Histone proteins are the major protein components of chromatin, the physiologically relevant form of the genome (or epigenome) in all eukaryotic cells. Chromatin is the substrate of many biological processes, such as gene regulation and transcription, replication, mitosis and apoptosis. Since histones are extensively post-translationally modified, the identification of these covalent marks on canonical and variant histones is crucial for the understanding of their biological significance. Many different biochemical techniques have been developed to purify and separate histone proteins. Here, we present standard protocols for acid extraction and salt extraction of histones from chromatin; separation of extracted histones by reversed-phase HPLC; analysis of histones and their specific post-translational modification profiles by acid urea (AU) gel electrophoresis and the additional separation of non-canonical histone variants by triton AU(TAU) and 2D TAU electrophoresis; and immunoblotting of isolated histone proteins with modification-specific antibodies.

Post-translational arginine methylation is responsible for regulation of many biological processes. The protein arginine methyltransferase 5 (PRMT5, also known as Hsl7, Jbp1, Skb1, Capsuleen, or Dart5) is the major enzyme responsible for mono- and symmetric dimethylation of arginine. An expanding literature demonstrates its critical biological function in a wide range of cellular processes. Histone and other protein methylation by PRMT5 regulate genome organization, transcription, stem cells, primordial germ cells, differentiation, the cell cycle, and spliceosome assembly. Metazoan PRMT5 is found in complex with the WD-repeat protein MEP50 (also known as Wdr77, androgen receptor coactivator p44, or Valois). PRMT5 also directly associates with a range of other protein factors, including pICln, Menin, CoPR5 and RioK1 that may alter its subcellular localization and protein substrate selection. Protein substrate and PRMT5–MEP50 post-translation modifications induce crosstalk to regulate PRMT5 activity. Crystal structures of C. elegans PRMT5 and human and frog PRMT5–MEP50 complexes provide substantial insight into the mechanisms of substrate recognition and procession to dimethylation. Enzymo-logical studies of PRMT5 have uncovered compelling insights essential for future development of specific PRMT5 inhibitors. In addition, newly accumulating evidence implicates PRMT5 and MEP50 expression levels and their methyltransferase activity in cancer tumorigenesis, and, significantly, as markers of poor clinical outcome, marking them as potential oncogenes. Here, we review the substantial new literature on PRMT5 and its partners to highlight the significance of understanding this essential enzyme in health and disease.

SummaryDNA double-stranded breaks present a serious challenge for eukaryotic cells. Inability to repair breaks leads to genomic instability, carcinogenesis, and cell death. During the DSB response, mammalian chromatin undergoes reorganization demarcated by H2A.X Ser139 phosphorylation (γ-H2A.X). However, the regulation of γ-H2A.X phosphorylation and its precise role in chromatin remodeling during the repair process remain unclear. Here, we report a novel regulatory mechanism mediated by WSTF, a component of the WICH ATP-dependent chromatin remodeling complex. We show that WSTF has intrinsic tyrosine kinase activity via a domain that shares no sequence homology to any known kinase fold. We show that WSTF phosphorylates Tyr142 of H2A.X and that WSTF activity plays an important role in regulating a number of events that are critical for the DNA damage response. Our work reveals a novel mechanism that regulates the DNA damage response and expands our knowledge of domains that contain intrinsic tyrosine kinase activity.

Timing of DNA replication initiation is dependent on S-phase-promoting kinase (SPK) activity at discrete origins and the simultaneous function of many replicons. DNA damage prevents origin firing through the ATM- and ATR-dependent inhibition of Cdk2 and Cdc7 SPKs. Here, we establish that modulation of ATM- and ATR-signalling pathways controls origin firing in the absence of DNA damage. Inhibition of ATM and ATR with caffeine or specific neutralizing antibodies, or upregulation of Cdk2 or Cdc7, promoted rapid and synchronous origin firing; conversely, inhibition of Cdc25A slowed DNA replication. Cdk2 was in equilibrium between active and inactive states, and the concentration of replication protein A (RPA)-bound single-stranded DNA (ssDNA) correlated with Chk1 activation and inhibition of origin firing. Furthermore, ATM was transiently activated during ongoing replication. We propose that ATR and ATM regulate SPK activity through a feedback mechanism originating at active replicons. Our observations establish that ATM- and ATR-signalling pathways operate during an unperturbed cell cycle to regulate initiation and progression of DNA synthesis, and are therefore poised to halt replication in the presence of DNA damage.