Multiple Mechanisms Inactivate the LIN-41 RNA-Binding Protein to Ensure A Robust Oocyte-to-Embryo Transition inCaenorhabditis elegans

Abstract

In the nematode Caenorhabditis elegans, the conserved LIN-41 RNA-binding protein is a translational repressor that coordinately controls oocyte growth and meiotic maturation. LIN-41 exerts these effects, at least in part, by preventing the premature activation of the cyclin-dependent kinase CDK-1. Here we investigate the mechanism by which LIN-41 is rapidly eliminated upon the onset of meiotic maturation. Elimination of LIN-41 requires the activities of CDK-1 and multiple SCF-type ubiquitin ligase subunits, including the conserved substrate adaptor protein SEL-10/Fbw7/Cdc4, suggesting that LIN-41 is a target of ubiquitin-mediated protein degradation. Within the LIN-41 protein, two non-overlapping regions, Deg-A and Deg-B, are individually necessary for LIN-41 degradation; both contain several potential phosphodegron sequences, and at least one of these sites is required for LIN-41 degradation. Finally, Deg-A and Deg-B are sufficient, in combination, to mediate SEL-10-dependent degradation when transplanted into a different oocyte protein. Although LIN-41 is a potent inhibitor of protein translation and M-phase entry, the failure to eliminate LIN-41 from early embryos does not result in the continued translational repression of LIN-41 oocyte mRNA targets. Based on these observations, we propose a molecular model for the elimination of LIN-41 by SCFSEL-10 and suggest that LIN-41 is inactivated before it is degraded. Furthermore, we provide evidence that another RNA-binding protein, the GLD-1 tumor suppressor, is regulated similarly. Redundant mechanisms to extinguish translational repression by RNA-binding proteins may both control and provide robustness to irreversible developmental transitions, including meiotic maturation and the oocyte-to-embryo transition.

Ubiquitin ligases and a processive proteasome facilitate protein clearance during the oocyte-to-embryo transition in Caenorhabditis elegans

“…Consequently, PUF-3::GFP and PUF-11::GFP expression appear to be largely absent from embryos (Figure 3A) or limited to a single young embryo (Figure 3D and Figure S3, A and B). A similar pattern is observed with GFP::LIN-41 and mSCARLET::LIN-41 (Spike et al . 2018; Figure 1A).…”

“…LIN-41 disappears rapidly upon the onset of meiotic maturation. We previously hypothesized that LIN-41 is phosphorylated at this stage and recognized by a SEL-10-containing E3 ubiquitin ligase that targets LIN-41 for proteolysis (Spike et al . 2018).…”

“…We therefore asked whether they are targeted for elimination by the same SCF E3 ubiquitin ligase. LIN-41 persists in early embryos after the RNAi-mediated knockdown of the highly similar Skp-related genes skr-1 and skr-2, the Cullin cul-1 , the F-box substrate recognition subunit sel-10 and in the sel-10(lf) mutants sel-10(ok1632) and sel-10(ar41) (Spike et al . 2018).…”

“…These somatic defects are not typically seen in lin-41(tn1541[gfp::s::lin-41]) animals at the same stage of adulthood (F), and the average brood size of lin-41(tn1541) animals is greater than 300 (Spike et al . 2014a, 2018). (I-N) GFP fluorescence (I, K, and M) and DIC (J, L, and N) images of 2-cell lin-41(tn2055[gfp::3xflag::lin-41]) embryos produced by otherwise wild-type (I and J), etc-1(gk5182) (K and L) and sel-10(ok1632) (M and N) parents.…”

“…2021b), suggesting its involvement as a component of a bistable developmental switch. Although it is not known whether LIN-41 can inhibit the let-7 pathway in C. elegans , it is clear that LIN-41 is a component of a bistable switch regulating oocyte meiotic maturation in this organism—LIN-41 inhibits CDK-1 activation, and in turn, active CDK-1 promotes the ubiquitin-mediated degradation of LIN-41 upon the onset of meiotic maturation (Spike et al . 2018).…”