Current Protein and Peptide Science - Volume 17, Issue 5, 2016

In eukaryotic cells, gene expressions on chromosome DNA are orchestrated by a dynamic chromosome structure state that is largely controlled by chromatin-regulating proteins, which regulate chromatin structures, release DNA from the nucleosome, and activate or suppress gene expression by modifying nucleosome histones or mobilizing DNA-histone structure. The two classes of chromatinregulating proteins are 1) enzymes that modify histones through methylation, acetylation, phosphorylation, adenosine diphosphate–ribosylation, glycosylation, sumoylation, or ubiquitylation and 2) enzymes that remodel DNA-histone structure with energy from ATP hydrolysis. Chromatin-regulating proteins, which modulate DNA-histone interaction, change chromatin conformation, and increase or decrease the binding of functional DNA-regulating protein complexes, have major functions in nuclear processes, including gene transcription and DNA replication, repair, and recombination. This review provides a general overview of chromatin-regulating proteins, including their classification, molecular functions, and interactions with the nucleosome in eukaryotic cells.

Chromatin structure regulating processes mediated by the adenosine triphosphate (ATP) – dependent chromatin remodeling complex and the covalent histone-modifying complexes are critical to gene transcriptional control and normal cellular processes, including cell stemness, differentiation, and proliferation. Gene mutations, structural abnormalities, and epigenetic modifications that lead to aberrant expression of chromatin structure regulating members have been observed in most of human malignancies. Advances in next-generation sequencing (NGS) technologies in recent years have allowed in-depth study of somatic mutations in human cancer samples. The Cancer Genome Atlas (TCGA) is the largest effort to date to characterize cancer genome using NGS technology. In this review, we summarize somatic mutations of chromatin-structure regulating genes from TCGA publications and other cancer genome studies, providing an overview of genomic alterations of chromatin regulating genes in human malignancies.

Posttranslational modifications of proteins critically regulate the function, localization, and stability of target proteins. Histone modification is one of the regulatory mechanisms that modulate the chromatin structure and thereby affect various DNA-templated processes, such as gene transcription, DNA replication, DNA recombination, and DNA repair in cells. These molecular processes contribute to basic cellular functions, including cell cycle, cell growth, and apoptosis. Histone modifications consist of acetylation, methylation, phosphorylation, ubiquitination, sumoylation biotination, citrullination, poly-ADPribosylation, and N-glycosylation. The modification status of histone is balanced by two enzyme families with opposing catalytic activities: histone modifying and de-modifying enzymes. Recent studies have shown that dysfunction of histone modification enzymes is a major cause for human cancer initiation and progression. In this review, we will summarize the functions of histone modification enzymes in cancer, and the mechanisms that histone modification enzymes use to drive or suppress human malignancies.

Chromatin remodeling complexes (chromatin remodelers) have been demonstrated as essential and powerful regulators for critical DNA-templated cellular processes, such as DNA replication, recombination, gene transcription/repression, and DNA damage repair. These molecular and genetic processes are important for a wide spectrum of cellular functions including, cell cycle, death, differentiation, pluripotency, and genome integrity. Not surprisingly, dysfunctions of chromatin remodeling proteins are observed in human developmental disorders and diseases. Specifically for human malignancies, genomic sequence analyses show that mutations and other genetic alternations of chromatin remodelers exist in almost all human tumor types. Using human cancer cell lines, xenograft models and genetically engineered mouse models, functions of chromatin remodelers in human cancers have been studied extensively in the last 10-15 years. In this review, we summarize the functional and mechanistic studies of chromatin remodelers in the initiation, progression and metastasis of human cancers.

Chromatin-regulating proteins modulate nucleosome structure by either modifying histones covalently or disrupting DNA-protein interaction directly with ATP hydrolysis. Evidence has shown that chromatin-regulating proteins play critical roles in regulation of molecular processes using DNA as template, including gene expression, DNA replication, DNA damage repair, and chromosome integrity. In most of human malignancies, chromatin-regulating proteins have been shown as functional oncogenes. In some scenarios, chromatin-regulating proteins also could have tumor suppressive functions. Thereby, small molecular inhibitors targeting chromatin-regulating proteins could be used for cancer therapies. Numerous small molecular inhibitors against chromatin-regulating proteins are recently developed by academic and industrial groups. These compounds are evaluated for antitumor effects in vitro and in vivo. Some of them have shown great potential to become a therapeutic drug for cancer, and is currently evaluated in clinical trials. A few compounds have been approved for clinical use in cancer treatment. In this review, we will focus on the recent progress on the development of small inhibitors of chromatin-regulating proteins for cancer therapy.

Human complement receptor type 2 (CR2; CD21) is a surface-associated glycoprotein which binds to a variety of endogenous ligands, including the complement component C3 fragments iC3b, C3dg and C3d, the low-affinity IgE receptor CD23, and the type I cytokine, interferon-alpha. CR2 links the innate complement-mediated immune response to pathogens and foreign antigens with the adaptive immune response by binding to C3d that is covalently attached to targets, and which results in a cell signalling phenomenon that lowers the threshold for B cell activation. Variations or deletions of the CR2 gene in humans, or the Cr2 gene in mice associate with a variety of autoimmune and inflammatory conditions. A number of infectious agents including Epstein-Barr virus (EBV), Human Immunodeficiency Virus (HIV) and prions also bind to CR2 either directly or indirectly by means of C3d-targeted immune complexes. In this review we discuss the interactions that CR2 undertakes with its best characterized ligands C3d, CD23 and the EBV gp350/220 envelope protein. To date only a single physiologically relevant complex of CR2 with one of its ligands, C3d, has been elucidated. By contrast, the interactions with CD23 and EBV gp350/220, while being important from physiologic and disease-associated standpoints, respectively, are only incompletely understood. A detailed knowledge of the structure-function relationships that CR2 undergoes with its ligands is necessary to understand the implications of using recombinant CR2 in therapeutic or imaging agents, or alternatively targeting CR2 to down-regulate the antibody mediated immune response in cases of autoimmunity.

The melanocortin receptor system consists of five closely related G-protein coupled receptors (MC1R, MC2R, MC3R, MC4R and MC5R). These receptors are involved in many of the key biological functions for multicellular animals, including human beings. The natural agonist ligands for these receptors are derived by processing of a primordial animal gene product, proopiomelanocortin (POMC). The ligand for the MC2R is ACTH (Adrenal Corticotropic Hormone), a larger processed peptide from POMC. The natural ligands for the other 4 melanocortin receptors are smaller peptides including α-melanocyte stimulating hormone (α-MSH) and related peptides from POMC (β-MSH and γ-MSH). They all contain the sequence His-Phe-Arg-Trp that is conserved throughout evolution. Thus, there has been considerable difficulty in developing highly selective ligands for the MC1R, MC3R, MC4R and MC5R. In this brief review, we discuss the various approaches that have been taken to design agonist and antagonist analogues and derivatives of the POMC peptides that are selective for the MC1R, MC3R, MC4R and MC5R receptors, via peptide, nonpeptide and peptidomimetic derivatives and analogues and their differential interactions with receptors that may help account for these selectivities.

The CW domain is a zinc binding domain, composed of approximately 50- 60 amino acid residues with four conserved cysteine (C) and two to four conserved tryptophan (W) residues. The members of the superfamily of CW domain containing proteins, comprised of 12 different eukaryotic nuclear protein families, are extensively expressed in vertebrates, vertebrate-infecting parasites and higher plants, where they are often involved in chromatin remodeling, methylation recognition, epigenetic regulation and early embryonic development. Since the first CW domain structure was determined 5 years ago, structures of five CW domains have been solved so far. In this review, we will discuss these recent advances in understanding the identification, definition, structure, and functions of the CW domain containing proteins.

Arginine, the most basic among the 20 amino acids, occurs less frequently than lysine in proteins despite being coded by six codons. Only a few important proteins of biological significance have been found to be abundant in arginine. It has been established that these arginine-rich proteins have been assigned important roles in the biological systems. Arginine-rich cationic proteins are known to stabilize macromolecular structures by establishing appropriate interactions (salt bridges, hydrogen bonds and cation-π interactions). These proteins are also known to be the key members of many regulatory pathways such as gene expression, chromatin stability, expurgation of introns from naïve mRNA, mRNA splicing, membrane-penetrating activity and pathogenesis-related defense, to name a few. Further, arginine occurs in various combinations with other amino acids (serine, lysine, proline, tryptophan, valine, glycine and glutamic acid) which diversify the potential functions of arginine-rich proteins. Arginine-rich proteins known till date from dietary sources have been described in terms of their structure and functional properties. A variety of activities such as bactericidal, membrane-penetrating, antimicrobial, anti-hypertensive, pro-angiogenic and others have been reported for arginine-rich proteins. This review attempts to collate the occurrence, functions and the biological significance of this unique class of proteins rich in arginine.