Characterization of SAP-1, a protein recruited by serum response factor to the c-fos serum response element

Pit-1 binding to specific DNA sites as a monomer or dimer determines gene-specific use of a tyrosine-dependent synergy domain

Transcriptional activation of the prolactin and growth hormone genes, occurring in a cell-specific fashion, requires short-range synergistic interactions between the pituitary-specific POU domain factor Pit-1 and other transcription factors, particularly nuclear receptors. Unexpectedly, we find that these events involve the gene-specific use of alternative Pit-1 synergy domains. Synergistic activation of the prolactin gene by Pit-1 and the estrogen receptor requires a Pit-1 amino-terminal 25-amino-acid domain that is not required for analogous synergistic activation of the growth hormone promoter. The action of this Pit-1 synergy domain is dependent on the presence of two of three tyrosine residues spaced by 6 amino acids and can be replaced by a comparable tyrosine-dependent trans-activation domain of an unrelated transcription factor (hLEF). The gene-specific utilization of this tyrosine-dependent synergy domain is conferred by specific Pit-1 DNA-binding sites that determine whether Pit-1 binds as a monomer or a dimer. Thus, the critical DNA site in the prolactin enhancer, where this domain is required, binds Pit-1 as a monomer, whereas the Pit-1 sites in the growth hormone gene, which do not utilize this synergy domain, bind Pit-1 as a dimer. The finding that the sequence of specific DNA sites dictates alternative Pit-1 synergy domain utilization based on monomeric or dimeric binding suggests an additional regulatory strategy for differential target gene activation in distinct cell types.

Structure of serum response factor core bound to DNA

The human serum response factor is a transcription factor belonging to the MADS domain protein family with members characterized from the plant and animal kingdoms. The X-ray crystal structure of the serum response factor core in a specific-recognition DNA complex shows that the functions of DNA binding, dimerization and accessory-factor interaction are compactly integrated into a novel protein unit. The intrinsic and induced conformation of the serum response element DNA is the principal DNA feature recognized in the specific complex.

Differential interactions of human Sos1 and Sos2 with Grb2

The guanine nucleotide exchange factor Son of sevenless (Sos) performs a crucial step in the coupling of receptor tyrosine kinases to Ras activation. Mammalian cells contain two related but distinct Sos proteins, Sos1 and Sos2. Although they share a high degree of overall similarity, it is not known to what extent their biological and biochemical properties overlap. In the present study, we have compared the interactions of the two human homologues of Sos, hSos1 and hSos2, with the adaptor protein Grb2. We show that hSos2 interacts with Grb2 via its proline-rich COOH-terminal domain and that this interaction is dependent on the SH3 domains of Grb2. In general, these characteristics are similar to the ones reported previously for the interaction of hSos1 with Grb2. However, the apparent binding affinity of hSos2 for Grb2 is significantly higher relative to that of hSos1 both in vitro and in vivo. The region conferring this higher binding affinity has been mapped to residues 1126-1242 of the hSos2 COOH-terminal domain. These results suggest that Sos1 and Sos2 may differentially contribute to receptor-mediated Ras activation.

DNA binding specificity determinants in MADS-box transcription factors

The MADS box is a conserved sequence motif found in the DNA binding domain of a family of transcription factors which possess related but distinct DNA binding specificities. We investigated the basis of differential sequence recognition by the MADS-box proteins serum response factor (SRF), MCM1, and MEF2A, using chimeric proteins and site-directed mutants in conjunction with gel mobility shift and binding site selection assays. Deletion of sequences immediately N terminal to the SRF MADS box alters its preferred binding site to that of MEF2A, although the resulting protein still weakly binds SRF-specific sites: exclusive binding to MEF2 sites requires further mutations, at MADS-box residues 11 to 15. In contrast to SRF, the sequence specificity of MCM1 (and of MEF2A) is determined entirely by sequences within its MADS box, and mutation of only SRF MADS-box residue 1 is sufficient to alter its binding specificity to that of MCM1. However, changes at both MADS-box positions 1 and 11 to 15 are necessary and sufficient to alter the specificity of the MCM1 MADS box to that of MEF2, and vice versa. The role of SRF MADS-box residues which differ from those present in the other proteins was investigated by selection of functional SRF variants in yeast cells. SRF MADS-box position 1 was always a glycine in the variants, but many different sequences at the other nonconserved MADS-box residues were compatible with efficient DNA binding. We discuss potential mechanisms of DNA recognition by MADS-box proteins.

Interaction of papillomavirus E6 oncoproteins with a putative calcium-binding protein

Human papillomaviruses (HPVs) are associated with the majority of cervical cancers and encode a transforming protein, E6, that interacts with the tumor suppressor protein p53. Because E6 has p53-independent transforming activity, the yeast two-hybrid system was used to search for other E6-binding proteins. One such protein, E6BP, interacted with cancer-associated HPV E6 and with bovine papillomavirus type 1 (BPV-1) E6. The transforming activity of BPV-1 E6 mutants correlated with their E6BP-binding ability. E6BP is identical to a putative calcium-binding protein, ERC-55, that appears to be localized in the endoplasmic reticulum.

Integration of MAP kinase signal transduction pathways at the serum response element

The ternary complex factor (TCF) subfamily of ETS-domain transcription factors bind with serum response factor (SRF) to the serum response element (SRE) and mediate increased gene expression. The TCF protein Elk-1 is phosphorylated by the JNK and ERK groups of mitogen-activated protein (MAP) kinases causing increased DNA binding, ternary complex formation, and transcriptional activation. Activated SRE-dependent gene expression is induced by JNK in cells treated with interleukin-1 and by ERK after treatment with phorbol ester. The Elk-1 transcription factor therefore integrates MAP kinase signaling pathways in vivo to coordinate biological responses to different extracellular stimuli.

DNA bending in the ternary nucleoprotein complex at the c-fos promoter

Transcriptional induction of the c-fos proto-oncogene in response to serum growth factors is mediated in part by a ternary complex that forms on the serum response element (SRE) within its promoter. This complex consists of Elk-1, serum response factor (SRF) and the SRE. Elk-1 is phosphorylated by MAP kinase, which correlates with the induction of c-fos transcription. In this study we have investigated the protein-induced DNA bending which occurs during the formation and post-translational modification of the ternary complex that forms at the c-fos SRE. Circular permutation analysis demonstrates that the minimal DNA-binding domain of SRF, which contains the MADS box, is sufficient to induce flexibility into the centre of its binding site within the SRE. Phasing analysis indicates that at least part of this flexibility results in the production of a directional bend towards the minor groove. The isolated ETS domains from Elk-1 and SAP-1 induce neither DNA bending nor increased DNA flexibility. Formation of ternary complexes by binding of Elk-1 to the binary SRF:SRE complex results in a change in the flexibility of the SRE. Phosphorylation of Elk-1 by MAP kinase (p42/ERK2) induces further minor changes in this DNA flexibility. However, phasing analysis reveals that the recruitment of Elk-1 to form the ternary complex affects the SRF-induced directional DNA bend in the SRE. The potential roles of DNA bending at the c-fos SRE are discussed.

ERK2/p42 MAP kinase stimulates both autonomous and SRF-dependent DNA binding by Elk-1

A ternary complex comprised of SRF, ternary complex factor (TCF) and the c-fos SRE is the target of several extracellular signal regulated pathways. Phosphorylation of the TCF Elk-1 is a key event in the activation of this complex. We demonstrate that ERK2/p42 phosphorylation of Elk-1 stimulates its recruitment into ternary complexes with SRF. Moreover, phosphorylation of Elk-1 also stimulates its autonomous SRF-independent binding to high affinity binding sites. Thus part of the effect of ERK2/p42 phosphorylation is to stimulate DNA-binding by the ETS DNA-binding domain of Elk-1.

The Rho family GTPases RhoA, Rac1, and CDC42Hs regulate transcriptional activation by SRF

The c-fos serum response element (SRE) forms a ternary complex with the transcription factors SRF (serum response factor) and TCF (ternary complex factor). By itself, SRF can mediate transcriptional activation induced by serum, lysophosphatidic acid, or intracellular activation of heterotrimeric G proteins. Activated forms of the Rho family GTPases RhoA, Rac1, and CDC42Hs also activate transcription via SRF and act synergistically at the SRE with signals that activate TCF. Functional Rho is required for signaling to SRF by several stimuli, but not by activated CDC42Hs or Rac1. Activation of the SRF-linked signaling pathway does not correlate with activation of the MAP kinases ERK, SAPK/JNK, or MPK2/p38. Functional Rho is required for regulated activity of the c-fos promoter. These results establish SRF as a nuclear target of a novel Rho-mediated signaling pathway.

Pip, a novel IRF family member, is a lymphoid-specific, PU.1-dependent transcriptional activator

The immunoglobulin light-chain gene enhancers E kappa 3', E lambda 2-4, and E lambda 3-1 contain a conserved cell type-specific composite element essential for their activities. This element binds a B cell-specific heterodimeric protein complex that consists of the Ets family member PU.1 and a second factor (NF-EM5), whose participation in the formation of the complex is dependent on the presence of DNA-bound PU.1. In this report we describe the cloning and characterization of Pip (PU.1 interaction partner), a lymphoid-specific protein that is most likely NF-EM5. As expected, the Pip protein binds the composite element only in the presence of PU.1; furthermore, the formation of this ternary complex is critically dependent on phosphorylation of PU.1 at serine-148. The Pip gene is expressed specifically in lymphoid tissues in both B- and T-cell lines. When coexpressed in NIH-3T3 cells, Pip and PU.1 function as mutually dependent transcription activators of the composite element. The amino-terminal DNA-binding domain of Pip exhibits a high degree of homology to the DNA-binding domains of members of the interferon regulatory factor (IRF) family, which includes IRF-1, IRF-2, ICSBP, and ISGF3 gamma.

Sequence-specific targeting of nuclear signal transduction pathways by homeodomain proteins

Cells translate extracellular signals into specific programs of gene expression that reflect their developmental history or identity. We present evidence that one way this interpretation may be performed is by cooperative interactions between serum response factor (SRF) and certain homeodomain proteins. We show that human and Drosophila homeodomain proteins of the paired class have the ability to recruit SRF to DNA sequences not efficiently recognized by SRF on its own, thereby imparting to a linked reporter gene the potential to respond to polypeptide growth factors. This activity requires both the DNA-binding activity of the homeodomain and putative protein-protein contact residues on the exposed surfaces of homeodomain helices 1 and 2. The ability of the homeodomain to impart signal responsiveness is DNA sequence specific, and this specificity differs from the simple DNA-binding specificity of the homeodomain in vitro. The homeodomain imparts response to a spectrum of signals characteristic of the natural SRF-binding site in the c-fos gene. Response to some of these signals is dependent on the secondary recruitment of SRF-dependent ternary complex factors, and we show directly that a homeodomain can promote the recruitment of one such factor, Elk1. We infer that SRF and homeodomains interact cooperatively on DNA and that formation of SRF-homeodomain complexes permits the recruitment of signal-responsive SRF accessory proteins. The ability to route extracellular signals to specific target genes is a novel activity of the homeodomain, which may contribute to the identity function displayed by many homeodomain genes.

Cell cycle-regulated transcription of the CLB2 gene is dependent on Mcm1 and a ternary complex factor

Clb2 is the major B-type mitotic cyclin required for entry into mitosis in the budding yeast Saccharomyces cerevisiae. We showed that accumulation of CLB2 transcripts in G2 cells is controlled at the transcriptional level and identified a 55-bp upstream activating sequence (UAS) containing an Mcm1 binding site as being necessary and sufficient for cell cycle regulation. Sequences within the cell cycle-regulated UAS were shown to bind Mcm1 in vitro, and mutation which abolished Mcm1-dependent DNA binding activity eliminated cell cycle-regulated transcription in vivo. A second protein with no autonomous DNA binding activity was also recruited into Mcm1-UAS complexes, generating a ternary complex. A point mutation in the CLB2 UAS which blocked ternary complex formation, but still allowed Mcm1 to bind, resulted in loss of cell cycle regulation in vivo, suggesting that the ternary complex factor is also important in control of CLB2 transcription. We discuss the possibility that the CLB2 gene is coregulated with other genes known to be regulated with the same periodicity and suggest that Mcm1 and the ternary complex factor may coordinately regulate several other G2-regulated transcripts.

The Drosophila E74 gene is required for metamorphosis and plays a role in the polytene chromosome puffing response to ecdysone

The steroid hormone ecdysone initiates Drosophila metamorphosis by reprogramming gene expression during late larval and prepupal development. The ecdysone-inducible gene E74, a member of the ets proto-oncogene family, has been proposed to play a key role in this process. E74 is encoded within the 74EF early puff and consists of two overlapping transcription units, E74A and E74B. To assess the function(s) of E74 during metamorphosis, we have isolated and characterized recessive loss-of-function mutations specific to each transcription unit. We find that mutations in E74A and E74B are predominantly lethal during prepupal and pupal development, consistent with a critical role for their gene products in metamorphosis. Phenotypic analysis reveals that E74 function is required for both pupariation and pupation, and for the metamorphosis of both larval and imaginal tissues. E74B mutants are defective in puparium formation and head eversion and die as prepupae or cryptocephalic pupae, while E74A mutants pupariate normally and die either as prepupae or pharate adults. We have also investigated the effects of the E74 mutations on gene expression by examining the puffing pattern of the salivary gland polytene chromosomes in newly formed mutant prepupae. Most puffs are only modestly affected by the E74B mutation, whereas a subset of late puffs are sub-maximally induced in E74A mutant prepupae. These observations are consistent with Ashburner's proposal that early puff proteins induce the formation of late puffs, and define E74A as a regulator of late puff activity. They also demonstrate that E74 plays a wide role in reshaping the insect during metamorphosis, affecting tissues other than the salivary gland in which it was originally identified.

The Drosophila E74 gene is required for the proper stage- and tissue-specific transcription of ecdysone-regulated genes at the onset of metamorphosis

The steroid hormone ecdysone directly induces a small set of early genes, visible as puffs in the larval salivary gland polytene chromosomes, as it signals the onset of Drosophila metamorphorsis. The products of these genes appear to function as regulators that both repress their own expression and induce a large set of secondary-response late genes. We have identified recessive loss-of-function mutations in the early gene E74, a member of the ets protooncogene family that encodes two related DNA-binding proteins, E74A and E74B. These mutations cause defects in pupariation and pupation, and result in lethality during metamorphosis. Here we extend our phenotypic characterization of the E74A and E74B mutant alleles to the molecular level by examining their effects on the transcription of over 30 ecdysone-regulated genes. We show that the transcription of most ecdysone primary-response genes during late larval and prepupal development is unaffected by the E74 mutations. Rather, we find that E74 is necessary for the appropriate regulation of many ecdysone secondary-response genes. E74B is required for the maximal induction of glue genes in mid third instar larval salivary glands, while E74A is required in early prepupae for the proper timing and maximal induction of a subset of late genes. E74 activity is also necessary for the correct regulation of genes expressed predominantly in the fat body, epidermis or imaginal discs. These observations confirm that E74 plays a critical role in regulating transcription during the early stages of Drosophila metamorphosis. In addition, the widespread effects of the E74 mutations on transcription indicate that E74 functions in regulatory hierarchies not only in the larval salivary gland, but throughout the entire organism.

Tissue-specific regulation of the insulin gene by a novel basic helix-loop-helix transcription factor

The insulin gene is one of the best paradigms of tissue-specific gene expression. It is developmentally regulated and is expressed exclusively in the pancreatic beta-cell. This restricted expression is directed by a tissue-specific enhancer, within the promoter, which contains an E-box sequence. The insulin E-box binds an islet-specific protein complex, termed 3a1. E-boxes bind proteins belonging to the basic helix-loop-helix (bHLH) family of transcription factors. The bHLH proteins function as potent transcriptional activators of tissue-specific genes by forming heterodimers between ubiquitous and cell-restricted family members. In addition, the cell-restricted bHLH members play an important role in specifying cell fate. To isolate the tissue-specific bHLH factor controlling insulin gene expression and study its role in islet cell differentiation, a modified yeast two-hybrid system was utilized to clone a novel bHLH factor, BETA2 (beta-cell E-box trans-activator 2), from a hamster insulin tumor (HIT) cell cDNA library. Northern analysis demonstrates that high-level expression of the BETA2 gene is restricted to pancreatic alpha- and beta-cell lines. As expected of tissue-specific bHLH members, BETA2 binds to the insulin E-box sequence with high affinity as a heterodimer with the ubiquitous bHLH factor E47. More importantly, antibody supershift experiments clearly show that BETA2 is a component of the native insulin E-box-binding complex. Transient transfection assays demonstrate that the BETA2/E47 heterodimer synergistically interacts with a neighboring beta-cell-specific complex to activate an insulin enhancer. In contrast, other bHLH factors such as MyoD and E47, which can bind to the insulin E-box with high affinity, fail to do so. Thus, a unique, cooperative interaction is the basis by which the insulin E-box enhancer discriminates between various bHLH factors to achieve tissue-specific activation of the insulin gene.

The solution structure of the RING finger domain from the acute promyelocytic leukaemia proto-oncoprotein PML

Acute promyelocytic leukaemia (APL) has been ascribed to a chromosomal translocation event which results in a fusion protein comprising the PML protein and the retinoic acid receptor alpha. PML is normally a component of a nuclear multiprotein complex (termed ND10, Kr bodies, nuclear bodies, PML oncogenic domains or PODs) which is disrupted in the APL disease state. PML contains a number of characterized motifs including a Zn2+ binding domain called the RING or C3HC4 finger. Here we describe the solution structure of the PML RING finger as solved by 1H NMR methods at physiological pH with r.m.s. deviations for backbone atoms of 0.88 and 1.39 A for all atoms. Additional biophysical studies including CD and optical spectroscopy, show that the PML RING finger requires Zn2+ for autonomous folding and that cysteines are used in metal ligation. A comparison of the structure with the previously solved equine herpes virus IE110 RING finger, shows significant differences suggesting that the RING motif is structurally diverse. The role of the RING domain in PML nuclear body formation was tested in vivo, by using site-directed mutagenesis and immunofluorescence on transiently transfected NIH 3T3 cells. Independently mutating two pairs of cysteines in each of the Zn2+ binding sites prevents PML nuclear body formation, suggesting that a fully folded RING domain is necessary for this process. These results suggest that the PML RING domain is probably involved in protein-protein interactions, a feature which may be common to other RING finger domains.

The MADS-box family of transcription factors

The MADS-box family of transcription factors has been defined on the basis of primary sequence similarity amongst numerous proteins from a diverse range of eukaryotic organisms including yeasts, plants, insects, amphibians and mammals. The MADS-box is a conserved motif found within the DNA-binding domains of these proteins and the name refers to four of the originally identified members: MCM1, AG, DEFA and SRF. Several proteins within this family have significant biological roles. For example, the human serum-response factor (SRF) is involved in co-ordinating transcription of the protooncogene c-fos, whilst MCM1 is central to the transcriptional control of cell-type specific genes and the pheromone response in the yeast Saccharomyces cerevisiae. The RSRF/MEF2 proteins comprise a sub-family of this class of transcription factors which are key components in muscle-specific gene regulation. Moreover, in plants, MADS-box proteins such as AG, DEFA and GLO play fundamental roles during flower development. The MADS-box is a contiguous conserved sequence of 56 amino acids, of which 9 are identical in all family members described so far. Several members have been shown to form dimers and consequently two functional regions within the MADS-box have been defined. The N-terminal half is the major determinant of DNA-binding specificity whilst the C-terminal half is necessary for dimerisation. This organisation allows the potential formation of numerous proteins, with subtly different DNA-binding specificities, from a limited number of genes by heterodimerisation between different MADS-box proteins. The majority of MADS-box proteins bind similar sites based on the consensus sequence CC(A/T)6GG although each protein apparently possesses a distinct binding specificity. Moreover, several MADS-box proteins specifically recruit other transcription factors into multi-component regulatory complexes. Such interactions with other proteins appears to be a common theme within this family and play a pivotal role in the regulation of target genes.

The MADS-box family of transcription factors

The MADS-box family of transcription factors has been defined on the basis of primary sequence similarity amongst numerous proteins from a diverse range of eukaryotic organisms including yeasts, plants, insects, amphibians and mammals. The MADS-box is a conserved motif found within the DNA-binding domains of these proteins and the name refers to four of the originally identified members: MCM1, AG, DEFA and SRF. Several proteins within this family have significant biological roles. For example, the human serum-response factor (SRF) is involved in co-ordinating transcription of the protooncogene c-fos, whilst MCM1 is central to the transcriptional control of cell-type specific genes and the pheromone response in the yeast Saccharomyces cerevisiae. The RSRF/MEF2 proteins comprise a sub-family of this class of transcription factors which are key components in muscle-specific gene regulation. Moreover, in plants, MADS-box proteins such as AG, DEFA and GLO play fundamental roles during flower development. The MADS-box is a contiguous conserved sequence of 56 amino acids, of which 9 are identical in all family members described so far. Several members have been shown to form dimers and consequently two functional regions within the MADS-box have been defined. The N-terminal half is the major determinant of DNA-binding specificity whilst the C-terminal half is necessary for dimerisation. This organisation allows the potential formation of numerous proteins, with subtly different DNA-binding specificities, from a limited number of genes by heterodimerisation between different MADS-box proteins. The majority of MADS-box proteins bind similar sites based on the consensus sequence CC(A/T)6GG although each protein apparently possesses a distinct binding specificity. Moreover, several MADS-box proteins specifically recruit other transcription factors into multi-component regulatory complexes. Such interactions with other proteins appears to be a common theme within this family and play a pivotal role in the regulation of target genes.

The smooth muscle alpha-actin gene promoter is differentially regulated in smooth muscle versus non-smooth muscle cells

To identify potential regulators of smooth muscle cell (SMC) differentiation, we studied the molecular mechanisms that control the tissue-specific transcriptional expression of SM alpha-actin, the most abundant protein in fully differentiated SMCs. A construct containing the region from -1 to -125 of the promoter (p125CAT) had high transcriptional activity in SMCs (57-fold > promoterless) and endothelial cells (ECs) (18-fold) but not in skeletal myoblasts or myotubes. Mutation of either of two highly conserved CC(AT-rich)6GG (CArG) motifs at -62 and -112 abolished the activity of p125CAT in SMCs but had no effect in ECs. In contrast, high transcriptional activity in skeletal myotubes, which also express SM alpha-actin, required at least 271 base pairs of the promoter (-1 to > or = -271). Constructs containing 547 base pairs or more of the promoter were transcriptionally active in SMCs and skeletal myotubes but had no activity in skeletal myoblasts or ECs, cell types that do not express SM alpha-actin. Electrophoretic mobility shift assays provided evidence for binding of a unique serum response factor-containing complex of factors to the CArG box elements in SMCs. Results indicate that: 1) transcriptional expression of SM alpha-actin in SMCs requires the interaction of the CArG boxes with SMC nucleoprotein(s); 2) expression of SM alpha-actin in skeletal myotubes requires different cis-elements and trans-factors than in SMCs; and 3) negative-acting cis-elements are important in restricting transcription in cells that do not express SM alpha-actin.

Cell cycle-regulated nuclear import and export of Cdc47, a protein essential for initiation of DNA replication in budding yeast

The CDC47 gene was isolated by complementation of a cdc47 temperature-sensitive mutant in Saccharomyces cerevisiae and was shown to encode a predicted polypeptide, Cdc47, of 845 aa. Cdc47 belongs to the Cdc46/Mcm family of proteins, previously shown to be essential for initiation of DNA replication. Using indirect immunofluorescence microscopy and subcellular fractionation techniques, we show that Cdc47 undergoes cell cycle-regulated changes in its subcellular localization. At mitosis, Cdc47 enters the nucleus, where it remains until soon after the initiation of DNA replication, when it is rapidly exported back into the cytoplasm. Cdc47 protein levels do not vary with the cell cycle, but expression of CDC47 and nascent synthesis of Cdc47 occur late in the cell cycle, coinciding with mitosis. Together, these results show that Cdc47 is not only imported into the nucleus at the end of mitosis but is also exported back into the cytoplasm at the beginning of S phase. The observation that Cdc47 is exported from the nucleus at the beginning of S phase has important implications for how initiation of DNA replication is controlled.

CpG methylation has differential effects on the binding of YY1 and ETS proteins to the bi-directional promoter of the Surf-1 and Surf-2 genes

The divergently transcribed Surf-1 and Surf-2 housekeeping genes are separated by a bi-directional, TATA-less promoter which lies within a CpG-rich island. Here we show that CpG methylation severely reduces transcription in the direction of both Surf-1 and Surf-2. Previous work has identified three promoter elements (Su1, Su2 and Su3) which are conserved between the human and mouse Surf-1/Surf-2 promoters. These elements bind transcription factors present in human and mouse cell nuclear extracts in vitro and mutations which prevent factor binding also reduce promoter activity in vivo. Transcription initiation factor YY1 binds to the Su1 site and stimulates transcription in the direction of Surf-1 and, to a lesser extent, Surf-2. Here we show that members of the ETS family of transcription factors bind to the Su2 site. Although the Su1 factor binding site contains three CpG dinucleotides, the binding of YY1 is not affected by CpG methylation. In contrast, CpG methylation abolishes the binding of ETS proteins to the Su2 site; methylation of a single cytosine, at position 3 of the consensus ETS site, is sufficient to prevent factor binding. This direct effect on the binding of ETS proteins is, however, not in itself sufficient to explain the repression of this promoter by CpG methylation. A mutation of the Su2 site which removes the sequence CpG, but which does not prevent ETS factor binding, fails to relieve this promoter from repression by CpG methylation.

Characterization of the Elk-1 ETS DNA-binding domain

The ETS domain family of transcription factors is comprised of several important proteins that are involved in controlling key cellular events such as proliferation, differentiation, and development. One such protein, Elk-1, regulates the activity of the c-fos promoter in response to extracellular stimuli. Elk-1 is representative of a subgroup of ETS domain proteins that utilize a bipartite recognition mechanism that is mediated by both protein-DNA and protein-protein interactions. In this study, we have overexpressed, purified, and characterized the ETS DNA-binding domain of Elk-1 (Elk-93). Elk-93 was expressed in Escherichia coli as a fusion protein with glutathione S-transferase and purified to homogeneity from both the soluble and insoluble fractions using a two-column protocol. A combination of CD, NMR, and fluorescence spectroscopy demonstrates that Elk-93 represents an independently folded domain of mixed alpha/beta structure in which the three conserved tryptophans appear to contribute to the hydrophobic core of the protein. Moreover, DNA binding studies demonstrate that Elk-93 binds DNA with both high affinity (Kd approximately 0.85 x 10(-10)M) and specificity. Circular permutation analysis indicates that DNA binding by Elk-93 does not induce significant bending of the DNA. Our results are discussed with respect to predictive models for the structure of the ETS DNA-binding domain.

Regulation of cell-type-specific interleukin-2 receptor alpha-chain gene expression: potential role of physical interactions between Elf-1, HMG-I(Y), and NF-kappa B family proteins

The interleukin 2 receptor alpha-chain (IL-2R alpha) gene is rapidly and potently induced in T cells in response to mitogenic stimuli. Previously, an inducible enhancer between nucleotides -299 and -228 that contains NF-kappa B and CArG motifs was identified. We now report the characterization of a second essential positive regulatory element located between nucleotides -137 and -64 that binds Elf-1 and HMG-I(Y). This element had maximal activity in lymphoid cells, paralleling the cell type specificity of Elf-1 expression. Transcription from the IL-2R alpha promoter was inhibited when either the Elf-1 or the HMG-I(Y) binding site was mutated. Coexpression of both proteins activated transcription of the -137 to -64 element in COS-7 cells. Elf-1 physically associated with HMG-I and with NF-kappa B p50 and c-Rel in vitro, suggesting that protein-protein interactions might functionally coordinate the actions of the upstream and downstream positive regulatory elements. This is the first report of a physical interaction between an Ets family member and NF-kappa B family proteins. These findings provide significant new insights into the protein-protein and protein-DNA interactions that regulate cell-type-specific and inducible IL-2R alpha gene expression and also have implications for other genes regulated by Elf-1 and NF-kappa B family proteins.

Protein-protein interactions: methods for detection and analysis

The function and activity of a protein are often modulated by other proteins with which it interacts. This review is intended as a practical guide to the analysis of such protein-protein interactions. We discuss biochemical methods such as protein affinity chromatography, affinity blotting, coimmunoprecipitation, and cross-linking; molecular biological methods such as protein probing, the two-hybrid system, and phage display: and genetic methods such as the isolation of extragenic suppressors, synthetic mutants, and unlinked noncomplementing mutants. We next describe how binding affinities can be evaluated by techniques including protein affinity chromatography, sedimentation, gel filtration, fluorescence methods, solid-phase sampling of equilibrium solutions, and surface plasmon resonance. Finally, three examples of well-characterized domains involved in multiple protein-protein interactions are examined. The emphasis of the discussion is on variations in the approaches, concerns in evaluating the results, and advantages and disadvantages of the techniques.

ADA3, a putative transcriptional adaptor, consists of two separable domains and interacts with ADA2 and GCN5 in a trimeric complex

Mutations in yeast ADA2, ADA3, and GCN5 weaken the activation potential of a subset of acidic activation domains. In this report, we show that their gene products form a heterotrimeric complex in vitro, with ADA2 as the linchpin holding ADA3 and GCN5 together. Further, activation by LexA-ADA3 fusions in vivo are regulated by the levels of ADA2. Combined with a prior observation that LexA-ADA2 fusions are regulated by the levels of ADA3 (N. Silverman, J. Agapite, and L. Guarente, Proc. Natl. Acad. Sci. USA 91:11665-11668, 1994), this finding suggests that these proteins also form a complex in cells. ADA3 can be separated into two nonoverlapping domains, an amino-terminal domain and a carboxyl-terminal domain, which do not separately complement the slow-growth phenotype or transcriptional defect of a delta ada3 strain but together supply full complementation. The carboxyl-terminal domain of ADA3 alone suffices for heterotrimeric complex formation in vitro and activation of LexA-ADA2 in vivo. We present a model depicting the ADA complex as a coactivator in which the ADA3 amino-terminal domain mediates an interaction between activation domains and the ADA complex.

Interaction with RAP74 subunit of TFIIF is required for transcriptional activation by serum response factor

A few general transcription factors, in particular TFIID and TFIIB, have been found to bind transcriptional activators. Here we show that the general transcription factor TFIIF is also a target for a transcriptional activator, namely serum response factor (SRF), which binds to the c-fos promoter. Using a yeast interaction assay, we find that SRF binds the RAP74 subunit of TFIIF and that SRF's transcriptional activation domain is the region involved in this binding. Further, RAP74's central charged cluster domain is required for binding to SRF's activation domain. Deletion of this domain impairs RAP74's ability to support SRF-activated transcription in vitro but has little effect on the protein's basal transcription activity or its ability to support SP1-activated transcription. The correlation of SRF-RAP74 binding with transcriptional activation suggests that RAP74 is a critical target for SRF-activated transcription.

The yeast two-hybrid system for studying protein-protein interactions

A great number of investigators are currently employing the yeast two-hybrid system to study protein-protein interactions. Recent applications and newer configurations of the technique include yeast strains with improved selectivity and screens for false-positive clones.

Does mitogen-activated-protein kinase have a role in insulin action? The cases for and against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

CD18 (beta 2 leukocyte integrin) promoter requires PU.1 transcription factor for myeloid activity

Normal cellular differentiation is linked to tightly regulated gene transcription. However, the DNA elements and trans-acting factors that regulate transcription in myeloid cells are poorly defined. CD18, the beta chain of the leukocyte integrins, is transcriptionally regulated during myeloid differentiation. The CD18 promoter is active after transfection into myeloid cells. We demonstrate that a region of the CD18 promoter that contains two binding sites for the PU.1 transcription factor is required for activity in myeloid cells. These sites are bound by in vitro translated PU.1 and by PU.1 from myeloid nuclear extracts. Mutagenesis of these sites abrogates binding by PU.1 and substantially decreases promoter activity in myeloid cells. Thus, the leukocyte-specific transcription factor PU.1 is required for myeloid activity of CD18.

NPI-1, the human homolog of SRP-1, interacts with influenza virus nucleoprotein

We used the yeast interactive trap system to identify a cellular protein which interacts with the nucleoprotein of influenza A viruses. This protein, nucleoprotein interactor 1 (NPI-1) is the human homolog of the yeast protein SRP1. SRP1 was previously identified as a suppressor of temperature-sensitive RNA polymerase I mutations (R. Yano, M. Oakes, M. Yamaghishi, J. Dodd, and M. Nomura, Mol. Cell. Biol. 12, 5640-5651, 1992). A full-length cDNA clone of NPI-1 was generated from HeLa cell poly A + RNA. The viral nucleoprotein, which had been partially purified from influenza A/PR/8/34 virus-infected embryonated eggs, could be coprecipitated from solution by glutathione agarose beads complexed with a bacterially expressed glutathione-S-transferase-NPI-1 fusion protein, confirming the results of the yeast genetic system. Antisera raised against NPI-1 identified a 60-kDa polypeptide from total cellular extracts of both HeLa and MDBK cells. The viral nucleoprotein was coimmunoprecipitated from influenza A/WSN/33 virus-infected MDBK cells by anti-NPI-1 sera, demonstrating an interaction of these two proteins in infected cells. Similarly, NPI-1 was coimmunoprecipitated from MDBK cells by anti-NP sera. These experiments suggest that NPI-1 plays a role during influenza virus replication.

Ras-Raf interaction: two-hybrid analysis

The identification of proteins mediating Ras effects, such as the serine/threonine kinases of the Raf family, has advanced our understanding of how signals are transmitted from the extracellular milieu to the nucleus. The modified two-hybrid system has proved to be a powerful tool for identifying specific protein interactions, such as those between Ras and Raf. We hope that the insight gained from the Ras screen, as well as insights from other two hybrid screens, will prove valuable in the application of this system to other enigmatic questions in biology.

The maf proto-oncogene stimulates transcription from multiple sites in a promoter that directs Purkinje neuron-specific gene expression

L7 is expressed in all adult cerebellar Purkinje cells, although during development it appears in a stereotyped spatial and temporal pattern that is manifested as parasagittal domains of neurons. Mutations of the L7 promoter in transgenic mice have established that these domains represent functional compartments of Purkinje neurons. Therefore, it is hoped that by defining the transcriptional control of the L7 gene insights into the mechanisms that control functional fate and organization in the nervous system can be gained. Fragments of the L7 promoter were introduced into a selectable reporter gene in Saccharomyces cerevisiae, and these strains were used to select for cerebellar cDNAs encoding proteins that can bind to, and activate transcription from, these elements. This assay identified the c-Maf proto-oncogene as activating transcription from two sites in the L7 promoter. We did a functional domain analysis of vertebrate c-Maf based upon transcriptional activation in S. cerevisiae and showed the requirement for a transactivation domain, leucine zipper, and DNA-binding region in c-Maf. The c-Maf interaction site was mapped to the sequence G/TGG/CNG/TNCT CAGNN in the L7 promoter, which represents an atypical 12-O-tetradecanoate-13-acetate-responsive element-type Maf-responsive element. However, neither Fos nor Jun, either alone or in combination with each other or c-Maf, altered transcription from this element. In contrast, a Maf-related protein, Nrl, completely mimicked c-Maf actions. These data suggest that Maf may interact with additional basic-zipper proteins that determine a subtype of Maf-responsive element binding.

Involvement of JunD in transcriptional activation of the orphan receptor gene nur77 by nerve growth factor and membrane depolarization in PC12 cells

nur77, an immediate-early gene that encodes an orphan nuclear receptor, is rapidly and transiently induced by nerve growth factor (NGF) stimulation or membrane depolarization in the rat pheochromocytoma-derived cell line PC12. The Nur77 protein can act as a potent transcription activator and may function to regulate the expression of downstream genes in response to extracellular stimuli. We show here that activation of nur77 by NGF treatment and membrane depolarization is signalled through distinct pathways. These distinct signals appear to converge on the same transcription factors acting on the same promoter elements. We show that nur77 activation by both processes requires two cis-acting AP1-like elements, NAP1 and NAP2, which contain the core sequence TGCGTCA centered at 67 and 38 nucleotides upstream of the transcription start site. The NAP elements can confer inducibility by NGF and membrane depolarization on an otherwise unresponsive heterologous promoter. We identified JunD as a key mediator of nur77 activation by reason of the following observations. (i) JunD, but not CREB or other members of the Fos/Jun family, is a component of NAP binding activity in PC12 cell nuclear extracts. (ii) JunD, but not other Fos/Jun family members, specifically transactivates the nur77 promoter through the NAP elements (iii) A dominant-negative mutant of JunD effectively abolishes the activation of nur77 by either NGF treatment or membrane depolarization. These data draw a contrast between the regulation of nur77 with that of c-fos, in which the sequence requirements for activation by NGF treatment and membrane depolarization appear separable, and CREB appears to play a role in activation by both NGF and membrane depolarization.

Molecular pathways to parallel evolution: I. Gene nexuses and their morphological correlates

Aspects of the regulatory interactions among genes are probably as old as most genes are themselves. Correspondingly, similar predispositions to changes in such interactions must have existed for long evolutionary periods. Features of the structure and the evolution of the system of gene regulation furnish the background necessary for a molecular understanding of parallel evolution. Patently "unrelated" organs, such as the fat body of a fly and the liver of a mammal, can exhibit fractional homology, a fraction expected to become subject to quantitation. This also seems to hold for different organs in the same organism, such as wings and legs of a fly. In informational macromolecules, on the other hand, homology is indeed all or none. In the quite different case of organs, analogy is expected usually to represent attenuated homology. Many instances of putative convergence are likely to turn out to be predominantly parallel evolution, presumably including the case of the vertebrate and cephalopod eyes. Homology in morphological features reflects a similarity in networks of active genes. Similar nexuses of active genes can be established in cells of different embryological origins. Thus, parallel development can be considered a counterpart to parallel evolution. Specific macromolecular interactions leading to the regulation of the c-fos gene are given as an example of a "controller node" defined as a regulatory unit. Quantitative changes in gene control are distinguished from relational changes, and frequent parallelism in quantitative changes is noted in Drosophila enzymes. Evolutionary reversions in quantitative gene expression are also expected. The evolution of relational patterns is attributed to several distinct mechanisms, notably the shuffling of protein domains. The growth of such patterns may in part be brought about by a particular process of compensation for "controller gene diseases," a process that would spontaneously tend to lead to increased regulatory and organismal complexity. Despite the inferred increase in gene interaction complexity, whose course over evolutionary time is unknown, the number of homology groups for the functional and structural protein units designated as domains has probably remained rather constant, even as, in some of its branches, evolution moved toward "higher" organisms. In connection with this process, the question is raised of parallel evolution within the purview of activating and repressing master switches and in regard to the number of levels into which the hierarchies of genic master switches will eventually be resolved.

Involvement of JunD in transcriptional activation of the orphan receptor gene nur77 by nerve growth factor and membrane depolarization in PC12 cells

nur77, an immediate-early gene that encodes an orphan nuclear receptor, is rapidly and transiently induced by nerve growth factor (NGF) stimulation or membrane depolarization in the rat pheochromocytoma-derived cell line PC12. The Nur77 protein can act as a potent transcription activator and may function to regulate the expression of downstream genes in response to extracellular stimuli. We show here that activation of nur77 by NGF treatment and membrane depolarization is signalled through distinct pathways. These distinct signals appear to converge on the same transcription factors acting on the same promoter elements. We show that nur77 activation by both processes requires two cis-acting AP1-like elements, NAP1 and NAP2, which contain the core sequence TGCGTCA centered at 67 and 38 nucleotides upstream of the transcription start site. The NAP elements can confer inducibility by NGF and membrane depolarization on an otherwise unresponsive heterologous promoter. We identified JunD as a key mediator of nur77 activation by reason of the following observations. (i) JunD, but not CREB or other members of the Fos/Jun family, is a component of NAP binding activity in PC12 cell nuclear extracts. (ii) JunD, but not other Fos/Jun family members, specifically transactivates the nur77 promoter through the NAP elements (iii) A dominant-negative mutant of JunD effectively abolishes the activation of nur77 by either NGF treatment or membrane depolarization. These data draw a contrast between the regulation of nur77 with that of c-fos, in which the sequence requirements for activation by NGF treatment and membrane depolarization appear separable, and CREB appears to play a role in activation by both NGF and membrane depolarization.

Activation of c-fos gene expression by a kinase-deficient epidermal growth factor receptor

The intrinsic tyrosine kinase activity of the epidermal growth factor receptor (EGFR) has been shown to be responsible for many of the pleiotropic intracellular effects resulting from ligand stimulation [W.S. Chen, C.S. Lazar, M. Poenie, R.Y. Tsien, G.N. Gill, and M.G. Rosenfeld, Nature (London) 328:820-823, 1987; A.M. Honegger, D. Szapary, A. Schmidt, R. Lyall, E. Van Obberghen, T.J. Dull, A. Ulrich, and J. Schlessinger, Mol. Cell. Biol. 7:4568-4571, 1987]. Recently, however, it has been shown that addition of ligand to cells expressing kinase-defective EGFR mutants can result in the phosphorylation of mitogen-activated protein kinase (R. Campos-González and J.R. Glenney, Jr., J. Biol. Chem. 267:14535-14538, 1992; E. Selva, D.L. Raden, and R.J. Davis, J. Biol. Chem. 268:2250-2254, 1993), as well as stimulation of DNA synthesis (K.J. Coker, J.V. Staros, and C.A. Guyer, Proc. Natl. Acad. Sci. USA 91:6967-6971, 1994). Moreover, mitogen-activated protein kinase has been shown to phosphorylate the transcription factor p62TCF in vitro, leading to enhanced ternary complex formation between p62TCF, p67SRF, and the c-fos serum response element (SRE) [H. Gille, A.D. Sharrocks, and P.E. Shaw, Nature (London) 358:414-417, 1992]. On the basis of these observations, we have investigated the possibility that the intrinsic tyrosine kinase activity of the EGFR may not be necessary for transcriptional activation mediated via p62TCF. Here, we demonstrate that a kinase-defective EGFR mutant can signal ligand-induced expression of c-fos protein and that a significant component of this induction appears to be mediated at the transcriptional level. Investigation of transcriptional activation mediated by the c-fos SRE shows that this response is impaired by mutations in the SRE which eliminate binding of p62(TCF). These data indicate that information inherent in the structure of the EGFR can be accessed by ligand stimulation independent of the receptor's catalytic kinase function.

Identification of ETS domain proteins in murine T lymphocytes that interact with the Moloney murine leukemia virus enhancer

The enhancer of Moloney murine leukemia virus (Mo-MuLV) contains an array of transcriptional control elements that direct viral gene expression in diverse cell types. The murine transcription factor Ets-1 was shown to bind to the LVb and LVc elements of the enhancer by DNase I protection and methylation interference assays. Enhancers containing disrupted Ets-1 binding sites were tested in transient expression assays in the murine T-cell line EL4.E1; alterations in the LVb element affected constitutive enhancer activity, while mutation of either the LVb or LVc element disrupted phorbol ester-induced enhancer activity. Members of the ets gene family of proteins display similar DNA-binding properties; therefore, we speculated that ets proteins other than Ets-1 also might bind these elements. Crude nuclear extracts of EL4.E1 cells were assayed to identify the protein(s) that potentially functions at the LVb element. The predominant binding activity was not Ets-1 but rather two independent DNA-protein complexes that comigrated in mobility shift assays. UV cross-linking and denaturing gel electrophoresis sized the two DNA-binding species, which we denoted p55 and p100. Immunoprecipitation combined with UV cross-linking identified p55 as the alpha subunit of GA-binding protein. The DNA-binding properties of p100 and several ets proteins were compared. Similarities suggested that p100 is also an ETS domain protein, possibly Elf-1. This strategy could be used to identify other ETS domain proteins in crude nuclear extracts. These findings suggest multiple ETS domain proteins could regulate gene expression of Mo-MuLV.

Activation of c-fos gene expression by a kinase-deficient epidermal growth factor receptor

The intrinsic tyrosine kinase activity of the epidermal growth factor receptor (EGFR) has been shown to be responsible for many of the pleiotropic intracellular effects resulting from ligand stimulation [W.S. Chen, C.S. Lazar, M. Poenie, R.Y. Tsien, G.N. Gill, and M.G. Rosenfeld, Nature (London) 328:820-823, 1987; A.M. Honegger, D. Szapary, A. Schmidt, R. Lyall, E. Van Obberghen, T.J. Dull, A. Ulrich, and J. Schlessinger, Mol. Cell. Biol. 7:4568-4571, 1987]. Recently, however, it has been shown that addition of ligand to cells expressing kinase-defective EGFR mutants can result in the phosphorylation of mitogen-activated protein kinase (R. Campos-González and J.R. Glenney, Jr., J. Biol. Chem. 267:14535-14538, 1992; E. Selva, D.L. Raden, and R.J. Davis, J. Biol. Chem. 268:2250-2254, 1993), as well as stimulation of DNA synthesis (K.J. Coker, J.V. Staros, and C.A. Guyer, Proc. Natl. Acad. Sci. USA 91:6967-6971, 1994). Moreover, mitogen-activated protein kinase has been shown to phosphorylate the transcription factor p62TCF in vitro, leading to enhanced ternary complex formation between p62TCF, p67SRF, and the c-fos serum response element (SRE) [H. Gille, A.D. Sharrocks, and P.E. Shaw, Nature (London) 358:414-417, 1992]. On the basis of these observations, we have investigated the possibility that the intrinsic tyrosine kinase activity of the EGFR may not be necessary for transcriptional activation mediated via p62TCF. Here, we demonstrate that a kinase-defective EGFR mutant can signal ligand-induced expression of c-fos protein and that a significant component of this induction appears to be mediated at the transcriptional level. Investigation of transcriptional activation mediated by the c-fos SRE shows that this response is impaired by mutations in the SRE which eliminate binding of p62(TCF). These data indicate that information inherent in the structure of the EGFR can be accessed by ligand stimulation independent of the receptor's catalytic kinase function.

Identification of ETS domain proteins in murine T lymphocytes that interact with the Moloney murine leukemia virus enhancer

The enhancer of Moloney murine leukemia virus (Mo-MuLV) contains an array of transcriptional control elements that direct viral gene expression in diverse cell types. The murine transcription factor Ets-1 was shown to bind to the LVb and LVc elements of the enhancer by DNase I protection and methylation interference assays. Enhancers containing disrupted Ets-1 binding sites were tested in transient expression assays in the murine T-cell line EL4.E1; alterations in the LVb element affected constitutive enhancer activity, while mutation of either the LVb or LVc element disrupted phorbol ester-induced enhancer activity. Members of the ets gene family of proteins display similar DNA-binding properties; therefore, we speculated that ets proteins other than Ets-1 also might bind these elements. Crude nuclear extracts of EL4.E1 cells were assayed to identify the protein(s) that potentially functions at the LVb element. The predominant binding activity was not Ets-1 but rather two independent DNA-protein complexes that comigrated in mobility shift assays. UV cross-linking and denaturing gel electrophoresis sized the two DNA-binding species, which we denoted p55 and p100. Immunoprecipitation combined with UV cross-linking identified p55 as the alpha subunit of GA-binding protein. The DNA-binding properties of p100 and several ets proteins were compared. Similarities suggested that p100 is also an ETS domain protein, possibly Elf-1. This strategy could be used to identify other ETS domain proteins in crude nuclear extracts. These findings suggest multiple ETS domain proteins could regulate gene expression of Mo-MuLV.

The amino-terminal region of the retinoblastoma gene product binds a novel nuclear matrix protein that co-localizes to centers for RNA processing

The tumor suppressing capacity of the retinoblastoma protein (p110RB) is dependent on interactions made with cellular proteins through its carboxy-terminal domains. How the p110RB amino-terminal region contributes to this activity is unclear, though evidence now indicates it is important for both growth suppression and regulation of the full-length protein. We have used the yeast two-hybrid system to screen for cellular proteins which bind to the first 300 amino acids of p110RB. The only gene isolated from this screen encodes a novel 84-kD nuclear matrix protein that localizes to subnuclear regions associated with RNA processing. This protein, p84, requires a structurally defined domain in the amino terminus of p110RB for binding. Furthermore, both in vivo and in vitro experiments demonstrate that p84 binds preferentially to the functionally active, hypophosphorylated form of p110RB. Thus, the amino terminus of p110RB may function in part to facilitate the binding of growth promoting factors at subnuclear regions actively involved in RNA metabolism.

Characterization of an interaction between insulin receptor substrate 1 and the insulin receptor by using the two-hybrid system

Insulin receptor substrate 1 (IRS-1) is a major substrate of the insulin receptor and has been implicated in insulin signaling. Although IRS-1 is thought to interact with the insulin receptor, the nature of the interaction has not been defined. In this study, we used the two-hybrid assay of protein-protein interaction in the yeast Saccharomyces cerevisiae to study the interaction between human IRS-1 and the insulin receptor. We demonstrate that IRS-1 forms a specific complex with the cytoplasmic domain of the insulin receptor when both are expressed as hybrid proteins in yeast cells. We show that the interaction is strictly dependent upon receptor tyrosine kinase activity, since IRS-1 shows no interaction with a kinase-inactive receptor hybrid containing a mutated ATP-binding site. Furthermore, mutation of receptor tyrosine 960 to phenylalanine eliminates IRS-1 interaction in the two-hybrid assay. These data suggest that the interaction between IRS-1 and the receptor is direct and provide evidence that the juxtamembrane domain of the receptor is involved. Furthermore, we show that a 356-amino-acid region encompassed by amino acids 160 through 516 of IRS-1 is sufficient for interaction with the receptor in the two-hybrid assay. Lastly, in agreement with our findings for yeast cells, we show that the insulin receptor is unable to phosphorylate an IRS-1 protein containing a deletion of amino acids 45 to 516 when expressed in COS cells. The two-hybrid assay should provide a facile means by which to pursue a detailed understanding of this interaction.

Characterization of an interaction between insulin receptor substrate 1 and the insulin receptor by using the two-hybrid system

Insulin receptor substrate 1 (IRS-1) is a major substrate of the insulin receptor and has been implicated in insulin signaling. Although IRS-1 is thought to interact with the insulin receptor, the nature of the interaction has not been defined. In this study, we used the two-hybrid assay of protein-protein interaction in the yeast Saccharomyces cerevisiae to study the interaction between human IRS-1 and the insulin receptor. We demonstrate that IRS-1 forms a specific complex with the cytoplasmic domain of the insulin receptor when both are expressed as hybrid proteins in yeast cells. We show that the interaction is strictly dependent upon receptor tyrosine kinase activity, since IRS-1 shows no interaction with a kinase-inactive receptor hybrid containing a mutated ATP-binding site. Furthermore, mutation of receptor tyrosine 960 to phenylalanine eliminates IRS-1 interaction in the two-hybrid assay. These data suggest that the interaction between IRS-1 and the receptor is direct and provide evidence that the juxtamembrane domain of the receptor is involved. Furthermore, we show that a 356-amino-acid region encompassed by amino acids 160 through 516 of IRS-1 is sufficient for interaction with the receptor in the two-hybrid assay. Lastly, in agreement with our findings for yeast cells, we show that the insulin receptor is unable to phosphorylate an IRS-1 protein containing a deletion of amino acids 45 to 516 when expressed in COS cells. The two-hybrid assay should provide a facile means by which to pursue a detailed understanding of this interaction.

Transient activation of RAF-1, MEK, and ERK2 coincides kinetically with ternary complex factor phosphorylation and immediate-early gene promoter activity in vivo

We have investigated the early in vivo signaling events triggered by serum that lead to activation of the c-fos proto-oncogene in HeLa cells. Both RAF-1 and MEK kinase activities are fully induced within 3 min of serum treatment and quickly decrease thereafter, slightly preceding the activation and inactivation of p42MAPK/ERK2. ERK2 activity correlates tightly with a transient phosphatase-sensitive modification of ternary complex factor (TCF), manifested by the slower electrophoretic mobility of TCF-containing protein-DNA complexes. These induced complexes in turn correlate with the activity of the c-fos, egr-1, and junB promoters. Phorbol ester treatment induces the same events but with slower and prolonged kinetics. Inhibition of serine/threonine phosphatase activities by okadaic acid treatment reverses the repression of the c-fos promoter either after induction or without induction. This corresponds to the presence of the induced complexes and of ERK2 activity, as well as to the activation of a number of other kinases. Inhibition of tyrosine phosphatase activities by sodium vanadate treatment delays but does not block ERK2 inactivation, TCF dephosphorylation, and c-fos repression. The tight linkage in vivo between the activity of MAP kinase, TCF phosphorylation, and immediate-early gene promoter activity is consistent with the notion that a stable ternary complex over the serum response element is a direct target for the MAP kinase signaling cascade. Furthermore, serine/threonine phosphatases are implicated in regulating the kinase cascade, as well as the state of TCF modification and c-fos promoter activity, in vivo.

Two pathways for serum regulation of the c-fos serum response element require specific sequence elements and a minimal domain of serum response factor

The c-fos serum response element (SRE) is necessary and sufficient for induction of the c-fos gene in response to serum and growth factors. This activation is dependent upon serum response factor (SRF), a transcriptional activator which binds the SRE. A factor, p62TCF, which binds in conjunction with SRF to the SRE and which is activated by mitogen-activated protein kinase, has also been implicated in c-fos regulation. By using a reporter gene system with weak SRE mutations that is dependent upon overexpression of SRF for serum induction, we have found that there are at least two pathways for serum induction that converge on the SRE. Loss of TCF binding by mutations in SRF and the SRE did not reduce serum induction of the reporter genes. We have found a pathway for serum induction that is sensitive to mutations in the A/T-containing central sequence of the SRE and which is independent of TCF. When this pathway was mutated, activation was dependent upon TCF binding, demonstrating that TCF can also function in serum induction. Both of the signalling pathways required a minimal domain of SRF. This domain, spanning SRF's DNA binding domain, was sufficient for serum induction when fused to a heterologous transcriptional activation domain.