- Polycomb Repressive Complexes 1 and 2 (PRC1 and PRC2) are two major multiprotein complexes that form the core of the Polycomb-group system of epigenetic gene regulation. These complexes play important roles in maintaining transcriptional repression, regulating developmental programs, preserving cellular identity, and organizing chromatin. Although PRC1 and PRC2 have distinct molecular compositions and enzymatic activities, they frequently cooperate to establish and maintain repressive chromatin states. Their activities are particularly important at genes involved in development and differentiation, where inappropriate gene activation can interfere with normal cell identity and tissue formation.
- PRC1 and PRC2 were originally characterized through genetic and biochemical studies of Polycomb-group proteins in Drosophila melanogaster. The discovery that different Polycomb proteins associate with one another to form stable protein complexes provided an important explanation for how individual Polycomb-group genes could participate in coordinated gene repression. Subsequent studies in mammals revealed considerably greater complexity, with multiple PRC1 and PRC2 variants containing different combinations of subunits. This diversity allows Polycomb complexes to operate in different genomic locations and cellular contexts.
- PRC2 is primarily responsible for establishing the histone modification H3K27me3, which refers to trimethylation of lysine 27 on histone H3. The catalytic activity of PRC2 is provided by the SET-domain-containing enzymes EZH1 or EZH2. These proteins transfer methyl groups to lysine 27 of histone H3. Depending on the degree of methylation, H3K27 can exist in mono-, di-, or trimethylated states, with H3K27me3 being particularly associated with Polycomb-mediated transcriptional repression.
- The core mammalian PRC2 complex contains EZH1 or EZH2, SUZ12, EED, and RBBP4 or RBBP7. EZH1 and EZH2 provide catalytic activity, whereas SUZ12 and EED are important for complex stability and regulation. EED contains a WD40-repeat domain that recognizes H3K27me3 and can promote further PRC2 activity. This creates an important mechanism through which existing Polycomb-associated chromatin can contribute to the propagation of the repressive state.
- PRC2 activity is closely connected to the concept of epigenetic memory. When cells divide, patterns of gene regulation must often be re-established on newly synthesized chromatin. Existing H3K27me3-containing nucleosomes can contribute to recruitment and activation of PRC2 on nearby chromatin. In this way, Polycomb repression can be maintained through successive cell divisions. However, Polycomb repression is not simply a permanent molecular imprint. Developmental signals and other regulatory mechanisms can alter Polycomb occupancy and allow previously repressed genes to become active.
- PRC1 is another major component of the Polycomb regulatory system. Its classical biochemical activity is the monoubiquitination of histone H2A at lysine 119, producing the modification known as H2AK119ub or H2AK119ub1. This reaction is catalyzed primarily by the RING1A or RING1B ubiquitin ligase proteins. H2AK119ub is strongly associated with Polycomb-mediated gene repression and can contribute to the formation of a chromatin environment that is unfavorable for transcription.
- Unlike PRC2, which has a relatively conserved core structure, PRC1 exists in numerous molecular forms. Mammalian PRC1 complexes can be broadly divided into canonical PRC1 and non-canonical PRC1 complexes. Canonical PRC1 complexes contain chromobox proteins such as CBX2, CBX4, CBX6, CBX7, or CBX8, which can recognize H3K27me3, together with RING1A or RING1B, PCGF proteins, and additional subunits. This arrangement provides a molecular connection between PRC2-generated H3K27me3 and PRC1 recruitment.
- Non-canonical PRC1 complexes differ from canonical PRC1 complexes in their composition and recruitment mechanisms. They generally lack the CBX proteins that characterize canonical PRC1 and instead contain other regulatory proteins, including RYBP or YAF2 in several complex types. Non-canonical PRC1 complexes can be recruited independently of pre-existing H3K27me3 and can contribute strongly to H2AK119ub deposition. Their activities demonstrate that Polycomb regulation does not always follow a simple PRC2-first, PRC1-second sequence.
- The relationship between PRC1 and PRC2 is therefore bidirectional and context dependent. In one classical model, PRC2 establishes H3K27me3, which is subsequently recognized by CBX-containing canonical PRC1 complexes. PRC1 then contributes H2AK119ub and chromatin compaction. However, non-canonical PRC1 complexes can independently deposit H2AK119ub, and this modification can influence PRC2 recruitment or activity. Consequently, PRC1 and PRC2 can reinforce one another through several molecular pathways rather than functioning as two strictly sequential complexes.
- The chromatin-binding properties of PRC1 and PRC2 are central to their functions. Polycomb complexes must be recruited to specific genomic regions rather than randomly modifying the genome. Recruitment can involve DNA sequence features, transcription factors, pre-existing histone modifications, chromatin-associated proteins, RNA molecules, and other epigenetic regulators. In mammals, many Polycomb-regulated genes are associated with CpG-rich promoters, particularly developmental genes containing CpG islands that remain relatively unmethylated. These genomic features can contribute to the establishment of Polycomb-associated chromatin domains.
- PRC1 and PRC2 are especially important for the regulation of developmental genes. During embryonic development, cells progressively acquire specialized identities by activating particular gene programs while repressing alternative developmental programs. Polycomb complexes help maintain repression of genes that are inappropriate for a particular developmental state. This regulation is essential because many developmental genes encode transcription factors capable of initiating extensive changes in cellular gene expression. Keeping these genes appropriately repressed helps prevent premature or inappropriate differentiation.
- Polycomb complexes also play important roles in stem cells. In pluripotent stem cells, many genes involved in differentiation are maintained in a repressed or poised state. PRC2-mediated H3K27me3 and PRC1-mediated H2AK119ub contribute to the repression of subsets of these developmental genes. When differentiation signals are received, Polycomb repression can be reduced or removed at genes that need to become active. This allows stem cells to transition into specialized cell types while retaining a controlled and coordinated developmental program.
- Polycomb repression can also occur at so-called bivalent chromatin domains, particularly in pluripotent and progenitor cells. These regions can contain both the repressive H3K27me3 modification and the transcription-associated H3K4me3 modification. Such chromatin states have historically been described as allowing developmental genes to remain repressed while retaining the potential for rapid activation. The molecular interpretation of bivalent chromatin is complex, but these domains illustrate the dynamic relationship between active and repressive chromatin mechanisms during development.
- PRC1 contributes to gene repression through mechanisms that extend beyond H2AK119ub. Some PRC1 complexes can promote chromatin compaction and influence nucleosome organization. The ability of Polycomb complexes to alter the physical properties of chromatin can reduce the accessibility of regulatory DNA to transcriptional machinery. PRC1-mediated chromatin compaction can therefore complement the biochemical effects of histone modification and contribute to the formation of stable repressive domains.
- PRC2 also interacts with a wide range of chromatin-regulating proteins. Its activity can be influenced by histone modifications, nucleosome composition, chromatin architecture, transcription factors, and other epigenetic complexes. PRC2 is not simply an enzyme that methylates histones independently of its surroundings. Instead, its activity is integrated into a broader network of chromatin signals that determine where and when H3K27 methylation occurs.
- The activities of PRC1 and PRC2 are also connected with three-dimensional genome organization. Polycomb-regulated genomic regions can interact with one another within the nucleus and form spatially organized chromatin domains. Such interactions can bring distant genomic regions into proximity and contribute to the coordinated regulation of multiple genes. This aspect of Polycomb biology highlights the importance of studying gene regulation not only at the level of individual promoters but also at the level of genome-wide chromatin architecture.
- Polycomb complexes must also be balanced against mechanisms that promote transcription. Active histone modifications, chromatin-remodeling complexes, transcription factors, and transcription-associated proteins can oppose Polycomb-mediated repression. The final expression state of a gene therefore reflects the combined effects of activating and repressive regulatory systems. Polycomb repression is particularly important when a gene needs to remain inactive despite the presence of regulatory elements that could otherwise promote transcription.
- Alterations in PRC1 and PRC2 function have important consequences for human health. Changes in the expression or activity of Polycomb proteins have been observed in a range of cancers. EZH2, the catalytic component of many PRC2 complexes, is one of the best-studied examples. Genetic alterations, increased expression, or abnormal activity of EZH2 can alter H3K27 methylation and contribute to abnormal gene regulation in particular cancers. Other Polycomb components can also be mutated, deleted, amplified, or otherwise dysregulated, demonstrating that the Polycomb system is an important component of cancer-associated epigenetic regulation.
- PRC1 abnormalities can likewise influence cancer development and progression by altering H2AK119ub, chromatin organization, and transcriptional regulation. Because PRC1 contains many different subunits, the biological consequences of altering one component can vary according to the cell type and the specific PRC1 complex involved. Some Polycomb proteins can function as tumor-promoting factors in particular contexts, whereas loss of other Polycomb components can also disrupt normal gene regulation. Polycomb biology in cancer is therefore highly context dependent.
- The importance of PRC1 and PRC2 has encouraged the development of drugs that target components of these complexes. EZH2 inhibitors are an important example of epigenetic therapies designed to interfere with abnormal PRC2 activity. By reducing EZH2 catalytic activity, these drugs can decrease H3K27 methylation and alter the expression of Polycomb-regulated genes. Research is also investigating other components of PRC1 and PRC2 and the possibility of targeting interactions between Polycomb complexes and their chromatin substrates.
- Despite extensive research, many aspects of PRC1 and PRC2 biology remain unresolved. Researchers continue to investigate how Polycomb complexes recognize specific genomic locations, how different complex variants are selected in particular cell types, how Polycomb repression is established during development, and how it is removed when genes need to be activated. The diversity of Polycomb complexes makes these questions particularly challenging because the function of a Polycomb protein can depend strongly on the other proteins with which it associates.
- PRC1 and PRC2 therefore represent interconnected but distinct components of the Polycomb-group regulatory system. PRC2 is primarily associated with H3K27 methylation, especially H3K27me3, whereas PRC1 is strongly associated with H2AK119 monoubiquitination and chromatin compaction. Their activities can reinforce one another, but they can also operate independently through alternative recruitment pathways and complex compositions. Together, they help establish and maintain transcriptionally repressive chromatin states across the genome.
- Overall, PRC1 and PRC2 provide an important molecular framework for understanding how cells maintain stable patterns of gene expression without changing the underlying DNA sequence. Through histone modification, chromatin organization, epigenetic memory, and interactions with other regulatory systems, these complexes help control development, differentiation, stem-cell states, and cellular identity. Their dysregulation can contribute to disease, particularly cancer, while their reversible activities make them important subjects of research in epigenetics, developmental biology, chromatin biology, and therapeutic development.
