Histones and Histone Deacetylases

Epigenetics is an increasingly big deal in biological discovery. We are regularly reading about the influence of actions peripheral to DNA in regulating DNA transcription and translation. We are learning that mice may fear what grandparent mice feared (Kelly’s blog ), due to heritable changes in DNA. In term of one of several mechanisms of epigenetic change, we are learning much about histone deacetylases and their role in gene regulation, as well as disease (Isobel’s blog ). In this blog, let’s take a step back and look at histones, and how they are influenced by acetylation/deacetylation.

The Role of Histones
Histones are proteins found in the nucleus of eukaryotic cells, where they package DNA into nucleosomes. Histones make up the main protein component of chromatin, acting as spool-like structures around which DNA wraps.

There are five major histone classes,three of these are core histones, the other two are called linker histones. Core histones comprise the core of the nucleosome, around which DNA is wrapped, while the linker histones bind at the entrance and exit sites of the DNA, so as to lock it into place. The linker histones also enable a higher order of structure. If you hold both ends of a rubber band, and twist one end, you’ll see that the rubber band twists and folds over itself; the end being held steady enables this twisting and folding: this is how the linker histones work. Histone-DNA structure is frequently represented as a beaded chain-type image (see figure).

DNA wrapping around histones in a bead and chain-like fashion.

DNA wrapping around histones in a bead and chain-like fashion.

Histone and DNA: Charged Interactions
Histone tails normally carry a positive charge due to amine groups present on their lysines and arginines. This positive charge is the means by which histone tails interact with and bind to the negatively-charged phosphate groups on the DNA backbone.

Histones are subject to post-translational modifications, primarily on their N-terminal tails, by enzymes. Such modifications include methylation, citrullination, acetylation, phosphorylation, SUMOylation, ubiquitination, and ADP-ribosylation. Such modifications can affect histone function in gene regulation. Acetylation is one of the most common post-translational modifications of histones (1). Continue reading

Use of Cell-Free Protein Expression for Epigenetics-Related Applications

Epigenetics is the study of the processes involved in the genetic development of an organism, especially the activation and deactivation of genes. One way that genes are regulated is through the remodeling of chromatin. Chromatin is the complex of DNA and the histone proteins with which it associates. The conformation of chromatin is profoundly influenced by the post-translational modification of the histone proteins. These modifications include acetylation, methylation, ubiquitylation, phosphorylation and sumolyation. The following references illustrate the use of cell-free expression to characterize this process.

Shao, Y. et al. (2010) Nucl. Acid. Res. 38, 2813–24.
Carbonic anhydrase IX (CAIX) plays an important role in the growth and survival of tumor cells.The MORC proteins contain a CW-type zinc finger domain and are predicted to have the function of regulating transcription, but no MORC2 target genes have been identified. CAIX mRNA to be down-regulated 8-fold when MORC2 was overexpressed. Moreover, MORC2 decreased the acetylation level of histone H3 at the CAIX promoter. Among the six HDACs tested, histone deacetylase 4 (HDAC4) had a much more prominent effect on CAIX repression. Assays showed that MORC2 and HDAC4 were assembled on the same region of the CAIX promoter. Interaction between MORC2 and HDAC 4 were confirmed by using cell free expression of MORC2 and GST-HDAC (GST pull-downs). Cell-free expression was also used to express MORC2 proteins to determine through gel shifts the binding location on the CAIX promoter region (gel shift experiments)

Denis, H. et al. (2009) Mol. Cell. Biol. 29, 4982–93.
The recent identification of enzymes that antagonize or remove histone methylation offers new opportunities to appreciate histone methylation plasticity in the regulation of epigenetic pathways. PAD4 was the first enzyme shown to antagonize histone methylation. Very little is known as to how PADI4 silences gene expression. Through the use of cell-free expression to express both PAD4 and HDAC1 proteins and E. coli expression of GST fusions of PAD4 and HDAC1, pulldown experiments confirmed by in vivo experiments that PADI4 associates with the histone deacetylase 1 (HDAC1), and the corresponding activities, associate cyclically and coordinately with the pS2 promoter during repression phases.

Brackertz, M. et al. (2006) Nucl. Acid. Res. 34, 397-406.
The Mi-2/NuRD complex is a multi-subunit protein complex with enzymatic activities involving chromatin remodeling and histone deacetylation. The function of p66α and of p66β within the multiple subunits has not been addressed. GST-fused histone tails of H2A, H2B, H3 and H4 were expressed in E. coli used in an in vitro pull-down assay with radioactively labeled p66-constructs expressed using cell free systems. Deletions at the C terminus noted reduced binding of p66 where as deletions at the N terminus did not affect binding. Also observed was that acetylation of histone tails reduces the association with both p66-proteins in vitro.

Zhou, R. et al. (2009) Nucl. Acids. Res. 37, 5183–96.
Lymphoid specific helicase (Lsh) belongs to the family of SNF2/helicases. Disruption of Lsh leads to developmental growth retardation and premature aging in mice. However, the specific effect of Lsh on human cellular senescence remains unknown. In vivo results noted that Lsh requires histone deacetylase (HDAC) activity to repress p16INK4a. Moreover, overexpression of Lsh is correlated with deacetylation of histone H3 at the p16 promoter. In vitro pull-downs using cell free expression and GST fusions from E. coli were used to collaborate interactions between Lsh, histone deacetylase 1 (HDAC1) and HDAC2 observed in vivo.