| The genome of eukaryotic cells is composed in nucleoprotein structure called 'chromatin'. The basic repeating unit of chromatin is nucleosome, which consists of two pairs of H2A-H2B dimmers, and one H3-H4 tetramer wrapped around 147 bp of double stranded DNA (dsDNA). Although, the structure of chromatin is necessary for hierarchical compaction of the entire genome, it is also an obstacle for some of the most important cellular processes like transcription, DNA repair, DNA replication. In order to alter the structure between histones and DNA, eukaryotic cells imply three basic mechanisms: chromatin remodeling, incorporation of histone variants, and post-translational modification of histones. Post-translational modification is considered as epigenetic modification, which plays a vital role in gene regulation as well as maintaining genome stability.;Although, many different post-translational modifications on histones are identified, histone acetylation mediated by histone acetyltransferases (HATs) and histone methylation mediated by histone methyltransferases (HMTs) are two major modifications. The detailed mechanistic processes on how these covalent modifications are introduced to the target amino acid is not clear, different modifications and combinations of modifications, the so-called 'histone code', produce entirely different cellular processes such as transcription activation, repression, DNA repair, DNA replication, and so on. Therefore, studying histone modifying enzymes can bring detailed understanding on the functions of different covalent modifications at the cellular level. SMYD3 is one of the histone methyltransferases that methylates histone H3-K4. Recent studies showed that SMYD3 is frequently overexpressed in different types of cancer cells, but how SMYD3 regulates the development and progression of these malignancies remains unknown. I report here the previously unrecognized role of SMYD3 in estrogen receptor (ER)-mediated transcription via its histone methyltransferase activity. I demonstrate that SMYD3 functions as a coactivator of ER and potentiates ER activity in response to ligand. SMYD3 directly interacts with the ligand binding domain (LBD) of ER and is recruited to the proximal promoter regions of ER target genes upon gene induction. Importantly, the chromatin immunoprecipitation analyses provided compelling evidence that SMYD3 is responsible for the accumulation of di- and trimethylation of H3--K4 at the induced ER target genes. Furthermore, RNA interference-directed down-regulation of SMYD3 reveals that SMYD3 is required for ER-regulated gene transcription in estrogen signaling pathway. Thus, these results identify SMYD3 as a new coactivator for ER-mediated transcription, providing a possible link between SMYD3 overexpression and breast cancer.;The exchange of histone variants is another mechanism, which can regulate transcription and other cellular processes. Unlike canonical histones, histone variants can be incorporated into the chromatin independent from DNA replication. Histone H2A and H3 variants have been introduced to the field of epigenetics for decades and numbers of different studies have shown the evidence of the histone variants involved in specific cellular processes causing different chromatin states.;Histone H3.3 is one of H3 'replacement' variants. Although, there are only 5 amino-acid sequence differences between human H3 and H3.3, the incorporation, localization, and the specification of H3.3 into the chromatin seems very distinct from the histone, H3. For instance, H3.3 is significantly recognized in transcriptionally active loci. However, it is not clear how it exerts its function such as whether H3.3 recognizes the active chromatin states or other proteins aid the incorporation of this variant.;A number of studies suggest that H3.3 plays a role in transcriptionally active chromatin and may be involved in the epigenetic maintenance of chromatin status.;It has been shown that H3.3 incorporation was triggered into HSP70 gene loci upon transcriptional activation in Drosophilla. However, the mechanistic details on how H3.3 is incorporated into the chromosome were not discussed such as whether H3.3 incorporation depends on other protein binding or how it targets to the active loci. In this study, we purified H3 and H3.3 specific mononucleosomes from a human cell line to search for proteins specifically preferential for H3.3 containing mononucleosomes. Interestingly, we observed preferential binding of one of the HP1 proteins, HP1., to H3.3 containing mononucleosome and we further investigated the function of HP1. and H3.3 in regulating HSP70 genes which affects the growth of breast cancer cell. |
