The germ line in C. elegans is organized in an assembly line fashion, with each region containing cells at a specific point in development. As cells move proximally, they progress further through the developmental stages of gametogenesis. Presumably, different factors are expressed in each region to execute each developmental stage. As cells move proximally, some factors must be turned on, while others are turned off. We previously demonstrated that proteasomal degradation of proteins is key for cells to properly exit the proliferative fate and enter into meiotic prophase. Reduced proteasomal function enhances over-proliferation in a sensitized genetic background (Macdonald et al. 2008). We hypothesized that the proteasome is involved in degrading proteins that either promote the proliferative fate, or inhibit meiotic entry. We refer to these as proliferation promoting proteins (PPPs). To understand how proteasomal degradation contributes to maintaining the balance between proliferation and differentiation, we sought to identify these PPPs. To do this, we first focused on identifying the substrate recognition subunits (SRSs) of the E3 ubiquitin ligases to find those that may target the PPPs for degradation. Since the SRSs directly bind their target proteins, we reasoned that if we genetically identify the SRSs involved, we could then biochemically identify the PPPs as interacting proteins. We screened 826 SRSs by RNAi in four sensitized genetic backgrounds and found five proteins that enhance over-proliferation when their activities are reduced. One of these, RFP-1 (putative E3 ligase), enhances
glp-1(
ar202gf) to form a germline tumor, albeit incomplete. To identify the target proteins of RFP-1, and potential PPPs, we screened the interacting proteins of RFP-1 and looked for suppression of the germline tumor when their activity was reduced. Through this, we identified the chromo-domain containing protein MRG-1.
mrg-1(0) partially suppresses over-proliferation in
glp-1(
ar202gf);
rfp-1(
ok572) animals, suggesting that it may function as a PPP. We found that MRG-1 levels increase in an
rfp-1 mutant, as well as when proteasomal function is decreased through genetic mutation or chemical inhibition, suggesting that its activity is regulated, at least in part, through its levels of accumulation that is likely controlled through proteasomal-mediated degradation (Gupta et al. 2015). Analysis in tissue culture cells further supports that RFP-1 directly targets MRG-1 for degradation by the proteasome. Therefore, we have identified MRG-1 as a key player in regulating the proliferation vs. differentiation decision and determined that its activity is controlled, at least in part, through proteasomal degradation.