Composition, remodeling and dynamics of the CPEB RNP

Abstract

Xenopus laevis oocytes are transcriptionally silent cells that require hormone stimulation for maturing into fecundation-competent eggs. Meiosis resumption underlying oocyte maturation is governed by sequential waves of protein synthesis, which in these cells is promoted by cytoplasmic polyadenylation, a mechanism that has been mostly studied for the CPEBs. CPEBs are a family of RNA binding proteins that can both promote translational repression in quiescent cells and translational activation in maturing oo- cytes. This dual activity has been shown to be regulated by post-translational modifica- tions that impact on their interactome, stability and aggregation properties.

Several studies have addressed the composition of the CPEB complexes in stage VI oo- cytes and their remodeling upon hormone stimulation. However, these studies show inconsistent and mutually exclusive results. In this regard, we have tailored an adapta- tion of the BioID methodology to identify the in vivo interactors of the CPEBs in both contexts. With this approach we have established novel links between the CPEBs and machineries associated to miRNA translational control, adenosine methylation and the CCR4-NOT complex. In addition, we show that all four CPEBs have a similar inter- actomic landscape in stage VI oocytes, though they have also interesting specificities.

To add another layer of complexity, we have characterized the phase separation proper- ties of the four CPEBs, a principle that is becoming increasingly relevant in all the pro- cesses regulating gene expression. We have found that CPEBl has clearly different LLPS properties compared to CPEB2-4, even though these three paralogs have also different material properties that need to be further addressed. We propose that these differences explain why there are four CPEBs with non-redundant functions in higher organisms.

Our findings pave the way for more specific research focused on the potential links between the CPEBs and other master translational regulators, both in repression and activation, as well as provide a starting point for a deeper understanding on how the composition and material properties of a condensate impact translational control.