Interaction of the Arabidopsis Polycomb group proteins FIE and MEA mediates their common phenotypes

Genes of the FERTILISATION INDEPENDENT SEED (FIS) class regulate cell proli feration during reproductive development in Arabidopsis [1-5], The FIS gene s FERTILISATION INDEPENDENT ENDOSPERM (FIE) and MEDEA (MEA) encode homologs of animal Polycomb group (Pc-G) proteins, transcriptional regulators that modify chromatin structure and are thought to form multimeric complexes [3- 11]. To test whether similarities in fis mutant phenotypes reflect interact ions between their protein products, we characterised FIE RNA and protein l ocalisation in vivo, and FIE protein interactions in yeast and in vitro. Ex pression of FIE mRNA overlaps with that of MEA during embryo sac and seed d evelopment and is unaffected in mea mutants. Results from the yeast two-hyb rid system and an in vitro pull-down assay indicate that MEA and FIE protei ns interact, The relevance of this interaction in vivo is supported by the finding that FIE and MEA co-localise in the nucleus in transfected plant ce lls. Interaction of MEA and FIE is mediated by the amino-terminal region of MEA. Despite sequence divergence in this domain, MEA can interact with its corresponding animal partner Extra sex combs (ESC) in the yeast two-hybrid system. We conclude that FIE and MEA act together as part of a multimeric complex and that this accounts for the similarities in mutant phenotypes. W e propose that an ancient mechanism for chromatin modification has been ind ependently recruited to different developmental processes in the two kingdo ms.