Dynamic protein interactions of the Polycomb Repressive Complex 2

Polycomb proteins assemble to form complexes with important roles in epigenetic regulation. The Polycomb Repressive Complex 2 (PRC2) modulates the di- and tri-methylation of lysine 27 on histone H3, each of which are associated with gene repression. Although three subunits, EZH1/2, SUZ12 and EED, form the catalytic core of PRC2, a wider group of proteins associate with low stoichiometry. This raises the question of whether dynamic variation of the PRC2 interactome results in alternative forms of the complex during differentiation. Here we compared the physical interactions of PRC2 in undifferentiated and differentiated states of NTERA2 pluripotent embryonic carcinoma cells. Label-free quantitative proteomics was used to assess endogenous immunoprecipitation of the EZH2 and SUZ12 subunits of PRC2. A high stringency dataset reflecting the endogenous state of PRC2 was produced that included all previously reported ‘core’ and ‘associated’ PRC2 components, and several novel interacting proteins. Comparison of the interactomes obtained in undifferentiated and differentiated cells revealed candidate proteins that favoured either one or other state. For example, SALL4 and ZNF281 associate with PRC2 in pluripotent cells, while PCL1 and SMAD3 more associate with PRC2 in differentiating cells. Analysis of the mRNA and protein levels of these factors found that their association with PRC2 correlated with their cell state-specific expression. Taken together, we propose that dynamic changes to the PRC2 interactome during differentiation may contribute to directing its activity during cell fate transitions.