Molecular Interactions Among Polycomb Protein CBX Family Members

Polycomb Group(PcG)proteins,a family of master epigenetic regulators,regulate gene expression mainly through repression of gene transcription.PcG proteins play crucial roles in controlling a variety of cellular physiological and pathological processes,such as maintenance of stemness,cell differentiation,cell cycle control,cell senescence,X chromosome inactivation,genomic imprinting,tumorigenesis etc.The best characterized protein complexes composed of PcG proteins are Polycomb Repressive Complexes 1 and 2(PRC1 and PRC2).They act synergistically in chromatin recognition and transcription regulation at their common target genes.As one of the core components in PRC1,CBX protein facilitates the recruitment and stabilization of PRC1 to target chromatin.In mammals,there are five CBX homologs.Although there are structural similarities and functional redundancies among CBX proteins,each member does have its unique features which result in their specific functions in a given cell type.In this study,we characterized the interactions between various CBX protein pairs by employing Bimolecular Fluorescence Complementation(BiFC)experiments.In addition,we cloned a novel splicing variant of mouse Cbx7 and analyzed its expression.The main results are listed as followed:(1)Various extents of BiFC fluorescent signals were detected between CBX2 protein and CBX7,CBX4,and CBX2 itself,suggesting that there were interactions between these factors;(2)The region in CBX7 that involed in its interaction to CBX2 was confirmed to be located in the ATHL domain;(3)The CDS of mCBX7 Isoform C was successfully cloned,the antibody specifically detecting this isoform was further prepared and the results from Western Blot analyses detected the protein product of this isoform.This research has laid the foundation of the exploration of different ways and significance of CBX proteinin the process of embryonic stem cell proliferation and differentiation.In addition,further clarification of each CBX protein in physiological processes such as embryonic development will help to explain the developmental defects and pathogenic mechanisms behind tumor formation triggered by the dysregulation of CBX protein,and provide valuable new targets for early diagnosis,treatment and prognosis of related human diseases in the near future.