The Core of the Polycomb Repressive Complex Is Compositionally and Functionally Conserved in Flies and Humans

The Polycomb group (PcG) genes are required to maintain homeotic genes in a silenced state during development in drosophila and mammals and are thought to form several distinct silencing complexes that maintain homeotic gene repression during development. Mutations in the PcG genes result in developmental defects and have been implicated in human cancer. Although some PcG protein domains are conserved between flies and humans, substantial regions of several PcG proteins are divergent and humans contain multiple versions of each PcG gene. To determine the effects of these changes on the composition and function of a PcG complex, we have purified a human Polycomb repressive complex from HeLa cells (hPRC-H) that contains homologues of PcG proteins found in drosophila embryonic PRC1 (dPRC1). hPRC-H was found to have fewer components than dPRC1, retaining the PcG core proteins of dPRC1 but lacking most non-PcG proteins. Preparations of hPRC-H contained either two or three different homologues of most of the core PcG proteins, including a new Ph homologue we have named HPH3. Despite differences in composition, dPRC1 and hPRC-H have similar functions: hPRC-H is able to efficiently block remodeling of nucleosomal arrays through a mechanism that does not block the ability of nucleases to access and cleave the arrays.