A pancreatic islet-specific glucose-6-phosphatase-related protein (IGRP) was cloned using a subtractive cDNA expression cloning procedure from mouse insulinoma tissue. Two alternatively spliced variants that differed by the presence or absence of a 118-bp exon (exon IV) were detected in normal balb/c mice, diabetic ob/ob mice, and insulinoma tissue. The longer, 1901-bp full-length cDNA encoded a 355-amino acid protein (molecular weight 40,684) structurally related (50% overall identity) to the liver glucose-6-phosphatase and exhibited similar predicted transmembrane topology, conservation of catalytically important residues, and the presence of an endoplasmic reticulum retention signal. The shorter transcript encoded two possible open reading frames (ORFs), neither of which possessed His174, a residue thought to be the phosphoryl acceptor (Pan CJ, Lei KJ, Annabi B, Hemrika W, Chou JY: Transmembrane topology of glucose-6-phosphatase. J Biol Chem 273:6144-6148, 1998). Northern blot and reverse transcription-polymerase chain reaction analysis showed that the mRNA was highly expressed in pancreatic islets and expressed more in beta-cell lines than in an alpha-cell line. It was notably absent in tissues and cell lines of non-islet neuroendocrine origin, and no other major tissue source of the mRNA was found. During development, it was expressed in parallel with insulin mRNA. The mRNA was efficiently translated and glycosylated in an in vitro translation/membrane translocation system and readily transcribed into COS 1, HIT, and CHO cells using cytomegalovirus or Rous sarcoma virus promoters. Whereas the liver glucose-6-phosphatase showed activity in these transfection systems, the IGRP failed to show glucose phosphotransferase or phosphatase activity with p-nitrophenol phosphate, inorganic pyrophosphate, or a range of sugar phosphates hydrolyzed by the liver enzyme. While the metabolic function of the enzyme is not resolved, its remarkable tissue-specific expression warrants further investigation, as does its transcriptional regulation in conditions where glucose responsiveness of the pancreatic islet is altered.

Recent data demonstrate that small synthetic compounds specifically targeting bromodomain proteins can modulate the expression of cancer-related or inflammatory genes. Although these studies have focused on the ability of bromodomains to recognize acetylated histones, it is increasingly becoming clear that histone-like modifications exist on other important proteins, such as transcription factors. However, our understanding of the molecular mechanisms through which these modifications modulate protein function is far from complete. The transcription factor GATA1 can be acetylated at lysine residues adjacent to the zinc finger domains, and this acetylation is essential for the normal chromatin occupancy of GATA1. We have recently identified the bromodomain-containing protein Brd3 as a cofactor that interacts with acetylated GATA1 and shown that this interaction is essential for the targeting of GATA1 to chromatin. Here we describe the structural basis for this interaction. Our data reveal for the first time the molecular details of an interaction between a transcription factor bearing multiple acetylation modifications and its cognate recognition module. We also show that this interaction can be inhibited by an acetyllysine mimic, highlighting the importance of further increasing the specificity of compounds that target bromodomain and extraterminal (BET) bromodomains in order to fully realize their therapeutic potential.Covalent posttranslational modifications (PTMs) on the histone proteins that package eukaryotic DNA have been shown to be essential for normal gene expression. The histone code hypothesis (28) posits that specific combinations of PTMs specify distinct transcriptional outcomes, and although it has been suggested that such a one-to-one correspondence is not supported by existing data (56), the biological significance of these PTMs in gene regulation is clear. The molecular mechanisms underlying the addition, removal, and recognition of these modifications are also now beginning to be understood. For example, the acetylation of lysine side chains in histone N-terminal tails has been strongly linked to the activation of nearby genes (32), and it has been shown that acetyllysinecontaining sequences are specifically recognized by bromodomains, helical bundles that carry an acetyllysine-specific binding pocket (45).A substantial body of work has also supported the hypothesis that inhibition of the addition, removal, or recognition of these PTMs could constitute a powerful method to treat a number of human disorders. Recently, for example, much effort has been focused on the specific inhibition of bromodomains from the bromodomain and extraterminal (ET) domain (BET) family, and several compounds based on a thienotriazolodiazepine platform (JQ1 and I-BET) have been demonstrated to mimic bromodomain targets and show considerable promise as therapeutic agents (17,49).Emerging data (reviewed in references 2, 15, 20, 35, and 57) indicate that certain transcription factors (TFs), in addition to histones, are s...

Highlightsd The NuRD complex has a 4:

Chromatin remodeling enzymes act to dynamically regulate gene accessibility. In many cases, these enzymes function as large multicomponent complexes that in general comprise a central ATP-dependent Snf2 family helicase that is decorated with a variable number of regulatory subunits. The nucleosome remodeling and deacetylase (NuRD) complex, which is essential for normal development in higher organisms, is one such macromolecular machine. The NuRD complex comprises ϳ10 subunits, including the histone deacetylases 1 and 2 (HDAC1 and HDAC2), and is defined by the presence of a CHD family remodeling enzyme, most commonly CHD4 (chromodomain helicase DNA-binding protein 4). The existing paradigm holds that CHD4 acts as the central hub upon which the complex is built. We show here that this paradigm does not, in fact, hold and that CHD4 is a peripheral component of the NuRD complex. A complex lacking CHD4 that has HDAC activity can exist as a stable species. The addition of recombinant CHD4 to this nucleosome deacetylase complex reconstitutes a NuRD complex with nucleosome remodeling activity. These data contribute to our understanding of the architecture of the NuRD complex.Nucleosomes effectively act as a roadblock to all aspects of genome biology. ATP-dependent chromatin remodeling enzymes solve this problem by using ATP-derived energy to alter the positions, occupancy and composition of nucleosomes. All remodelers possess a highly related ATPase motor domain from the helicase family and are classified into four subfamilies (INO80, ISWI, SWR1, and CHD) based on sequence similarity (1). Each subfamily is represented in nearly all eukaryotes, suggesting that they catalyze different remodeling events. For example, ISWI proteins reposition (or slide) nucleosomes to create regularly spaced arrays; this periodic organization is a key characteristic of DNA at the start of genes (2). SWR1 and INO80 enzymes have opposing roles in histone variant dynamics; the former incorporates these histone variants (e.g. H2A.Z), whereas the latter removes them. These variants set up specific chromatin structures that modulate transcription and replication, although the roles of many variants are still under debate (3). Fundamentally, these remodeling enzymes all alter the accessibility of DNA to other DNA-binding factors and thereby broadly underpin genome biology.Remodelers frequently act in the context of large multisubunit complexes, and in general, the "mixing and matching" of complex composition can generate complexes with varying activities; the human ISWI protein Snf2h for instance has been identified in six distinct complexes (4). Likewise, the accessory subunits can also modulate remodeler activity. For example, the paralogous methyl-CpG-binding domain proteins 2 and 3 (MBD2 and MBD3) subunits of the nucleosome remodeling and deacetylase (NuRD) 6 complex are mutually exclusive (5); MBD2 recognizes 5-methylcytosine-modified DNA, whereas MBD3 instead binds to 5-hydroxymethylated DNA (6, 7). Unsurprisingly, it has been observed tha...

The Nucleosome Remodeling and Deacetylase (NuRD) complex is essential for development in complex animals but has been refractory to biochemical analysis. We present the first integrated analysis of the architecture of the native mammalian NuRD complex, combining quantitative mass spectrometry, covalent cross-linking, protein biochemistry and electron microscopy. NuRD is built around a 2:2:4 pseudo-symmetric deacetylase module comprising MTA, HDAC and RBBP subunits. This module interacts asymmetrically with a remodeling module comprising one copy each of MBD, GATAD2 and CHD subunits. The previously enigmatic GATAD2 controls the asymmetry of the complex and directly recruits the ATP-dependent CHD remodeler. Unexpectedly, the MTA-MBD interaction acts as a point of functional switching. The transcriptional regulator PWWP2A modulates NuRD assembly by competing directly with MBD for binding to the MTA-HDAC-RBBP subcomplex, forming a 'moonlighting' PWWP2A-MTA-HDAC-RBBP complex that likely directs deacetylase activity to PWWP2A target sites. Taken together, our data describe the overall architecture of the intact NuRD complex and reveal aspects of its structural dynamics and functional plasticity. INO80(1, 2) and SWR1 complexes (3), as well as to the Snf2 (4) and Chromodomain-Helicase-DNA-binding 1 (CHD1) remodelers (5), our understanding of how such enzymes bring about remodeling is still underdeveloped. This is particularly true for the nucleosome remodeling and deacetylase (NuRD) complex.The NuRD complex is widely distributed among Metazoans and is expressed in most, if not all, tissues. It is essential for normal development (6, 7) and is a key regulator in the reprogramming of differentiated cells into pluripotent stem cells (8-10). Age-related reductions in NuRD subunit levels are strongly associated with memory loss, metastatic potential in human cancers (11), and the accumulation of chromatin defects (12, 13).The mammalian NuRD complex comprises at least six subunits (Figure S1a), and for each subunit there are at least two paralogues, giving the potential for significant compositional heterogeneity. CHD4 (and its paralogues CHD3 and -5) is the ATP-dependent DNA translocase in the complex and harbours several regulatory and targeting domains. For example, the PHD domains of CHD4 can recognize histone H3 N-terminal tails bearing methyllysine marks (14-16), and the HMG domain has been shown to bind to poly-ADP(ribose) (17). What distinguishes NuRD from many other remodelers is that it harbours a second catalytic activity, imparted by the histone deacetylases HDAC1 and -2. MBD2 and -3 can bind hydroxymethylated and/or methylated , and RBBP4 and -7 can each bind histone tails (21) and other transcriptional regulators (22,23). The metastasis-associated proteins MTA1, -2 and -3 contain several domains that are associated with nucleosome recognition, whereas GATAD2A and GATAD2B bind to both 25) and CHD proteins (25, 26) but otherwise do not have known functions. Some structural information is available for portions of t...