Showing papers in "Progress in Molecular Biology and Translational Science in 2009"

Chapter 9 Maturation and Degradation of Ribosomal RNA in Bacteria

Abstract: Ribosomal RNAs are the major components of ribosomes and are responsible for their catalytic activity. The three bacterial rRNAs (16S, 23S, and 5S) are cotranscribed as a single molecule that must be converted to the mature, functioning species through a series of nucleolytic processing events and base and sugar modifications that occur in the context of the assembling ribosome. One focus of this review is to examine the reactions that lead from the rRNA precursor to the mature species and to describe the ribonucleases (RNases) that carry out these processing reactions. rRNA, although usually stable in growing cells, also can be degraded if its assembly into ribosomes is aberrant or in response to certain stress conditions, such as starvation. The second focus of this review is to describe these degradative reactions, the RNases that carry them out, and the conditions that initiate the turnover process.

Endonucleolytic initiation of mRNA decay in Escherichia coli.

Abstract: Instability is a fundamental property of mRNA that is necessary for the regulation of gene expression. In E. coli, the turnover of mRNA involves multiple, redundant pathways involving 3'-exoribonucleases, endoribonucleases, and a variety of other enzymes that modify RNA covalently or affect its conformation. Endoribonucleases are thought to initiate or accelerate the process of mRNA degradation. A major endoribonuclease in this process is RNase E, which is a key component of the degradative machinery amongst the Proteobacteria. RNase E is the central element in a multienzyme complex known as the RNA degradosome. Structural and functional data are converging on models for the mechanism of activation and regulation of RNase E and its paralog, RNase G. Here, we discuss current models for mRNA degradation in E. coli and we present current thinking on the structure and function of RNase E based on recent crystal structures of its catalytic core.

Regulation of translation by stress granules and processing bodies.

Abstract: Stress necessitates rapid reprogramming of translation in order to facilitate an adaptive response and promote survival. Cytoplasmic stress granules (SGs) and processing bodies (PBs) are dynamic structures that form in response to stress-induced translational arrest. PBs are linked to mRNA silencing and decay, while SGs are more closely linked to translation and the sorting of specific mRNAs for different fates. While they share some components and can interact physically, SGs and PBs are regulated independently, house separate functions, and contain unique markers. SG formation is associated with numerous disease states, and the expanding list of SG-associated proteins integrates SG formation with other processes such as transcription, splicing, and survival. Growing evidence suggests that SG assembly is initiated by translational arrest, and mediates cross talk with many other signaling pathways.

The making of tRNAs and more - RNase P and tRNase Z.

Abstract: Transfer-RNA (tRNA) molecules are essential players in protein biosynthesis. They are transcribed as precursors, which have to be extensively processed at both ends to become functional adaptors in protein synthesis. Two endonucleases that directly interact with the tRNA moiety, RNase P and tRNase Z, remove extraneous nucleotides on the molecule's 5'- and 3'-side, respectively. The ribonucleoprotein enzyme RNase P was identified almost 40 years ago and is considered a vestige from the "RNA world". Here, we present the state of affairs on prokaryotic RNase P, with a focus on recent findings on its role in RNA metabolism. tRNase Z was only identified 6 years ago, and we do not yet have a comprehensive understanding of its function. The current knowledge on prokaryotic tRNase Z in tRNA 3'-processing is reviewed here. A second, tRNase Z-independent pathway of tRNA 3'-end maturation involving 3'-exonucleases will also be discussed.

Chapter 12 mRNA Interferases, Sequence‐Specific Endoribonucleases from the Toxin–Antitoxin Systems

Abstract: Escherichia coli contains a large number of suicide or toxin genes, whose expression leads to cell growth arrest and eventual cell death. One such toxin, MazF, is an ACA‐specific endoribonuclease, termed “mRNA interferase.” E. coli contains other mRNA interferases with different sequence specificities, which are considered to play important roles in growth regulation under stress conditions, and also in eliminating stress‐damaged cells from a population. Recently, MazF homologues with 5‐base recognition sequences have been identified, for example, those from Mycobacterium tuberculosis . These sequences are significantly underrepresented in the genes for protein families playing a role in the immunity and pathogenesis of M. tuberculosis . An mRNA interferase in Myxococcus xanthus is essential for programmed cell death during fruiting body formation. We propose that mRNA interferases play roles not only in cell growth regulation and programmed cell death, but also in regulation of specific gene expression (either positively or negatively) in bacteria.

Ribosomal protein S6 kinase from TOP mRNAs to cell size.

Abstract: Ribosomal protein S6 kinase (S6K) has been implicated in the phosphorylation of multiple substrates and is subject to activation by a wide variety of signals that converge at mammalian target of rapamycin (mTOR) In the course of the search for its physiological role, it was proposed that S6K activation and ribosomal protein S6 (rpS6) phosphorylation account for the translational activation of a subgroup of transcripts, the TOP mRNAs The structural hallmark of these mRNAs is an oligopyrimidine tract at their 5′-terminus, known as the 5′-TOP motif TOP mRNAs consists of about 90 members that encode multiple components of the translational machinery, such as ribosomal proteins and translation factors The translation efficiency of TOP mRNAs indeed correlates with S6K activation and rpS6 phosphorylation, yet recent biochemical and genetic studies have established that, although S6K and TOP mRNAs respond to similar signals and are regulated by mTOR, they maintain no cause and effect relationship Instead, S6K is primarily involved in regulation of cell size, and affects glucose homeostasis, but is dispensable for global protein synthesis, whereas translational efficiency of TOP mRNAs is a determinant of the cellular protein synthesis capacity Despite extensive studies of their function and mode of regulation, the mechanism underlying the effect of S6K on the cell size, as well as the trans-acting factor that mediates the translational control of TOP mRNAs, still await their identification

The role of 3'-5' exoribonucleases in RNA degradation.

Abstract: RNA degradation is a major process controlling RNA levels and plays a central role in cell metabolism. From the labile messenger RNA to the more stable noncoding RNAs (mostly rRNA and tRNA, but also the expanding class of small regulatory RNAs) all molecules are eventually degraded. Elimination of superfluous transcripts includes RNAs whose expression is no longer required, but also the removal of defective RNAs. Consequently, RNA degradation is an inherent step in RNA quality control mechanisms. Furthermore, it contributes to the recycling of the nucleotide pool in the cell. Escherichia coli has eight 3'-5' exoribonucleases, which are involved in multiple RNA metabolic pathways. However, only four exoribonucleases appear to accomplish all RNA degradative activities: polynucleotide phosphorylase (PNPase), ribonuclease II (RNase II), RNase R, and oligoribonuclease. Here, we summarize the available information on the role of bacterial 3'-5' exoribonucleases in the degradation of different substrates, highlighting the most recent data that have contributed to the understanding of the diverse modes of operation of these degradative enzymes.

Calcium-sensing receptor and associated diseases.

Abstract: The calcium-sensing receptor (CASR) is expressed in parathyroid hormone (PTH)-secreting cells of the parathyroid gland and cells lining the renal tubule. The activated CASR modulates intracellular signaling pathways altering PTH secretion and renal cation and water handling. Inherited abnormalities of the CASR gene give rise to a variety of disorders of mineral ion homeostasis. Heterozygous loss-of-function mutations cause familial (benign) hypocalciuric hypercalcemia (FHH) in which the lifelong mild hypercalcemia is generally asymptomatic. Homozygous inactivating mutations give rise to neonatal severe hyperparathyroidism (NSHPT) with extreme hypercalcemia and marked skeletal changes. Heterozygous activating mutations of the CASR cause autosomal dominant hypocalcemia (ADH) that may be asymptomatic or present with seizures in the neonatal period or childhood or later in life. Phenocopies of FHH or ADH are due to circulating CASR inactivating or activating autoantibodies, respectively. The CASR is the target of small molecule allosteric modifiers, either activators, calcimimetics, or inhibitors, calcilytics.

RNA polyadenylation and decay in mitochondria and chloroplasts.

Abstract: Mitochondria and chloroplasts were originally acquired by eukaryotic cells through endosymbiotic events and retain their own gene expression machinery. One hallmark of gene regulation in these two organelles is the predominance of posttranscriptional control, which is exerted both at the gene-specific and global levels. This review focuses on their mechanisms of RNA degradation, and therefore mainly on the polyadenylation-stimulated degradation pathway. Overall, mitochondria and chloroplasts have retained the prokaryotic RNA decay system, despite evolution in the number and character of the enzymes involved. However, several significant differences exist, of which the presence of stable poly(A) tails, and the location of PNPase in the intermembrane space in animal mitochondria, are perhaps the most remarkable. The known and predicted proteins taking part in polyadenylation-stimulated degradation pathways are described, both in chloroplasts and four mitochondrial types: plant, yeast, trypanosome, and animal.

Mutations in melanocortin-4 receptor and human obesity.

Abstract: Multiple lines of investigations demonstrated that the melanocortin-4 receptor (MC4R) is a critical regulator of energy homeostasis from fishes to humans. Clinical studies in humans showed that mutations in the MC4R gene are the most prevalent form of monogenic obesity. More than 150 mutations have been identified from subjects of different ethnic backgrounds. Functional analyses of the mutant MC4Rs revealed multiple defects, including cell-surface expression, ligand binding, and signaling. Based on the defects, the mutants can be classified into five classes. Potential therapeutic implications from the analyses of the naturally occurring MC4R mutations, such as novel ligands and pharmacological chaperones, are highlighted.

Cell signaling in protein synthesis ribosome biogenesis and translation initiation and elongation.

Abstract: Protein synthesis is a highly energy-consuming process that must be tightly regulated. Signal transduction cascades respond to extracellular and intracellular cues to phosphorylate proteins involved in ribosomal biogenesis and translation initiation and elongation. These phosphorylation events regulate the timing and rate of translation of both specific and total mRNAs. Alterations in this regulation can result in dysfunction and disease. While many signaling pathways intersect to control protein synthesis, the mTOR and MAPK pathways appear to be key players. This chapter briefly reviews the mTOR and MAPK pathways and then focuses on individual phosphorylation events that directly control ribosome biogenesis and translation.

Killer and protective ribosomes.

Abstract: In prokaryotes, translation influences mRNA decay. The breakdown of most Escherichia coli mRNAs is initiated by RNase E, a 5′‐dependent endonuclease. Some mRNAs are protected by ribosomes even if these are located far upstream of cleavage sites (“protection at a distance”), whereas others require direct shielding of these sites. I argue that these situations reflect different modes of interaction of RNase E with mRNAs. Protection at a distance is most impressive in Bacilli, where ribosomes can protect kilobases of unstable downstream sequences. I propose that this protection reflects the role in mRNA decay of RNase J1, a 5′→3′ exonuclease with no E. coli equivalent. Finally, recent years have shown that besides their protective role, ribosomes can also cleave their mRNA under circumstances that cause ribosome stalling. The endonuclease associated with this “killing” activity, which has a eukaryotic counterpart (“no‐go decay”), is not characterized; it may be borne by the distressed ribosome itself.

Poly(A)-assisted RNA decay and modulators of RNA stability.

Abstract: In Escherichia coli, RNA degradation is orchestrated by the degradosome with the assistance of complementary pathways and regulatory cofactors described in this chapter. They control the stability of each transcript and regulate the expression of many genes involved in environmental adaptation. The poly(A)-dependent degradation machinery has diverse functions such as the degradation of decay intermediates generated by endoribonucleases, the control of the stability of regulatory non coding RNAs (ncRNAs) and the quality control of stable RNA. The metabolism of poly(A) and mechanism of poly(A)-assisted degradation are beginning to be understood. Regulatory factors, exemplified by RraA and RraB, control the decay rates of subsets of transcripts by binding to RNase E, in contrast to regulatory ncRNAs which, assisted by Hfq, target RNase E to specific transcripts. Destabilization is often consecutive to the translational inactivation of mRNA. However, there are examples where RNA degradation is the primary regulatory step.

Chapter 4 Structure and Function of Regulator of G Protein Signaling Homology Domains

Abstract: All regulator of G protein signaling (RGS) proteins contain a conserved domain of approximately 130 amino acids that binds to activated heterotrimeric G protein α subunits (Gα) and accelerates their rate of GTP hydrolysis. Homologous domains are found in at least six other protein families, including a family of Rho guanine nucleotide exchange factors (RhoGEFs) and the G protein-coupled receptor kinases (GRKs). Although some of the RhoGEF and GRK RGS-like domains can also bind to activated Gα subunits, they do so in distinct ways and with much lower levels of GTPase activation. In other protein families, the domains have as of yet no obvious relationship to heterotrimeric G protein signaling. These RGS homology (RH) domains are now recognized as mediators of extraordinarily diverse protein-protein interactions. Through these interactions, they play roles that range from enzyme to molecular scaffold to signal transducing module. In this review, the atomic structures of RH domains from RGS proteins, Axins, RhoGEFs, and GRKs are compared in light of what is currently known about their functional roles.

RNA degradation in Archaea and Gram-negative bacteria different from Escherichia coli.

Abstract: Exoribonucleolytic and endoribonucleolytic activities are important for controlled degradation of RNA and contribute to the regulation of gene expression at the posttranscriptional level by influencing the half-lives of specific messenger RNAs The RNA half-lives are determined by the characteristics of the RNA substrates and by the availability and the properties of the involved proteins-ribonucleases and assisting polypeptides Much is known about RNA degradation in Eukarya and Bacteria, but there is limited information about RNA-degrading enzymes and RNA destabilizing or stabilizing elements in the domain of the Archaea The recent progress in the understanding of the structure and function of the archaeal exosome, a protein complex with RNA-degrading and RNA-tailing capabilities, has given some first insights into the mechanisms of RNA degradation in the third domain of life and into the evolution of RNA-degrading enzymes Moreover, other archaeal RNases with degrading potential have been described and a new mechanism for protection of the 5'-end of RNA in Archaea was discovered Here, we summarize the current knowledge on RNA degradation in the Archaea Additionally, RNA degradation mechanisms in Rhodobacter capsulatus and Pseudomonas syringae are compared to those in the major model organism for Gram-negatives, Escherichia coli, which dominates our view on RNA degradation in Bacteria

Messenger RNA decay and maturation in Bacillus subtilis.

Abstract: Our understanding of the ribonucleases that act to process and turn over RNA in Bacillus subtilis , a model Gram‐positive organism, has increased greatly in recent years. This chapter discusses characteristics of B. subtilis ribonucleases that have been shown to participate in messenger RNA maturation and decay. Distinct features of a recently discovered ribonuclease, RNase J1, are reviewed, and are put in the context of a mechanism for the mRNA decay process in B. subtilis that differs greatly from the classical model developed for E. coli . This chapter is divided according to three parts of an mRNA—5′ end, body, and 3′ end—that could theoretically serve as sites for initiation of decay. How 5′‐proximal elements affect mRNA half‐life, and especially how these elements interface with RNase J1, forms the basis for a set of “rules” that may be useful in predicting mRNA stability.

Rhodopsin-mediated retinitis pigmentosa.

Abstract: Retinitis pigmentosa (RP) is a genetically and phenotypically heterogeneous group of diseases that cause blindness. Mutations within the rhodopsin gene account for approximately 25% of autosomal dominantly inherited RP cases. Therefore, understanding the mechanisms causing rhodopsin-mediated RP has a significant health impact. To date, results from multiple labs indicate that rhodopsin-mediated RP pathogenesis does not share a common mechanism of degeneration. There is strong evidence that multiple mechanisms are involved, including protein misfolding, mislocalization, release of toxic products, and aberrant signaling. Development of effective treatments requires investigation of the mechanism involved in the different rhodopsin mutations. This chapter focuses on the mechanisms by which rhodopsin mutations cause retinal degeneration, as well as potential therapeutic strategies to treat the disease.

Diseases associated with mutations of the human lutropin receptor.

Abstract: The human lutropin receptor (LHCGR) plays an integral role in male and female reproductive physiology. In response to either placental hCG or pituitary LH, gonadal LHCGR mediates its effects primarily through Gs activation. Heterozygous mutations leading to constitutive activation of the LHCGR cause gonadotropin-independent precocious puberty in males, but have no detectable effects on prepubertal or postpubertal females. Homozygous or compound heterozygous inactivating mutations of the LHCGR cause gonadal resistance to hCG and LH, where the clinical phenotypes associated with these mutations are closely correlated with the severity of the mutation. Inactivating mutations in 46,XY individuals cause Leydig cell hypoplasia and impairments in the differentiation of male external genitalia, the development of secondary sexual characteristics and sperm production. 46,XX siblings with inactivating LHCGR mutations exhibit infertility and varying degrees of menstrual irregularities.

A phylogenetic view of bacterial ribonucleases.

Abstract: A phylogenetic analysis of bacterial genomes shows them to comprise persistent genes, the "paleome" (Greek: palaios, ancient, reminiscent of the origin of life), associated with genes permitting development of life in a particular niche, the "cenome" (from koinos, common, a radical often used in ecology). Most ribonucleases belong to the former, demonstrating their central position in core life processes. These enzymes appear to have often (but not always) evolved through consistent scenarios, generally grouping bacteria into well-defined clades. The evolution of phosphorylases (which salvage energy) is particularly revealing, resulting in diverse complex structures whose function is to degrade RNA. The degradosome of the gamma-Proteobacteria is a paradigm of such complex structures that emphasizes the essential role of energy in degradative processes. The A+T-rich Firmicutes behave in a highly original manner, where many ribonucleases and related proteins coevolve as a group. The recent identification of novel activities in these organisms, stresses the (underestimated) importance of degradation of very short RNAs, as well as 5'-3' degradative processes in Bacteria.

Translational regulatory mechanisms in synaptic plasticity and memory storage.

Abstract: Synaptic activity-dependent protein synthesis is required to convert a labile short-term memory (STM) into a persistent long-term memory (LTM). Indeed, genetic or pharmacological inhibition of translation impairs LTM, but not STM. Long-lasting biochemical and morphological changes of synapses, which underlie learning and memory, also require new protein synthesis. In recent years, a large number of experiments have yielded much new information about the processes that govern translational control of synaptic plasticity during learning and memory processes. Signaling pathways that modulate mRNA translation play critical roles in these processes. In this chapter, we review the mechanisms by which certain translational regulators including eIF2alpha, 4E-BP, S6K, and CPEB control long-term synaptic plasticity and memory consolidation and their involvement in neurologic disease.

V2R mutations and nephrogenic diabetes insipidus.

Abstract: Nephrogenic diabetes insipidus (NDI), which can be inherited or acquired, is characterized by an inability to concentrate urine despite normal or elevated plasma concentrations of the antidiuretic hormone, arginine vasopressin (AVP). Polyuria, with hyposthenuria, and polydipsia are the cardinal clinical manifestations of the disease. Nephrogenic failure to concentrate urine maximally may be due to a defect in vasopressin-induced water permeability of the distal tubules and collecting ducts, to insufficient buildup of the corticopapillary interstitial osmotic gradient, or to a combination of these two factors. Thus, the broadest definition of the term NDI embraces any antidiuretic hormone-resistant urinary-concentrating defect, including medullary disease with low interstitial osmolality, renal failure, and osmotic diuresis. About 90% of patients with congenital NDI are males with X-linked recessive NDI (OMIM 304800)(1) and have mutations in the AVP receptor 2 (AVPR2) gene that codes for the vasopressin V(2) receptor; the gene is located in chromosome region Xq28. In about 10% of the families studied, congenital NDI has an autosomal recessive or autosomal dominant mode of inheritance (OMIM 222000 and 125800)(1). Mutations have been identified in the aquaporin-2 gene (AQP2, OMIM 107777)(1), which is located in chromosome region 12q13 and codes for the vasopressin-sensitive water channel. NDI is clinically distinguishable from neurohypophyseal diabetes insipidus (OMIM 125700(1); also referred to as central or neurogenic diabetes insipidus) by a lack of response to exogenous AVP and by plasma levels of AVP that rise normally with increase in plasma osmolality. Hereditary neurohypophyseal diabetes insipidus is secondary to mutations in the gene encoding AVP (OMIM 192340)(1). Neurohypophyseal diabetes insipidus is also a component of autosomal recessive Wolfram syndrome 1 or DIDMOAD syndrome (diabetes insipidus, diabetes mellitus, optic atrophy, and deafness) (OMIM 222300)(1), an autosomal recessive disorder. Other inherited disorders with complex polyuro-polydipsic syndrome with loss of water, sodium, chloride, calcium, magnesium, and potassium include Bartter syndrome (OMIM 601678)(1) and cystinosis (OMIM 219800)(1), while long-term lithium administration is the main cause of acquired NDI. Here, we use the gene symbols approved by the HUGO Gene Nomenclature Committee (http://www.gene.ucl.ac.uk/nomenclature) and provide OMIM entry numbers [OMIM (Online Mendelian Inheritance in Man)(1); McKusick-Nathans Institute for Genetic Medicine, Johns Hopkins University (Baltimore, MD) and National Center for Biotechnology Information, National Library of Medicine (Bethesda, MD), 2000; World Wide Web URL: http://www.ncbi.nlm.nih.gov/omim/].

Roles of histone H3-lysine 4 methyltransferase complexes in NR-mediated gene transcription.

Abstract: Transcriptional regulation by nuclear hormone receptors (NRs) requires multiple coregulators that modulate chromatin structures by catalyzing a diverse array of posttranslational modifications of histones. Different combinations of these modifications yield dynamic functional outcomes, constituting an epigenetic histone code. This code is inscribed by histone-modifying enzymes and decoded by effector proteins that recognize specific covalent marks. One important modification associated with active chromatin structures is methylation of histone H3-lysine 4 (H3K4). Crucial roles for this modification in NR transactivation have been recently highlighted through our purification and subsequent characterization of a steady-state complex associated with ASC-2, a coactivator of NRs and other transcription factors. This complex, designated ASCOM for ASC-2 complex, contains H3K4-methyltransferase MLL3/HALR or its paralogue MLL4/ALR and represents the first Set1-like H3K4-methyltransferase complex to be reported in vertebrates. This review focuses on recent progress in our understanding of how ASCOM-MLL3 and ASCOM-MLL4 influence NR-mediated gene transcription and of their physiological function.

Regulators of G protein signaling in neuropsychiatric disorders.

Abstract: Regulators of G protein signaling (RGS) comprise a diverse group of about 40 proteins which determine signaling amplitude and duration via modulation of receptor/G protein or receptor/effector coupling. Several members of the RGS family are expressed in the brain, where they have precise roles in regulation of important physiological processes. The unique functions of each RGS can be attributed to its structure, distinct pattern of expression, and regulation, and its preferential interactions with receptors, Gα subunits and other signaling proteins. Evidence suggests dysfunction of RGS proteins is related to several neuropathological conditions. Moreover, clinical and preclinical work reveals that the efficacy and/or side effects of treatments are highly influenced by RGS activity. This article summarizes findings on RGS proteins in vulnerability to several neuropsychiatric disorders, the mechanism via which RGS proteins control neuronal responses and their potential use as drug targets.

Regulation of immune function by G protein-coupled receptors, trimeric G proteins, and RGS proteins.

Abstract: Receptors for chemokines and a variety of ligands such as histamine, nucleosides, and bioactive lipids signal through heterotrimeric G proteins and play critical roles in immune function. Heterotrimeric G protein signaling pathways are subjected to many layers of regulation including regulators of G protein signaling (RGS) proteins that mainly function to attenuate these signaling pathways. This review focuses on the overall importance of G protein-coupled receptor-heterotrimeric G protein-RGS protein signaling in immune function with emphasis on lymphocyte trafficking and motility. Considerable portion is devoted to discussing mechanisms by which chemoattractant receptors activate downstream signaling pathways that function during leukocyte migration. Studies using intravital imaging techniques to monitor lymphocyte trafficking and motility as well as ones probing intracellular spatiotemporal dynamics of trimeric signaling components are also discussed as they increasingly provide mechanistic insights into trimeric G protein signaling networks.

Structure, Function, and Localization of Gβ5–RGS Complexes

Abstract: Members of the R7 subfamily of regulator of G protein signaling (RGS) proteins (RGS6, 7, 9, and 11) exist as heterodimers with the G protein beta subunit Gβ5. These protein complexes are only found in neurons and are defined by the presence of three domains: DEP/DHEX, Gβ5/GGL, and RGS. This article summarizes published work in the following areas: (1) the functional significance of structural organization of Gβ5–R7 complexes, (2) regional distribution of Gβ5–R7 in the nervous system and regulation of R7 family expression, (3) subcellular localization of Gβ5–R7 complexes, and (4) novel binding partners of Gβ5–R7 proteins. The review points out some contradictions between observations made by different research groups and highlights the importance of using alternative experimental approaches to obtain conclusive information about Gβ5–R7 function in vivo.

The melanocortin-1 receptor gene polymorphism and association with human skin cancer.

Abstract: The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

Chapter 3 Regulators of G Protein Signaling Proteins as Central Components of G Protein‐Coupled Receptor Signaling Complexes

Abstract: The regulators of G protein signaling (RGS) proteins bind directly to G protein alpha (Gα) subunits to regulate the signaling functions of Gα and their linked G protein‐coupled receptors (GPCRs) Recent studies indicate that RGS proteins also interact with GPCRs, not just G proteins, to form preferred functional pairs Interactions between GPCRs and RGS proteins may be direct or indirect (via a linker protein) and are dictated by the receptors, rather than the linked G proteins Emerging models suggest that GPCRs serve as platforms for assembling an overlapping and distinct constellation of signaling proteins that perform receptor‐specific signaling tasks Compelling evidence now indicates that RGS proteins are central components of these GPCR signaling complexes This review will outline recent discoveries of GPCR/RGS pairs as well as new data in support of the idea that GPCRs serve as platforms for the formation of multiprotein signaling complexes

Diseases associated with growth hormone-releasing hormone receptor (GHRHR) mutations.

Abstract: The growth hormone (GH)-releasing hormone (GHRH) receptor (GHRHR) belongs to the G protein-coupled receptors family. It is expressed almost exclusively in the anterior pituitary, where it is necessary for somatotroph cells proliferation and for GH synthesis and secretion. Mutations in the human GHRHR gene (GHRHR) can impair ligand binding and signal transduction, and have been estimated to cause about 10% of autosomal recessive familial isolated growth hormone deficiency (IGHD). Mutations reported to date include five splice donor site mutations, two microdeletions, two nonsense mutations, seven missense mutations, and one mutation in the promoter. These mutations have an autosomal recessive mode of inheritance, and heterozygous individuals do not show signs of IGHD, although the presence of an intermediate phenotype has been hypothesized. Conversely, patients with biallelic mutations have low serum insulin-like growth factor-1 and GH levels (with absent or reduced GH response to exogenous stimuli), resulting--if not treated--in proportionate dwarfism. This chapter reviews the biology of the GHRHR, the mutations that affect its gene and their effects in homozygous and heterozygous individuals.

RNA processing and decay in bacteriophage T4.

Abstract: Bacteriophage T4 is the archetype of virulent phage. It has evolved very efficient strategies to subvert host functions to its benefit and to impose the expression of its genome. T4 utilizes a combination of host and phage-encoded RNases and factors to degrade its mRNAs in a stage-dependent manner. The host endonuclease RNase E is used throughout the phage development. The sequence-specific, T4-encoded RegB endoribonuclease functions in association with the ribosomal protein S1 to functionally inactivate early transcripts and expedite their degradation. T4 polynucleotide kinase plays a role in this process. Later, the viral factor Dmd protects middle and late mRNAs from degradation by the host RNase LS. T4 codes for a set of eight tRNAs and two small, stable RNA of unknown function that may contribute to phage virulence. Their maturation is assured by host enzymes, but one phage factor, Cef, is required for the biogenesis of some of them. The tRNA gene cluster also codes for a homing DNA endonuclease, SegB, responsible for spreading the tRNA genes to other T4-related phage.

Follicle stimulating hormone receptor mutations and reproductive disorders.

Abstract: The follicle stimulating hormone receptor (FSHR) plays a critical role in reproductive function. In the males, FSH supports spermatogenesis, whereas in females, FSH is absolutely required for ovarian follicle growth. In females, inactivating mutations in the FSHR result in ovarian dysgenesis with amenorrhea and infertility. The few males reported with severe inactivating mutations exhibited varying spermatogenic defects, but not azoospermia. While these findings may potentially suggest that FSH action is not absolutely required for spermatogenesis, it cannot be ruled out that these individuals have some residual FSHR activity. Gain-of-function mutations in the FSHR cause spontaneous ovarian hyperstimulation syndrome in females due to the inappropriate stimulation of the mutant FSHR by human choriogonadotropin.