Function
B-cell-specific Moloney leukemia virus insertion site 1 (BMI1) is a ring finger protein that is component of a , which epigenetically repress regulatory genes, such as Hox genes, related to embryonic development and self-renewal of somatic stem-cells. BMI1 is central in the DNA damage repair, being the aberrant expression associated with numerous cancers and resistance to some types of chemotherapies.
PcG PRC1 is responsible for chromatin remodeling and histone modification, mediating monoubiquitination of histone H2A at Lysine 119, rendering chromatin heritably changed in its expressibility. BMI1 regulates the E3 ubiquitin-protein ligase activity of RING1b, which is also a protein component of the PcG PRC1 complex. Other monomers such as CBX2, CBX4, CBX8, PHC1, PHC2 and RNF2/RING2 are capable of forming a PRC1-like complex replacing the RING1b, also forming a heterodimer with E3 ubiquitin activity. In the complex, and form a heterodimer and only RING1b interacts with UbcH5c, which is a Ubiquitin Conjugating Enzyme E2.
Disease
Relevance
Structure
In the PRC1 complex, BMI1 and RING1b heterodimerize via their N-terminal RING domains, forming an active E3 ubiquitin ligase. The heterodimerization occurs by the wrapping of the N-terminal arm of Ring1b around a groove on the surface of Bmi1 (ref Buchwald et al, 2006; Li et al, 2006), which stabilizes the structure of Ring1b and greatly stimulates ligase activity (ref Wang et al, 2004). Is worth noting that RING-domains, such as the found in BMI1 and RING1b, are structurally stabilized through the binding of two zinc atoms (ref Jackson et al, 2000). X-ray crystallography shows prominent salt bridges being formed by the Bmi1/Ring1b pairs of Asp72/Arg70, Glu11/Lys112, Lys81/Glu48, and Thr41/Arg26. (ref EMBOJ 2011)
A RING-domain E3 is responsible for promoting the transfer of ubiquitin from the active site of the E2 ubiquitin-conjugating enzyme to an acceptor lysine residue in the substrate. The PRC1 complex promote the monoubiquitination of histone H2A on lysine 119 (uH2A), which leads to the stalling of the RNA polymerase at the promotor of the monoubiquitinated gene, hence the transcription doesn't occur. HoxC13 gene is one of the Hox gene regulated by the PRC1 complex during development.
How can the PRC1 complex promote such a epigenetic modification? The complex binds to the duplex of DNA through a surface patch unique to the BMI1/RING1b RING–RING dimer. However only the RING1b interacts with the E2 enzyme UbcH5c. The ubiquitination is a cascade of events that involve a E1 activating enzyme, a E2 ubiquitin-conjugating enzyme and a E3 ubiquitin-protein ligase. First, E1 receives the ubiquitin through the covalent attaching of the ubiquitin C terminus to a cysteine of the E1 through a thioester bond, only possible due to ATP hydrolysis. Then, the E1 ubiquitin conjugate binds to an E2, catalyzing the transfer of the ubiquitin onto the E2 active site cysteine, also by a thio ester bond. Finally, the E3 ligase brings together the loaded E2 and the substrate, catalyzing the transfer of ubiquitin from the active site cysteine of the E2 to the amino group of a lysine side chain, forming an isopeptide bond . Therefore, the PRC1 complex promotes the transfer of the ubiquitin attached to E2 enzyme UbcH5c to the lysine 119 on histone H2A. Computational modelling suggests that the heterodimer BMI1/RING1b interacts with both nucleosomal DNA and an acidic patch on histone H4 to promote the specific monoubiquitination of H2A. (ref EMBOJ 2011)
In humans, there are multiple E2 ubiquitin-conjugating enzymes, but only UbcH5 and UbcH6 interact with the BMI1/RING heterodimer and promote the monoubiquitination of histone H2A at Lysine 119. The subtype UbcH5c creates more polyubiquitin chains then UbcH5a and UbcH5b, in contrast UbcH6 doesn't create polyubiquitin chains. (ref Buchwalld 2006)
