Mechanistic studies of Polycomb group proteins

Most cells within multicellular organisms contain the same genetic information, yet the appropriate tissue-specific expression of genes is required for the proper formation of adult tissues. Genes can either be "turned on" or "turned off" from the initial zygotic state and maintained during subsequent cell divisions. Maintaining the correct expression profiles during cell divisions is accomplished by a number of different nuclear factors. One of the key families of proteins that maintains the repression of target genes during development is the Polycomb group (PcG) of proteins. PcG proteins form a number of different multi-subunit protein complexes that interact with specific regions of chromatin and direct the repression of nearby genes by reducing transcription. One PcG complex, Polycomb repressive complex 1 (PRC1), inhibits transcription and nucleosome remodeling as well as compacts chromatin, both in vivo and in vitro. The in vitro repressive activities map mainly to one subunit of Drosophila PRC1--the Posterior sex combs (PSC) protein. The PRC1 complex is conserved in many other organisms including mammals. To better understand the mechanisms involved in PcG mediated repression we undertook a biochemical structure/function analysis of mouse PRC1. In chapter one, I review the current understanding of PcG biology and a rationale for the dissertation is provided. In chapter two, data are presented that argues that a mouse PRC1 protein, M33/Cbx2, which is non-homologous to PSC, is responsible for chromatin compaction and repression of nucleosome remodeling. Data are presented that suggests these activities are localized to a basic, natively unfolded region of M33/Cbx2. In chapter three, we extend the findings from chapter two in an attempt to predict whether homologous PcG proteins from other species besides fly and mouse have biochemical activity. In agreement with predictions, a panel of recombinant PcG proteins was generated and data are presented that shows the predicted active PcG proteins are capable of both inhibition of nucleosome remodeling and compaction of chromatin. Finally, in chapter four, the implications of the data presented are discussed, and directions for further inquiry are explored.