) signalosome. Here, we find that the MPN (Mpr1/Pad1 N-terminal) metalloprotease domain of CSN-5 interacts with the PUF RNA-binding domain of FBFs and the interaction is conserved for human homologs hPUM1 and hCSN5. This interaction likely takes place outside of the COP9 holoenzyme. csn-5 mutation results in destabilization of FBF proteins in germline stem cells leading to phenotypes consistent with a partial FBF loss of function. We propose that a COP9-independent role of CSN-5 in the PUF protein stability and function may be conserved across species, with implications for human stem cell biology and cancer.

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