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The corepressor mSin3A regulates phosphorylation-induced activation, intranuclear location, and stability of AML1.

The AML1 (RUNX1) gene, one of the most frequent targets of translocations associated with human leukemias, encodes a DNA-binding protein that plays pivotal roles in myeloid differentiation through transcriptional regulation of various genes. Previously, we reported that AML1 is phosphorylated on two serine residues with dependence on activation of extracellular signal-regulated kinase, which positively regulates the transcriptional activity of AML1. Here, we demonstrate that the interaction between AML1 and the corepressor mSin3A is regulated by phosphorylation of AML1 and that release of AML1 from mSin3A induced by phosphorylation activates its transcriptional activity. Furthermore, phosphorylation of AML1 regulates its intranuclear location and disrupts colocalization of AML1 with mSin3A in the nuclear matrix. PEBP2 beta/CBF beta, a heterodimeric partner of AML1, was shown to play a role in protecting AML1 from proteasome-mediated degradation. We show that mSin3A also protects AML1 from proteasome-mediated degradation and that phosphorylation-induced release of AML1 from mSin3A results in degradation of AML1 in a time-dependent manner. This study provides a novel regulatory mechanism for the function of transcription factors mediated by protein modification and interaction with cofactors.

Animals↗

Common fold in helix-hairpin-helix proteins.

Helix-hairpin-helix (HhH) is a widespread motif involved in non-sequence-specific DNA binding. The majority of HhH motifs function as DNA-binding modules, however, some of them are used to mediate protein-protein interactions or have acquired enzymatic activity by incorporating catalytic residues (DNA glycosylases). From sequence and structural analysis of HhH-containing proteins we conclude that most HhH motifs are integrated as a part of a five-helical domain, termed (HhH)(2) domain here. It typically consists of two consecutive HhH motifs that are linked by a connector helix and displays pseudo-2-fold symmetry. (HhH)(2) domains show clear structural integrity and a conserved hydrophobic core composed of seven residues, one residue from each alpha-helix and each hairpin, and deserves recognition as a distinct protein fold. In addition to known HhH in the structures of RuvA, RadA, MutY and DNA-polymerases, we have detected new HhH motifs in sterile alpha motif and barrier-to-autointegration factor domains, the alpha-subunit of Escherichia coli RNA-polymerase, DNA-helicase PcrA and DNA glycosylases. Statistically significant sequence similarity of HhH motifs and pronounced structural conservation argue for homology between (HhH)(2) domains in different protein families. Our analysis helps to clarify how non-symmetric protein motifs bind to the double helix of DNA through the formation of a pseudo-2-fold symmetric (HhH)(2) functional unit.

Amino Acid Sequence↗

Regulation of gene expression in cultured embryonic mouse mandibular mesenchyme by serotonin antagonists.

During murine embryogenesis, uptake sites for the neurotransmitter serotonin (5-HT) are transiently expressed in craniofacial epithelial structures. Based on malformations produced in cultured mouse embryos exposed to uptake inhibitors or receptor ligands, we have proposed that 5-HT acts as a dose-dependent morphogenetic signal during critical periods of craniofacial development. Several 5-HT receptor subtypes are co-distributed with tenascin and the calcium binding protein S-100 beta in developing craniofacial mesenchyme. Since these molecules are thought to be important for craniofacial development, their regulation by 5-HT could mediate some of its morphogenetic actions. Mandibular mesenchyme cells, from E12 mouse embryos (plug day = E1), grown in micromass cultures were used as an in vitro model to investigate whether 5-HT regulates expression of these molecules. Immunocytochemistry revealed expression of S-100 beta, tenascin, cartilage proteoglycan core protein (a component of the cartilage matrix) and a variety of 5-HT receptors in these cultures. To block the actions of 5-HT (from serum in the culture medium), cultures were exposed to one of these selective 5-HT receptor antagonists and effects on expression were investigated using quantitative immunobinding and in situ hybridization assays. These antagonists differentially regulated expression of cartilage core protein, S-100 beta and tenascin. Antagonism of 5-HT3 receptors by Zofran or 5-HT1A receptors by NAN-190 reduced the amount of core protein, whereas antagonism of 5-HT2A-C receptors by mianserin had no significant effect. All three antagonists stimulated levels of tenascin mRNA and protein. Expression of S-100 beta mRNA and protein was inhibited by Zofran and stimulated by mianserin, whereas NAN-190 had no significant effect. The differential effects of antagonists suggest that in vivo, 5-HT could: (1) promote expression of cartilage core protein by activation of 5-HT3 or 5-HT1A receptors, (2) inhibit production of tenascin by activation of multiple receptors, (3) promote or inhibit synthesis of S-100 beta by activation of 5-HT3 or 5-HT2 receptors, respectively. These actions may be important components of the morphogenetic functions of 5-HT during craniofacial development.

Aggrecans↗

Genistein and zinc synergistically enhance gene expression and mineralization in osteoblastic MC3T3-E1 cells.

Genistein, a phytoestrogen, and zinc, an essential trace element, have an anabolic effect on bone components. We investigated whether the combination of genistein and zinc has additive and synergistic effects in osteoblastic cells in vitro. Osteoblastic MC3T3-E1 cells with subconfluency were cultured for 48 h in medium containing either vehicle, genistein (10(-6) or 10(-5) M), zinc sulfate (10(-5) or 10(-4) M) or genistein (10(-6) or 10(-5) M) plus zinc sulfate (10(-5) or 10(-4) M). The protein content in the osteoblastic cells was significantly increased in the presence of genistein (10(-5) M) or zinc (10(-4) M). This increase was significantly enhanced with a culture of genistein (10(-5) M) plus zinc (10(-4) M). The expression of mRNAs of alpha1(I) collagen and osteocalcin, which are bone matrix protein, in osteoblastic cells was demonstrated using reverse transcription-polymerase chain reaction (RT-PCR) analysis with a specific primer. This expression was not significantly increased with genistein (10(-6) or 10(-5) M) or zinc (10(-5) or 10(-4) M). The combination of genistein (10(-6) or 10(-5) M) and zinc (10(-5) or 10(-4) M) caused a significant increase in alpha1(I) collagen and osteocalcin mRNA expression in osteoblastic cells. Alkaline phosphatase mRNA expression in osteoblastic cells was significantly enhanced with a culture of genistein (10(-5) M) plus zinc (10(-4) M). The effect of genistein (10(-5) M) plus zinc (10(-4) M) in enhancing the expression of alpha1(I) collagen, alkaline phosphatase, or osteocalcin mRNAs in osteoblastic cells was completely prevented with a culture of cycloheximide (10(-7) M), an inhibitor of protein synthesis, or 5,6-dichloro-1-beta-D-ribofuranosylbenimidazole (DRB) (10(-6) M), an inhibitor of transcriptional activity. Moreover, a culture with genistein (10(-5) M) or zinc (10(-4) M) for 14 or 21 days caused a significant increase in mineralization. This increase was markedly enhanced with a culture of the combination of genistein (10(-5) M) and zinc (10(-4) M). This study demonstrates that the combination of genistein and zinc can synergistically enhance gene expression and mineralization in osteoblastic cells.

Alkaline Phosphatase↗

Fusion of the TEL gene on 12p13 to the AML1 gene on 21q22 in acute lymphoblastic leukemia.

Chromosomal rearrangements involving band 12p13 are found in a wide variety of human leukemias but are particularly common in childhood acute lymphoblastic leukemia. The genes involved in these rearrangements, however, have not been identified. We now report the cloning of a t(12;21) translocation breakpoint involving 12p13 and 21q22 in two cases of childhood pre-B acute lymphoblastic leukemia, in which t(12;21) rearrangements were not initially apparent. The consequence of the translocation is fusion of the helix-loop-helix domain of TEL, an ETS-like putative transcription factor, to the DNA-binding and transactivation domains of the transcription factor AML1. These data show that TEL, previously shown to be fused to the platelet-derived growth factor receptor beta in chronic myelomonocytic leukemia, can be implicated in the pathogenesis of leukemia through its fusion to either a receptor tyrosine kinase or a transcription factor. The TEL-AML1 fusion also indicates that translocations affecting the AML1 gene can be associated with lymphoid, as well as myeloid, malignancy.

Amino Acid Sequence↗

A coiled-coil from the RNA polymerase beta' subunit allosterically induces selective nontemplate strand binding by sigma(70).

For transcription to initiate, RNA polymerase must recognize and melt promoters. Selective binding to the nontemplate strand of the -10 region of the promoter is central to this process. We show that a 48 amino acid (aa) coiled-coil from the beta' subunit (aa 262--309) induces sigma(70) to perform this function almost as efficiently as core RNA polymerase itself. We provide evidence that interaction between the beta' coiled-coil and region 2.2 of sigma(70) promotes an allosteric transition that allows sigma(70) to selectively recognize the nontemplate strand. As the beta' 262--309 peptide can function with the previously crystallized portion of sigma(70), nontemplate recognition can be reconstituted with only 47 kDa, or 1/10 of holoenzyme.

Allosteric Regulation↗

RUNX expression and function in human B cells.

RUNX1 and RUNX3 are expressed at many stages of B-cell differentiation, suggesting that they play a role in the development and functions of this lineage. Transgenic mice lacking expression of RUNX1 or the RUNX protein-binding partner, CBFbeta, have defective B-cell development, with differentiation blocked at an early stage. Specific knockout of RUNX1 in adult hematopoietic cells also caused a decrease in the number of mature B cells, supporting a role for RUNX1 in both developmental and adult hematopoiesis. Furthermore, RUNX proteins have been shown to regulate several B-cell-specific genes and play an important role in TGF-beta-induced immunoglobulin class switching to IgA. The importance of RUNX1 in B-cell development is additionally demonstrated by its dysregulation in the t(12;21) translocation, which is the most frequent translocation found in acute lymphocytic leukemia. Epstein Barr virus immortalized human B lymphoblastoid cell lines express RUNX3, and cross-regulation of RUNX1 by RUNX3 occurs in these cells. Knockdown of RUNX3 in these cells induces RUNX1 expression and inhibits cell proliferation, directly showing that RUNX proteins can regulate B-cell growth.

B-Lymphocytes↗

A RUNX2/PEBP2alpha A/CBFA1 mutation displaying impaired transactivation and Smad interaction in cleidocranial dysplasia.

Cleidocranial dysplasia (CCD), an autosomal-dominant human bone disease, is thought to be caused by heterozygous mutations in runt-related gene 2 (RUNX2)/polyomavirus enhancer binding protein 2alphaA (PEBP2alphaA)/core-binding factor A1 (CBFA1). To understand the mechanism underlying the pathogenesis of CCD, we studied a novel mutant of RUNX2, CCDalphaA376, originally identified in a CCD patient. The nonsense mutation, which resulted in a truncated RUNX2 protein, severely impaired RUNX2 transactivation activity. We show that signal transducers of transforming growth factor beta superfamily receptors, Smads, interact with RUNX2 in vivo and in vitro and enhance the transactivation ability of this factor. The truncated RUNX2 protein failed to interact with and respond to Smads and was unable to induce the osteoblast-like phenotype in C2C12 myoblasts on stimulation by bone morphogenetic protein. Therefore, the pathogenesis of CCD may be related to the impaired Smad signaling of transforming growth factor beta/bone morphogenetic protein pathways that target the activity of RUNX2 during bone formation.

3T3 Cells↗

Structural and molecular studies of human chorionic gonadotropin and its receptor.

Human chorionic gonadotropin (hCG) is a placental hormone that stimulates secretion of the pregnancy-sustaining steroid progesterone. It and other glycoprotein hormones are disulfide-rich heterodimers that share a common alpha chain and distinctive beta chains specific to their particular G protein-linked receptors. We determined the structure of partially deglycosylated hCG at 2.6 A resolution from multiwavelength anomalous diffraction (MAD) measurements of a selenomethionyl hCG crystal. We have also begun three- and four-dimensional structural studies on the biologically active hormone and have determined the structure of the carbohydrate attached to the alpha-subunit. Despite little sequence similarity limited to 10% identity, the alpha and beta subunits of hCG maintain strikingly similar tertiary folds, with cystine-knot motifs at cores of extended hairpin loops. Structural and sequence comparisons indicate an evolutionary homology between the glycoprotein hormone chains and other cystine-knot proteins, notably PDGF, TGF-beta, and NGF. This structural similarity has led us to speculate that early hCG secretion has a broader role than solely the stimulation of the corpus luteum; indeed, levels of hCG, which rise rapidly in the circulation after implantation, are greater than the levels necessary for corpus luteum function. One such role of hCG or its subunits could be as a growth factor that facilitates endometrial receptivity. Our studies of hCG have also identified structural variants, notably in the carbohydrate moiety, that are distinctive for patients with a variety of disorders of pregnancy, including hydatidiform mole and choriocarcinoma. We have also focused our efforts on using information gleaned from the structure of hCG for the design of drug-like molecules that might serve as either agonists or antagonists of hCG. To facilitate these experiments, we have designed a rapid screen for the identification of molecules that might bind the hCG receptor by identifying compounds that disrupt binding of hCG to its receptor. This screen employs a filamentous phage that displays the extracellular domain of the hCG receptor on its surface. Thus far, we have identified a few compounds that disrupt binding of hCG with its receptor at a concentration of approximately 1 micromolar. These "lead" molecules are currently being modified in an attempt to identify a molecule that can disrupt binding of hCG at nanomolar concentrations.

Amino Acid Sequence↗

The structure of GABPalpha/beta: an ETS domain- ankyrin repeat heterodimer bound to DNA.

GA-binding protein (GABP) is a transcriptional regulator composed of two structurally dissimilar subunits. The alpha subunit contains a DNA-binding domain that is a member of the ETS family, whereas the beta subunit contains a series of ankyrin repeats. The crystal structure of a ternary complex containing a GABPalpha/beta ETS domain-ankyrin repeat heterodimer bound to DNA was determined at 2. 15 angstrom resolution. The structure shows how an ETS domain protein can recruit a partner protein using both the ETS domain and a carboxyl-terminal extension and provides a view of an extensive protein-protein interface formed by a set of ankyrin repeats. The structure also reveals how the GABPalpha ETS domain binds to its core GGA DNA-recognition motif.

Amino Acid Sequence↗

Human DNA Polymerase-beta Promoter: Phorbol Ester Activation Is Mediated through the cAMP Response Element and cAMP-Response-Element-Binding Protein.

That mammalian DNA polymerase-beta (beta-pol) gene transcription is upregulated by activated ras and also by phorbol ester (TPA) treatment suggests the involvement of protein kinase C in the gene expression control for this DNA repair enzyme. Yet, the core promoters of the human, bovine and rodent beta-pol genes do not have a TPA response element or other binding site for the transcriptional activator AP-1. Instead, these beta-pol promoters appear to be regulated mainly by proteins binding to the cAMP response element (CRE) centered within 50 bp 5' of the transcriptional start site. In this study, the CRE in the human beta-pol promoter was found to mediate TPA upregulation of the cloned promoter in HeLa cell transient expression experiments. To further examine the role of this CRE in TPA stimulation, we used several mutated promoters that were either deficient in protein binding to the CRE or contained extra CRE sites arranged as tandem repeats. All constructs with at least one functional CRE were upregulated by TPA, whereas mutants lacking CRE protein-binding function were not TPA upregulated. Analyses of HeLa nuclear extract DNA-binding proteins indicated that the beta-pol CRE was bound by CRE-binding protein (CREB) family members CREB-1 and activating transcription factor-1, but not by AP-1 or complexes containg AP-1 subunits. These results suggest that CREB, rather than AP-1 proteins, are required for the CRE-mediated TPA activation of the beta-pol promoter.

Journal Article↗

Spatial and temporal expression pattern of Runx3 (Aml2) and Runx1 (Aml1) indicates non-redundant functions during mouse embryogenesis.

The human RUNX3/AML2 gene belongs to the 'runt domain' family of transcription factors that act as gene expression regulators in major developmental pathways. Here, we describe the expression pattern of Runx3 during mouse embryogenesis compared to the expression pattern of Runx1. E10.5 and E14.5-E16.5 embryos were analyzed using both immunohistochemistry and beta-galactosidase activity of targeted Runx3 and Runx1 loci. We found that Runx3 expression overlapped with that of Runx1 in the hematopoietic system, whereas in sensory ganglia, epidermal appendages, and developing skeletal elements, their expression was confined to different compartments. These data provide new insights into the function of Runx3 and Runx1 in organogenesis and support the possibility that cross-regulation between them plays a role in embryogenesis.

Animals↗

AML1a but not AML1b inhibits erythroid differentiation induced by sodium butyrate and enhances the megakaryocytic differentiation of K562 leukemia cells.

AML1 may play a role in growth and differentiation of cells along erythroid and/or megakaryocytic lineages, because a significant level of the AML1 gene is expressed in these cells. We overexpressed AML1a (without the transcription-activating domain) and AML1b (with the domain) proteins in K562 leukemia cells, which can be induced to differentiate into hemoglobin-producing cells and megakaryocytes. The AML1a-transfected K562 cells had a reduced capacity to differentiate in the presence of sodium n-butyrate but not in the presence of other inducers, such as hemin, 1-beta-D-arabinofuranosylcytosine, and herbimycin A. The AML1 antisense oligodeoxynucleotide but not the sense oligomer recovered its differentiation-inducing capacity in the presence of butyrate. On the other hand, AML1b conferred a similar differentiation-inducing capacity upon K562 cells transfected with vector alone. AML1a expression was associated with enhanced sensitivity to megakaryocytic differentiation induced by phorbol ester. These results provide evidence that AML1 proteins play a role in erythroid and megakaryocytic differentiation.

Antigens, CD↗

Regulation of G protein-mediated signal transduction by RGS proteins.

RGS proteins form a new family of regulatory proteins of G protein signaling. They contain homologous core domains (RGS domains) of about 120 amino acids. RGS domains interact with activated Galpha subunits. Several RGS proteins have been shown biochemically to act as GTPase activating proteins (GAPs) for their interacting Galpha subunits. Other than RGS domains, RGS proteins differ significantly in size, amino acid sequences, and tissue distribution. In addition, many RGS proteins have other protein-protein interaction motifs involved in cell signaling. We have shown that p115RhoGEF, a newly identified GEF(guanine nucleotide exchange factor) for RhoGTPase, has a RGS domain at its N-terminal region and this domain acts as a specific GAP for Galpha12 and Galpha13. Furthermore, binding of activated Galpha13 to this RGS domain stimulated GEF activity of p115RhoGEF. Activated Galpha12 inhibited Galpha13-stimulated GEF activity. Thus p115RhoGEF is a direct link between heterotrimeric G protein and RhoGTPase and it functions as an effector for Galpha12 and Galpha13 in addition to acting as their GAP. We also found that RGS domain at N-terminal regions of G protein receptor kinase 2 (GRK2) specifically interacts with Galphaq/11 and inhibits Galphaq-mediated activation of PLC-beta, apparently through sequestration of activated Galphaq. However, unlike other RGS proteins, this RGS domain did not show significant GAP activity to Galphaq. These results indicate that RGS proteins have far more diverse functions than acting simply as GAPs and the characterization of function of each RGS protein is crucial to understand the G protein signaling network in cells.

Animals↗

Structure of the NF-kappa B p50 homodimer bound to DNA.

The structure of a large fragment of the p50 subunit of the human transcription factor NF-kappa B, bound as a homodimer to DNA, reveals that the Rel-homology region has two beta-barrel domains that grip DNA in the major groove. Both domains contact the DNA backbone. The amino-terminal specificity domain contains a recognition loop that interacts with DNA bases; the carboxy-terminal dimerization domain bears the site of I-kappa B interaction. The folds of these domains are related to immunoglobulin-like modules. The amino-terminal domain also resembles the core domain of p53.

Amino Acid Sequence↗

A mutation in the yeast mitochondrial ribosomal protein Rml2p is associated with a defect in catalase gene expression.

Yeast strains containing a new temperature-sensitive allele of the RML2 gene, encoding a component of the large subunit of the mitochondrial ribosome, display normal growth on acetate, slowed growth on glycerol and an inability to grow on oleic acid. These cells, denoted rml2(fat21), have an apparent inability to induce peroxisomal function, as evidenced by a deficiency in oleic acid induction of beta-oxidation. However, the oleic acid regulation of genes encoding core enzymes of peroxisomal beta-oxidation is normal. In contrast, up-regulation of CTA1 (catalase) mRNA expression and enzyme activity is interrupted. Upon comparison of the induction requirements of catalase and the genes of beta-oxidation, we hypothesized that the rml2(fat21) mutation alters the activity of the transcription factor Adr1p. In support of this hypothesis, over-expression of ADR1 in rml2(fat21) cells restores CTA1 induction. Several assays of mitochondria from rml2(fat21) strains suggest normal mitochondrial function. Thus, the modulation of Adr1p-associated gene regulation is not due to overt mitochondrial dysfunction.

Alleles↗

Stability of AML1 (core) site enhancer mutations in T lymphomas induced by attenuated SL3-3 murine leukemia virus mutants.

Murine retrovirus SL3-3 is highly T lymphomagenic. Its pathogenic properties are determined by the transcriptional enhancer of the U3 repeat region which shows preferential activity in T cells. Within the U3 repeats, the major determinant of T-cell specificity has been mapped to binding sites for the AML1 transcription factor family (also known as the core binding factor [CBF], polyomavirus enhancer binding protein 2 [PEBP2], and SL3-3 enhancer factor 1 [SEF-1]). SL3-3 viruses with AML1 site mutations have lost a major determinant of T-cell-specific enhancer function but have been found to retain a lymphomagenic potential, although disease induction is slower than for the SL3-3 wild type. To compare the specificities and mechanisms of disease induction of wild-type and mutant viruses, we have examined lymphomas induced by mutant viruses harboring transversions of three consecutive base pairs critical to AML1 site function (B. Hallberg, J. Schmidt, A. Luz, F. S. Pedersen, and T. Grundström. J. Virol. 65:4177-4181, 1991). Our results show that the mutated AML1 sites are genetically stable during lymphomagenesis and that ecotropic provirus numbers in DNA of tumors induced by wild-type and mutant viruses fall within the same range. Moreover, proviruses were found to be integrated at the c-myc locus in similar proportions of wild-type and mutant SL3-3-induced tumors, and the mutated AML1 sites of proviruses at c-myc are unaltered. In some cases, however, including one c-myc-integrated provirus, a single-base pair change was detected in a second, weaker AML1 binding site. By DNA rearrangement analysis of the T-cell receptor beta-locus, tumors induced by the AML1 site mutants are found to be of the T-cell type. Thus, although the AML1 site mutants have weakened T-cell-specific enhancers they are T-lymphomagenic, and wild-type- and mutant-virus-induced tumor DNAs are similar with respect to the number of overall ecotropic and c-myc-integrated clonal proviruses. The SL3-3 wild-type and AML1 site mutant viruses may therefore induce disease by similar mechanisms.

Animals↗

The Hypocrea jecorina HAP 2/3/5 protein complex binds to the inverted CCAAT-box (ATTGG) within the cbh2 (cellobiohydrolase II-gene) activating element.

The 5' regulatory region of the chh2 gene, encoding cellobiohydrolase II, of the filamentous fungus Hypocrea jecorina contains the cbh2 activating element (CAE) which is essential for cbh2 expression. The CAE consists of two separate, adjacent motifs, a CCAAT box on the template strand (ATTGG) and a GTAATA box on the coding strand, which co-operate in the induction of the gene by cellulose or sophorose. EMSA supershift experiments using an antibody against Aspergillus nidulans HAPC suggested that the complex which binds to the H. jecorina CCAAT box contains a HAPC homolog. To obtain direct evidence for this, we have cloned the hap2, hap3 and hap5 genes from H. jecorina. They encode proteins whose core regions display great similarity to Aspergillus HAPB, HAPC and HAPE and to known HAP homologs from other organisms. All three genes are transcribed in a carbon source-independent manner. A. nidulans deltahap strains were functionally complemented in vitro by the overexpressed H. jecorina HAP2, HAP3 and HAP5 proteins, and they thus represent subunits of the CCAAT-binding complex. Furthermore, all three proteins (HAP2, HAP3 and HAP5) were needed to bind to the CAE in the H. jecorina cbh2 gene promoter in vitro. We conclude that the CCAAT box on the template strand in CAE is bound by the H. jecorina equivalent of the HAP protein complex.

Amino Acid Sequence↗