Hemoglobin subunit theta-1 is a protein that in humans is encoded by the HBQ1 gene. Theta-globin mRNA is originally found in human fetal erythroid tissue but not in adult erythroid or other nonerythroid tissue. Theta-1 is a member of the human alpha-globin gene cluster that includes five functional genes and two pseudogenes. Research supports a transcriptionally active role for the gene and a functional role for the peptide in specific cells, possibly those of early erythroid tissue. Hemoglobin has a quaternary structure characteristically composed of many multi-subunit globular proteins. Most of the amino acids in hemoglobin form alpha helices, connected by short non-helical segments. Hydrogen bonds stabilize the helical sections inside this protein, causing attractions within the molecule, folding each polypeptide chain into a specific shape. Hemoglobin's quaternary structure comes from its four subunits in roughly a tetrahedral arrangement.
Hematopoietic lineage cell-specific protein (HCLS1) is a protein that in humans is encoded by the HCLS1 gene. It is expressed only in tissues and cells of hematopoietic origin. It is substrate of the antigen receptor-coupled tyrosine kinase and plays a role in antigen receptor signaling for both clonal expansion and deletion in lymphoid cells. It may also be involved in the regulation of gene expression.
Acetylation of the histone tail causes chromatin to adopt an "open" conformation, allowing increased accessibility of transcription factors to DNA. The identification of histone acetyltransferases (HATs) and their large multiprotein complexes has yielded important insights into how these enzymes regulate transcription. HAT complexes interact with sequence-specific activator proteins to target specific genes. In addition to histones, HATs can acetylate nonhistone proteins, suggesting multiple roles for these enzymes. In contrast, histone deacetylation promotes a "closed" chromatin conformation and typically leads to repression of gene activity. Mammalian histone deacetylases can be divided into three classes on the basis of their similarity to various yeast deacetylases. Class I proteins (HDACs 1, 2, 3, and 8) are related to the yeast Rpd3-like proteins, those in class II (HDACs 4, 5, 6, 7, 9, and 10) are related to yeast Hda1-like proteins, and class III proteins are related to the yeast protein Sir2. Inhibitors of HDAC activity are now being explored as potential therapeutic cancer agents.
HDAC4 (histone deacetylase 4), belongs to class II of the histone deacetylase/acuc/apha family. Histone Deacetylases (HDACs) are a group of enzymes closely related to sirtuins. They catalyze the removal of acetyl groups from lysine residues in histones and non-histone proteins, resulting in transcriptional repression. In general, they do not act autonomously but as components of large multiprotein complexes, such as pRb-E2F and mSin3A, that mediate important transcription regulatory pathways. There are three classes of HDACs; classes 1, 2 and 4, which are closely related Zn2+-dependent enzymes. HDACs are ubiquitously expressed and they can exist in the nucleus or cytosol. Their subcellular localization is effected by protein-protein interactions and by the class to which they belong. HDACs have a role in cell growth arrest, differentiation and death and this has led to substantial interest in HDAC inhibitors as possible antineoplastic agents. HDAC4 possesses histone deacetylase activity and represses transcription when tethered to a promoter. It does not bind DNA directly, but through transcription factors MEF2C and MEF2D. HDAC4 seems to interact in a multiprotein complex with RbAp48 and HDAC3.
Histone deacetylase 8, also known as HDAC8 and HDACL1, is a nucleus and cytoplasm protein which belongs to thehistone deacetylase family and HD type 1 subfamily. Histone deacetylases (HDACs) are a growing family of enzymes implicated in transcriptional regulation by affecting the acetylation state of core histones in the nucleus of cells.HDAC8 / HDACL1 is weakly expressed in most tissues. It expressed at higher level in heart, brain, kidney and pancreas and also in liver, lung, placenta, prostate and kidney. HDAC8 / HDACL1 is responsible for the deacetylation of lysine residues on the N-terminal part of the core histones ( H2A, H2B, H3 and H4 ). Histone deacetylation gives a tag for epigenetic repression and plays an important role in transcriptional regulation, cell cycle progression and developmental events. Histone deacetylases act via the formation of large multiprotein complexes. HDAC8 / HDACL1 may play a role in smooth muscle cell contractility. HDAC8 / HDACL1 may be a potential drug target for neuroblastoma differentiation therapy using selective inhibitors, avoiding unspecific side effects.
Hepatoma-Derived Growth Factor is a original member of the HDGF family. HDGF is a cytoplasmic protein and contains one PWWP domain. HDGF expression levels are high in the nucleus and cytoplasm of smooth muscle and endothelial cells. HDGF has proliferative, angiogenic and neurotrophic activity. HDGF was initially characterized as a secreted mitogen from the Huh-7 human hepatoma cell line. As a heparin-binding protein, which is highly expressed in tumor cells where it stimulates proliferation. HDGF has mitogenic activity for fibroblasts and acts as a transcriptional repressor. It has been shown that HDGF is linked with tumorigenesis and the growth of cancer.
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