Abstract
Polycomb group (PcG) proteins are evolutionarily conserved chromatin modifiers that regulate developmental pathways in plants. PcGs form nuclear multi-subunit Polycomb Repressive Complexes (PRCs). The PRC2 complex mediates gene repression via methylation of lysine 27 on histone H3, which consequently leads to chromatin condensation. In Arabidopsis thaliana, several PRC2 complexes with different compositions were identified, each controlling a particular developmental program.The core subunit FIE is crucial for PRC2 function throughout the plant life cycle, yet accurate information on its spatial and temporal localization was absent. This study focused on identifying FIE accumulation patterns, using microscopy and biochemical approaches. Analysing endogenous FIE and transgenic gFIE-green fluorescent protein fusion protein (gFIE-GFP) showed that FIE accumulates in the nuclei of every cell type examined. Interestingly, gFIE-GFP, as well as the endogenous FIE, also localized to the cytoplasm in all examined tissues. In both vegetative and reproductive organs, FIE formed cytoplasmic high-molecular-mass complexes, in parallel to the nuclear PRC2 complexes. Moreover, size-exclusion chromatography and bimolecular fluorescence complementation assays indicated that in inflorescences FIE formed a cytoplasmic complex with MEA, a PRC2 histone methyltransferase subunit. In contrast, CLF and SWN histone methyltransferases were strictly nuclear. Presence of PRC2 subunits in cytoplasmic complexes has not been previously described in plants. Our findings are in agreement with accumulating evidence demonstrating cytoplasmic localization and function of PcGs in metazoa. The cytosolic accumulation of PRC2 components in plants supports the model that PcGs have alternative non-nuclear functions that go beyond chromatin methylation.
Highlights
Polycomb group proteins (PcGs) are highly conserved chromatin modifiers that regulate key developmental pathways in plants (Kohler and Villar, 2008; Derkacheva and Hennig, 2014) and animals (Schwartz and Pirrotta, 2007, 2008), selectively controlling temporal and spatial expression of numerous genes
FERTILIZATION INDEPENDENT ENDOSPERM (FIE) transcript was detected during ovule development and after fertilization in both the embryo and endosperm (Kiyosue et al, 1999)
In Arabidopsis, members of the three PRC2 complexes were shown to play a vital role in nearly every stage of the plant life cycle, as evident from the severe phenotypes of pcg mutants (Mozgova et al, 2015)
Summary
Polycomb group proteins (PcGs) are highly conserved chromatin modifiers that regulate key developmental pathways in plants (Kohler and Villar, 2008; Derkacheva and Hennig, 2014) and animals (Schwartz and Pirrotta, 2007, 2008), selectively controlling temporal and spatial expression of numerous genes. The PRC2 complex, conserved in both animals (Korf et al, 1998; Tie et al, 2001; Poux et al, 2001) and plants (Goodrich et al, 1997; Grossniklaus et al, 1998; Luo et al, 1999; Ohad et al, 1999), is responsible for initiating gene silencing by catalysing trimethylation of H3K27 on target loci (Czermin et al, 2002; Muller et al, 2002; Cao et al, 2002, 2008; Kuzmichev et al, 2002; Bouyer et al, 2011; Kim et al, 2012). Four core components of the PRC2 complex were identified in Drosophila: enhancer of zeste (E(z)); extra sex comb (ESC); suppressor of zeste 12 (Su(z)12) and the nucleosome remodeling factor 55-kDa (p55) (Martinez-Balbas et al, 1998; Czermin et al, 2002; Muller et al, 2002)
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