In the eukaryotic cell, DNA is associated with protein factors to form chromatin. The fundamental repeating unit of chromatin is called the nucleosome where 146 base pairs of DNA is wrapped around two copies of each histone protein (H3, H4, H2A, and H2B). An important role for histone proteins is to help in the compaction of our genome into the nucleus of the cell. However, this compaction of DNA can restrict nuclear factors from gaining access to the DNA template. Therefore, this inherently restrictive environment must be regulated and organized to allow permissive cellular processes such as gene transcription, replication, recombination, repair and chromosomal segregation. The mechanisms that regulate chromatin structure and function are histone modifying complexes that posttranslationally modify histones. Generally, all of the histone modifications have been located on the N- and C-terminal tail domains. However, recent evidence has indicated novel modification sites within the central part of the histone called the histone fold-domain. Since posttranslational modifications on histones such as acetylation, phosphorylation, ubiquitination, and/or methylation can influence the chromatin environment and gene expression, we are interested in studying the machinery that mediates these modifications and how mis-regulation of these enzymes can lead to a disease state.
Posttranslational modifications on histones. Specific amino acid sites of posttranslational modifications (acetylation, phosphorylation, ubiquitination and methylation) that are known to occur on histones are indicated by colored symbols. Half of the structure of the nucleosome core particle H3 (yellow), H4 (blue), H2A (red) and H2B (green) are shown in color. The other half is represented in grey
Histone methylation and regulation
Work on histone methylation has lead to the identification of histone methylation sites and their corresponding methyltransferases. Histone methylation has now been identified on lysine (Lys) and arginine (Arg) residues on histone H3 and H4 (see figure). The catalytic core for some but not all lysine histone methyltransferases (HMTs) resides in the SET domain. A conserved domain named for its appearance in Su(var) 3-9 (suppressor of position effect variegation), E(z) (enhancer of zeste), and Trx (trithorax). In contrast to lysine HMTs, arginine HMTs do not contain a SET domain but have highly conserved non-contiguous amino acid residues that are essential for forming its catalytic core. Histone lysine methylation is a unique posttranslational modification since it can exist in three different methyl states (mono-, di- and trimethyl). In S. cerevisiae, all three methylation states are catalyzed by the same enzyme. For example, Set1, Set2 and Dot1 are responsible for catalyzing mono-, di- and trimethylation on histone H3 at Lys4, Lys36, and Lys79, respectively. Interestingly, histone methylation can organize chromatin into an active or repressed state depending on the site of methylation and methyl-binding protein. My lab has largely exploited the strengths of yeast and mammalian model organisms in combination with biochemistry and molecular biology techniques. We are currently using budding yeast as a model system to understand how Set1, Set2 and Dot1 methyltransferases and their sites of histone modification function.
Methyltransferases and cancer
Several histone methyltransferases and demethylases are found either mutated, chromosomal translocated, or over-expressed when isolated from oncogenic cells suggesting that they play an important regulatory role in the cell. Therefore, we are interested in determining how mis-regulation and/or aberrant expression of these methyltransferases can lead to an oncogenic event and how aberrant histone methylation may play a role in oncogenesis. Therefore, understanding the mechanism of how histone methyltransferases and demethylases function will provide key insights into designing small molecule inhibitors for potential novel chemotherapeutic drugs.