Ubiquityl-Histone H2B (Lys120) (D11) XP^® Rabbit mAb (BSA and Azide Free) #33291

The nucleosome, made up of four core histone proteins (H2A, H2B, H3 and H4), is the primary building block of chromatin. Originally thought to function as a static scaffold for DNA packaging, histones have now been shown to be dynamic proteins, undergoing multiple types of post-translational modifications, including acetylation, phosphorylation, methylation, and ubiquitylation (1). Ubiquitin is a conserved 76 amino acid peptide unit that can be covalently linked to many cellular proteins by the ubiquitylation process. Three components are involved in this protein-ubiquitin conjugation process. Ubiquitin is first activated by forming a thiolester complex with the activation component E1; the activated ubiquitin is subsequently transferred to the ubiquitin-carrier protein E2, then from E2 to ubiquitin ligase E3 for final delivery to the epsilon-NH₂ of the target protein lysine residue (2). Histone H2B is mono-ubiquitylated on lysine 120 during transcriptional activation by the RAD6 E2 protein in conjunction with the BRE1A/BRE1B E3 ligase (also known as RNF20/RNF40) (3). The RAD6/BRE1 complex is recruited to gene promoters during activation by the PAF complex, an RNA polymerase II-associated protein complex that regulates transcriptional elongation (3-5). Mono-ubiquitylated histone H2B lysine 120 is associated with the transcribed region of active genes (3,6). Mono-ubiquitylation of histone H2B stimulates transcriptional elongation by facilitating FACT-dependent chromatin remodeling (7,8). In addition, it is essential for subsequent methylation of histone H3 lysines 4 and 79, two additional histone modifications that regulate transcriptional initiation and elongation (9). Interestingly, de-ubiquitylation of histone H2B lysine 120 by USP22, a subunit of the human SAGA histone acetyltransferase complex, is a required step in transcriptional activation (10). Thus, it appears that the ubiquitylation state of histone H2B is dynamic during transcription and may serve as an intermediate step in transcriptional activation.