Functional analysis of a growth factor-responsive transcription factor complex

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.

Mouse Elk oncogene maps to chromosome X and a novel Elk oncogene (Elk3) maps to chromosome 10

The Elk protein is a member of the Ets family found in both vertebrates and invertebrates. Human ELK1 encoded by ELK1 binds alone or together with serum response factor to DNA and regulates gene expression in a variety of biological processes. Using a panel of interspecific backcross mice, we have mapped the Elk oncogene (Elk) and a novel type Elk oncogene (Elk3), closely related to ELK1. Elk maps to Chr X, and Elk3 maps to the proximal region of Chr 10.

MEK-2, a Caenorhabditis elegans MAP kinase kinase, functions in Ras-mediated vulval induction and other developmental events

Activated Ras initiates a cascade of sequential phosphorylation events, including the protein kinases Raf, MEK, and MAP kinase. The Let-60 Ras-mediated signal transduction pathway controls vulval induction in Caenorhabditis elegans. Both Lin-45 Raf and Sur-1 MAP kinase have been determined to be essential factors during vulval induction; however, the C. elegans mek gene has not been identified. In this paper, we have cloned a C. elegans mek gene, mek-2, and demonstrated that the MEK-2 protein possesses the biochemical properties of MAP kinase kinases: The C. elegans MEK-2 protein can phosphorylate and activate a human MAP kinase (ERK1), and MEK-2 itself can be phosphorylated and activated by immunoprecipitated mammalian Raf. The mek-2 gene plays a key role in the let-60 ras-mediated vulval induction pathway, as loss-of-function mutations in the gene (ku114 and h294) significantly reduce the signal transmitted through Ras. mek-2(ku114) completely suppressed the Multivulva (Muv) phenotype of a hyperactive let-60 ras mutation, and animals homozygous for mek-2(ku114) also displayed a partial larval lethal phenotype. Animals homozygous for mek-2(h294) exhibited a highly penetrant sterile and Vulvaless phenotype. Microinjection of a gain-of-function mek-2 mutation resulted in Muv and other mutant phenotypes, whereas microinjection of a dominant-negative mutation not only suppressed the Muv phenotype of an activated let-60 ras mutation but also caused an egg-laying defective phenotype in otherwise wild type animals. Our results demonstrate that mek-2 acts between lin-45 raf and sur-1/mpk-1 in a signal transduction pathway used in the control of vulval differentiation and other developmental events.

Multiple Ras functions can contribute to mammalian cell transformation

We have developed a generalized approach, using two hybrid interactions, to isolate Ha-Ras effector loop mutations that separate the ability of Ha-Ras to interact with different downstream effectors. These mutations attenuate or eliminate Ha-ras(G12V) transformation of mammalian cells, but retain complementary activity, as demonstrated by synergistic induction of foci of growth-transformed cells, and by the ability to activate different downstream components. The transformation defect of Ha-ras(G12V, E37G) is rescued by a mutant, raf1, that restores interaction. These results indicate that multiple cellular components, including Raf1, are activated by Ha-Ras and contribute to Ha-Ras-induced mammalian cell transformation.

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.

Regulation of Raf-1 kinase activity by the 14-3-3 family of proteins

We have identified the beta (beta) isoform of the 14-3-3 family of proteins as an activator of the Raf-1 protein kinase. 14-3-3 was isolated in a yeast two-hybrid screen for Raf-1 kinase domain binding proteins. Purified bovine brain 14-3-3 interacted specifically with both c-Raf-1 and the isolated Raf-1 kinase domain. Association was sensitive to the activation status of Raf-1; 14-3-3 bound to unactivated Raf-1, but not Raf-1 activated by protein kinase C alpha or Ras and Lck. The significance of these interactions under physiological conditions was demonstrated by co-immunoprecipitation of Raf-1 and 14-3-3 from extracts of quiescent, but not mitogen-stimulated, NIH 3T3 cells. 14-3-3 was not a preferred Raf-1 substrate in vitro and did not significantly affect Raf-1 kinase activity in a purified system. However, in cell-free extracts 14-3-3 acted as a Ras-independent activator of both c-Raf-1 and the Raf-1 kinase domain. The same results were obtained in vivo using transfection assays; 14-3-3 enhanced both c-Raf-1- and Raf-1 kinase domain-stimulated expression of AP-1- and NF-kappa B-dependent reporter genes and accelerated Raf-1 kinase domain-triggered differentiation of PC12 cells. We conclude that 14-3-3 is a latent co-activator bound to unactivated Raf-1 in quiescent cells and mediates mitogen-triggered but Ras-independent regulatory effects aimed directly at the kinase domain.

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.

Regulation of c-fos expression in transgenic mice requires multiple interdependent transcription control elements

Transcription control regions of eukaryotic genes contain multiple sequence elements proposed to function independently to regulate transcription. We developed transgenic mice carrying fos-lacZ fusion genes with clustered point mutations in each of several distinct regulatory sequences: the sis-inducible element, the serum response element, the fos AP-1 site, and the calcium/cAMP response element. Analysis of Fos-lacZ expression in the CNS and in cultured cells demonstrated that all of the regulatory elements tested were required in concert for tissue- and stimulus-specific regulation of the c-fos promoter. This implies that the regulation of c-fos expression requires the concerted action of multiple control elements that direct the assembly of an interdependent transcription complex.

A ternary complex factor-dependent mechanism mediates induction of egr-1 through selective serum response elements following antigen receptor cross-linking in B lymphocytes

Induction of the primary response gene egr-1 occurs rapidly following antigen receptor cross-linking in B lymphocytes. Antisense studies have demonstrated that this induction is necessary for their subsequent activation to this signal. The present study examines the molecular mechanism whereby the receptor-generated signals interact with the egr-1 promoter to elicit transcription. Deletion mapping and point mutations have indicated that two of the five serum response elements (SREs) in the egr-1 promoter can mediate induction. Of the two critical SREs, both are capable of mediating maximal induction even in the absence of the other SRE. Our results also indicate that adjacent Ets motifs are necessary for induction. Like the c-fos SRE, the egr-1 SRE/Ets sites are occupied by a multiprotein (ternary) complex containing a homodimer of serum response factor and an unidentified member of the Ets family of transcription factors. The identification of a ternary complex-dependent mechanism of egr-1 induction, along with selective utilization of SREs in B lymphocytes, suggests that a complicated array of signaling cascades interacts with unique combinations of regulatory elements in the egr-1 promoter in different cell types.

Transcriptional repression mediated by the serum response factor

The serum response element (SRE) contributes to transcriptional repression of the c-fos proto-oncogene. We show that the transcription factor SRF is able to repress SRE-dependent transcription, apparently by sequestering a co-activator. Only the DNA-binding core region is required for this SRE-dependent repression. Furthermore the phosphorylation status at potential casein kinase II sites within an N-terminal repression domain affects SRE-independent transcription. SRF may thus pleiotropically influence cellular transcription, representing a novel aspect of SRF function.

Serum response element associated transcription factors in mouse embryos: serum response factor, YY1, and PEA3 factor

Many mammalian transcription factors, including human and mouse serum response factors (SRFs), are post-translationally modified with O-linked N-acetylglucosamine monosaccharides on multiple serine and/or threonine residues. Nuclear extracts were prepared from 9.5 to 19 days postcoitum mouse embryos and subsequently were fractionated by wheat germ agglutinin (WGA)-agarose affinity chromatography. SRF binds WGA-agarose and apparently is O-glycosylated. On the other hand, the low molecular weight serum response element (SRE)-binding proteins, including the previously named band I and band II factors, did not bind WGA-agarose. Furthermore, we showed that the fastest migrating complex contains the Yin-Yang 1 (YY1) factor. YY1 binds to the c-fos SRE and skeletal alpha-actin muscle regulatory element (MRE), but not the cardiac alpha-actin MRE. Nuclear extracts from NIH/3T3 fibroblasts contain similar, if not identical, SRE-binding complexes. Besides these SRE-binding factors, mouse PEA3-binding factor, presumably an ETS domain-containing protein, was found to bind SRF protein. This physical interaction, between SRF and ETS domain proteins, was shown to involve the DNA-binding domain-containing region of SRF and not the carboxyl-terminal transactivation domain.

Ras-dependent and -independent pathways target the mitogen-activated protein kinase network in macrophages

Mitogen-activated protein kinases (MAPKs) are activated upon a variety of extracellular stimuli in different cells. In macrophages, colony-stimulating factor 1 (CSF-1) stimulates proliferation, while bacterial lipopolysaccharide (LPS) inhibits cell growth and causes differentiation and activation. Both CSF-1 and LPS rapidly activate the MAPK network and induce the phosphorylation of two distinct ternary complex factors (TCFs), TCF/Elk and TCF/SAP. CSF-1, but not LPS, stimulated the formation of p21ras. GTP complexes. Expression of a dominant negative ras mutant reduced, but did not abolish, CSF-1-mediated stimulation of MEK and MAPK. In contrast, activation of the MEK kinase Raf-1 was Ras independent. Treatment with the phosphatidylcholine-specific phospholipase C inhibitor D609 suppressed LPS-mediated, but not CSF-1-mediated, activation of Raf-1, MEK, and MAPK. Similarly, down-regulation or inhibition of protein kinase C blocked MEK and MAPK induction by LPS but not that by CSF-1. Phorbol 12-myristate 13-acetate pretreatment led to the sustained activation of the Raf-1 kinase but not that of MEK and MAPK. Thus, activated Raf-1 alone does not support MEK/MAPK activation in macrophages. Phosphorylation of TCF/Elk but not that of TCF/SAP was blocked by all treatments that interfered with MAPK activation, implying that TCF/SAP was targeted by a MAPK-independent pathway. Therefore, CSF-1 and LPS target the MAPK network by two alternative pathways, both of which induce Raf-1 activation. The mitogenic pathway depends on Ras activity, while the differentiation signal relies on protein kinase C and phosphatidylcholine-specific phospholipase C activation.

Activation of the p21ras pathway couples antigen receptor stimulation to induction of the primary response gene egr-1 in B lymphocytes

The primary response gene egr-1 encodes a sequence-specific transcription factor whose expression is necessary for antigen receptor-stimulated activation of B lymphocytes. The molecular processes involved in linking egr-1 induction to antigen receptor signaling have not been defined. The present study demonstrates that expression of an activated form of p21ras results in egr-1 induction similar to that previously shown after antigen receptor cross-linking. In addition, both antigen receptor cross-linking and p21ras use the same element in the egr-1 promoter to exert their effects. Using dominant-negative mutants of p21ras and raf-1, we demonstrate that induction of egr-1 after antigen receptor cross-linking is mediated by activation of the p21ras/mitogen-activated protein kinase signaling pathway. While regulation of the p21ras pathway during B cell activation has been intensively studied, this report represents the first description of a biologically relevant event associated with its activation.

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.

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.

Nuclear import of serum response factor (SRF) requires a short amino-terminal nuclear localization sequence and is independent of the casein kinase II phosphorylation site

Serum stimulation of resting cells is mediated at least in part at the transcriptional level by the activation of numerous genes among which c-fos constitutes a model. Serum response factor (SRF) forms a ternary complex at the c-fos serum response element (SRE) with an accessory protein p62TCF/Elk-1. Both proteins are the targets of multiple phosphorylation events and their role is still unknown in the amino terminus of SRF. While the transcriptional activation domain has been mapped between amino acids 339 and 508, the DNA-binding and the dimerization domains have been mapped to between amino acids 133–235 and 168–235, respectively, no role has been proposed for the amino-terminal portion of the molecule. We demonstrate in the present work that amino acids 95 to 100 contain a stretch of basic amino acids that are sufficient to target a reporter protein to the nucleus. Moreover, this sequence appears to be the only nuclear localization signal operating in SRF. Finally, whereas the global structure around this putative nuclear location signal is reminiscent of what is found in the SV40 T antigen, the casein kinase II phosphorylation site does not determine the rate of cyto-nuclear protein transport of this protein.

Serum response factor has functional roles both in indirect binding to the CArG box and in the transcriptional activation function of human T-cell leukemia virus type I Tax

We previously reported that Tax1 of human T-cell leukemia virus type I interacts directly with serum response factor (SRF) and that Tax1 activates the transcription of several cellular immediate-early genes through the SRF binding site (CArG box). This activation required the transcriptional activation function of Tax1, identified as an activity of GALTax (a chimeric Tax1 with the yeast transcription factor GAL4) at the GAL4-binding site. In this study, we examined whether SRF plays a role in the transcriptional activation function of Tax1. Expression of Tax1 suppressed the GALTax activity at the GAL4 site as a result of squelching, and the suppressed activity was recovered by the overexpression of SRF, suggesting that SRF is a factor that is required for GALTax activity and that is inhibited by competition with Tax1. The expression of antisense SRF RNA specifically inhibited GALTax activity to less than 20%. Deletion of the Tax1 interaction domain of SRF at its C terminus converted SRF from an activator of GALTax to an inhibitor. These results suggest that SRF is an essential component of the transcriptional activation of Tax1 in addition to a mediator of CArG box binding.

Positive regulation of the cAMP-responsive activator CREM by the p70 S6 kinase: an alternative route to mitogen-induced gene expression

Activation of the adenylyl cyclase signaling pathway elicits the induction of genes via activators binding to cAMP-responsive elements (CREs). Nuclear factor CRE modulator (CREM) is activated by PKA-mediated phosphorylation on a serine at position 117. We show that Ser-117 is also phosphorylated by the mitogen-activated p70 S6 kinase (p70S6K) in vitro. Activation of cellular p70S6K by serum factors enhances Ser-117 phosphorylation and CREM transactivation. Coexpression of p70S6K significantly increases transactivation by a GAL4-CREM fusion. The macrolide rapamycin, a potent and specific inhibitor of p70S6K in vivo, completely blocks CREM activation induced by serum and by p70S6K. Thus, CREM constitutes a target for mitogenic signaling through p70S6K and may acts as a nuclear effector in which transduction pathways may converge and cross-talk.

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.

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.

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.

Ras/MAP kinase-dependent and -independent signaling pathways target distinct ternary complex factors

Transcriptional activation of the immediate early genes c-fos and egr-1 by extracellular signals appears to be mediated by ternary complex factors (TCFs). In BAC-1 macrophages, growth factor stimulation leads to the retardation of protein-DNA complexes containing distinct TCFs. One TCF is recognized by Elk-1 antisera, whereas the other is immunologically related to SAP-1. The appearance and decay of hyperphosphorylated TCF/Elk-1-containing complexes after stimulation coincide with the activation of mitogen-activated protein kinase (MAPK) and the induction and repression of c-fos and egr-1, whereas modified TCF/SAP-1-containing complexes decay more slowly. Suppression of MAPK activation in macrophages and fibroblasts correlates with the failure to induce TCF/Elk-1 hyperphosphorylation without blocking TCF/SAP-1 modification. Accordingly the modified Elk-1 complex is generated in vitro by activated MAPK, whereas that of SAP-1 is not. Expression of a dominant-negative Ras mutant (RasAsn17) in BAC-1 cells does not affect CSF-1-induced TCF/SAP-1 modification while suppressing TCF/Elk-1 phosphorylation. Neither PKC down-regulation by TPA nor inhibition of Gi proteins by pertussis toxin pretreatment influences CSF-1-induced signaling to TCFs. These data indicate the existence of two separate signaling pathways for the modification of distinct TCFs: one dependent on Ras and MAPK and converging on TCF/Elk-1, and the other targeting TCF/SAP-1 independently of Ras and MAPK.

Novel interactions between human T-cell leukemia virus type I Tax and activating transcription factor 3 at a cyclic AMP-responsive element

Human proenkephalin gene transcription is transactivated by human T-cell leukemia virus type I (HTLV-I) Tax in human Jurkat T lymphocytes. This transactivation was further enhanced in Jurkat cells treated with concanavalin A, cyclic AMP, or 12-O-tetradecanoylphorbol-13-acetate. Deletion and cis-element transfer analyses of the human proenkephalin promoter identified a cyclic AMP-responsive AP-1 element (-92 to -86) as both necessary and sufficient to confer Tax-dependent transactivation. Different AP-1 or cyclic AMP-responsive element-binding protein (CREB)/activating transcription factor (ATF) proteins which bind this element were expressed in murine teratocarcinoma F9 cells to identify those capable of mediating Tax-dependent transactivation of human proenkephalin gene transcription. Although CREB, c-Fos, c-Jun, and JunD did not have significant effects, JunB inhibited the Tax-dependent transactivation. In contrast, ATF3 dramatically induced Tax-dependent transactivation, which was further enhanced by protein kinase A. Electrophoretic mobility shift assays with recombinant fusion proteins expressed and purified from bacteria indicate that the DNA-binding activity of ATF3 is also dramatically enhanced by Tax. Chimeric fusion proteins consisting of the DNA-binding domain of the yeast transcription factor Gal4 and the amino-terminal domain (residues 1 to 66) of ATF3 were able to mediate Tax-dependent transactivation of a Gal4-responsive promoter, which suggests a direct involvement of this region of ATF3. Recombinant fusion proteins of glutathione S-transferase with either the amino- or carboxy-terminal (residues 139 to 181) domain of ATF3 were able to specifically interact with Tax. Furthermore, specific antisera directed against Tax coimmunoprecipitated ATF3 only in the presence of Tax.

Dual modes of control of c-fos mRNA induction by intracellular calcium in T cells

Cytoplasmic calcium is a nearly universal second messenger in eukaryotes. In many cell types, elevated intracellular calcium interacts synergistically with inducers of protein kinase C to elicit activation of complete biological programs normally induced by extracellular signals. In T cells, elevated cytoplasmic calcium is a critical mediator of activation in response to stimulation of the antigen receptor, and in some T-cell lines, treatment with a combination of calcium ionophore and protein kinase C activator mimics authentic antigen treatment. The synergistic interaction of calcium and protein kinase C in T cells is also observed at the level of gene expression. Here we examine the molecular mechanisms through which these agents exert synergistic control over the expression of the c-fos proto-oncogene in a T-cell hybridoma. We find that the principal effect of calcium is on the elongation of c-fos transcripts. This step constitutes the major control of c-fos mRNA accumulation in these cells. In addition, calcium regulates the initiation of c-fos transcription. This effect requires the serum response element of the c-fos gene and an additional sequence immediately 3' to this element. Thus, calcium regulates c-fos expression through at least two distinct molecular pathways.

Inhibition of v-raf-dependent c-fos expression and transformation by a kinase-defective mutant of the mitogen-activated protein kinase Erk2

Receptor-bound growth factors elicit intracellular signals that lead to the phosphorylation and activation of numerous intracellular kinases and transcription factors with consequent changes in patterns of gene expression. Several oncogene products are able to mimic these signals, resulting in cell transformation and proliferation. For example, the introduction of oncogenic forms of Raf-1 kinase into fibroblasts induces transformation and leads to the constitutive expression of, among others, the c-fos proto-oncogene. Here it is shown that the elevation of c-fos promoter activity brought about by v-raf is mediated by TCF/Elk-1, which forms a ternary complex with SRF at the serum response element and is a substrate for mitogen-activating protein kinases in vitro. In NIH 3T3 fibroblasts, v-raf activates Erk2, and overexpression of an interfering mutant of Erk2 both blocks the ability of v-raf to activate the c-fos promoter and suppresses transformation. Mutation of individual mitogen-activating protein kinase phosphoacceptor sites in TCF/Elk-1 also compromises v-raf-activated expression of a Gal-Elk/Gal-chloramphenicol acetyltransferase reporter system. However, in at least one instance the introduction of glutamate, but not aspartate, at a phosphoacceptor site is compatible with activation. These results provide compelling evidence that phosphorylation of TCF/Elk-1 by Erk2 is a major link in the Raf-1 kinase-dependent signal transduction pathway that activates c-fos expression.

The POZ domain: a conserved protein-protein interaction motif

We describe a novel zinc finger protein, ZID (zinc finger protein with interaction domain). At its amino terminus ZID contains a 120-amino-acid conserved motif present in a large family of proteins that includes both the otherwise unrelated zinc finger proteins, such as Ttk, GAGA, and ZF5, and a group of poxvirus proteins: We therefore refer to this domain as the POZ (poxvirus and zinc finger) domain. The POZ domains of ZID, Ttk, and GAGA act to inhibit the interaction of their associated finger regions with DNA. This inhibitory effect is not dependent on interactions with other proteins and does not appear dependent on specific interactions between the POZ domain and the finger region. The POZ domain acts as a specific protein-protein interaction domain: The POZ domains of ZID and Ttk can interact with themselves but not with each other, POZ domains from ZF5, or the viral protein SalF17R. However, the POZ domain of GAGA can interact efficiently with the POZ domain of Ttk. In transfection experiments, the ZID POZ domain inhibits DNA binding in NIH-3T3 cells and appears to localize the protein to discrete regions of the nucleus. We discuss the implications of multimerization for the function of POZ domain proteins.

Oxygen and the control of gene expression

The respiration of oxygen, while essential to aerobic organisms for the generation of energy, leads to the formation of reactive oxygen intermediates (ROIs) as harmful byproducts. ROIs damage nucleic acids, lipids and proteins. Therefore, protective mechanisms against elevated intracellular ROI levels, referred to as oxidative stress, have evolved. These include the activation of transcription factors which elevate the expression of protective enzymes. Eukaryotic cells have also evolved the ability to specifically generate ROIs following stimulation with various agents. In these cases, ROIs are used as second messengers to activate gene expression. Here we will discuss both prokaryotic and eukaryotic transcription factors that respond to ROIs. In addition, transcription factors will be described that are activated by either exposure to antioxidants, which reduce the intracellular ROI concentration, or by hypoxia, the absence of oxygen.

Novel interactions between human T-cell leukemia virus type I Tax and activating transcription factor 3 at a cyclic AMP-responsive element

Human proenkephalin gene transcription is transactivated by human T-cell leukemia virus type I (HTLV-I) Tax in human Jurkat T lymphocytes. This transactivation was further enhanced in Jurkat cells treated with concanavalin A, cyclic AMP, or 12-O-tetradecanoylphorbol-13-acetate. Deletion and cis-element transfer analyses of the human proenkephalin promoter identified a cyclic AMP-responsive AP-1 element (-92 to -86) as both necessary and sufficient to confer Tax-dependent transactivation. Different AP-1 or cyclic AMP-responsive element-binding protein (CREB)/activating transcription factor (ATF) proteins which bind this element were expressed in murine teratocarcinoma F9 cells to identify those capable of mediating Tax-dependent transactivation of human proenkephalin gene transcription. Although CREB, c-Fos, c-Jun, and JunD did not have significant effects, JunB inhibited the Tax-dependent transactivation. In contrast, ATF3 dramatically induced Tax-dependent transactivation, which was further enhanced by protein kinase A. Electrophoretic mobility shift assays with recombinant fusion proteins expressed and purified from bacteria indicate that the DNA-binding activity of ATF3 is also dramatically enhanced by Tax. Chimeric fusion proteins consisting of the DNA-binding domain of the yeast transcription factor Gal4 and the amino-terminal domain (residues 1 to 66) of ATF3 were able to mediate Tax-dependent transactivation of a Gal4-responsive promoter, which suggests a direct involvement of this region of ATF3. Recombinant fusion proteins of glutathione S-transferase with either the amino- or carboxy-terminal (residues 139 to 181) domain of ATF3 were able to specifically interact with Tax. Furthermore, specific antisera directed against Tax coimmunoprecipitated ATF3 only in the presence of Tax.

Cell signalling and the control of gene transcription

Recent discoveries have led to a better understanding of how signals generated by growth factors, cytokines and hormones elicit changes in gene expression in mammalian cells. Three general strategies of information pathways from the cell surface to the nucleus can be defined, all of which involve protein phosphorylation: (1) activation and translocation of cytoplasmic kinases to the nucleus, leading to changes in transcription-factor functions; (2) direct activation of latent cytoplasmic transcription factors by phosphorylation; and (3) release of transcription factors from cytoplasmic anchor proteins. These information conduits are illustrated by the mitogen-activated protein (MAP) kinase pathway, signal transducer and activator of transcription (Stat) proteins and nuclear factor-kappa B (NF-kappa B), respectively, and are discussed in this article by Dylan Edwards. Regulation of nucleo-cytoplasmic compartmentation emerges as a key aspect of signal transfer.

Net, a new ets transcription factor that is activated by Ras

Ras signaling appears to be mediated in part by transcription factors that belong to the ets gene family. To identify downstream targets for the Ras signal transduction pathway, we have used Ras-transformed mouse fibroblasts to isolate a new member of the ets gene family, net. Net has sequence similarity in three regions with the ets factors Elk1 and SAP1, which have been implicated in the serum response of the fos promoter. Net shares various properties with these proteins, including the ability to bind to ets DNA motifs through the Ets domain of the protein and form ternary complexes with the serum response factor SRF on the fos serum response element, SRE. However, Net differs from Elk1 and SAP1 in a number of ways. The pattern of net RNA expression in adult mouse tissues is different. Net has negative effects on transcription in a number of assays, unlike Elk1. Strikingly, Ras, Src, and Mos expression switch Net activity to positive. The study of Net should help in understanding the interplay between Net and other members of the Elk subfamily and their contribution to signal transduction through Ras to the nucleus.

Dual modes of control of c-fos mRNA induction by intracellular calcium in T cells

Cytoplasmic calcium is a nearly universal second messenger in eukaryotes. In many cell types, elevated intracellular calcium interacts synergistically with inducers of protein kinase C to elicit activation of complete biological programs normally induced by extracellular signals. In T cells, elevated cytoplasmic calcium is a critical mediator of activation in response to stimulation of the antigen receptor, and in some T-cell lines, treatment with a combination of calcium ionophore and protein kinase C activator mimics authentic antigen treatment. The synergistic interaction of calcium and protein kinase C in T cells is also observed at the level of gene expression. Here we examine the molecular mechanisms through which these agents exert synergistic control over the expression of the c-fos proto-oncogene in a T-cell hybridoma. We find that the principal effect of calcium is on the elongation of c-fos transcripts. This step constitutes the major control of c-fos mRNA accumulation in these cells. In addition, calcium regulates the initiation of c-fos transcription. This effect requires the serum response element of the c-fos gene and an additional sequence immediately 3' to this element. Thus, calcium regulates c-fos expression through at least two distinct molecular pathways.

Inhibition of v-raf-dependent c-fos expression and transformation by a kinase-defective mutant of the mitogen-activated protein kinase Erk2

Receptor-bound growth factors elicit intracellular signals that lead to the phosphorylation and activation of numerous intracellular kinases and transcription factors with consequent changes in patterns of gene expression. Several oncogene products are able to mimic these signals, resulting in cell transformation and proliferation. For example, the introduction of oncogenic forms of Raf-1 kinase into fibroblasts induces transformation and leads to the constitutive expression of, among others, the c-fos proto-oncogene. Here it is shown that the elevation of c-fos promoter activity brought about by v-raf is mediated by TCF/Elk-1, which forms a ternary complex with SRF at the serum response element and is a substrate for mitogen-activating protein kinases in vitro. In NIH 3T3 fibroblasts, v-raf activates Erk2, and overexpression of an interfering mutant of Erk2 both blocks the ability of v-raf to activate the c-fos promoter and suppresses transformation. Mutation of individual mitogen-activating protein kinase phosphoacceptor sites in TCF/Elk-1 also compromises v-raf-activated expression of a Gal-Elk/Gal-chloramphenicol acetyltransferase reporter system. However, in at least one instance the introduction of glutamate, but not aspartate, at a phosphoacceptor site is compatible with activation. These results provide compelling evidence that phosphorylation of TCF/Elk-1 by Erk2 is a major link in the Raf-1 kinase-dependent signal transduction pathway that activates c-fos expression.

Oncogenic Ras activates c-Jun via a separate pathway from the activation of extracellular signal-regulated kinases

c-Jun transcriptional activity is augmented by expression of oncogenic Ras and Raf proteins. This study demonstrates a direct correlation between Ras transforming activity and c-Jun activation, supporting an important role for c-Jun in transformation by Ras. Since we observed that Ras activated c-Jun transcriptional activity by increasing phosphorylation of the c-Jun activation domain at residues Ser-63/Ser-73 and that oncogenic Ras proteins activated extracellular signal-regulated protein kinases (ERK1 and ERK2) (also known as mitogen-activated protein kinases), we evaluated the possibility that ERKs were directly responsible for c-Jun activation. Coexpression of wild-type ERKs with oncogenic Ras proteins potentiated, while kinase-defective ERKs inhibited, Ras-induced transcriptional activation from the Ras-responsive element (Ets-1/AP-1) present in the NVL-3 enhancer and the serum-response element in the c-fos promoter. In contrast, coexpression of either wild-type or kinase-defective ERKs inhibited Ras and Raf activation of c-Jun transcriptional activity. Thus, although activation of both ERK and c-Jun are downstream consequences of activation of the Ras signal transduction pathway, our results suggest that Ras-induced c-Jun phosphorylation and transcriptional activation are not a direct consequence of ERK1 and ERK2 activation.

Signal transduction from the cell surface to the nucleus through the phosphorylation of transcription factors

The activation of several transcription factors in response to extracellular stimuli is fairly well understood. In all cases, activation is mediated through phosphorylation by signal responsive protein kinases. The specificity of transcription factor phosphorylation is likely to be ensured by physical interactions between the protein kinases and their substrates. Distinct biological responses are likely to be mediated through activation of different constellations of protein kinases, resulting in phosphorylation of different transcription factors.

The transcription factors Elk-1 and serum response factor interact by direct protein-protein contacts mediated by a short region of Elk-1

Transcriptional induction of the c-fos gene in response to epidermal growth factor stimulation is mediated in part by a ternary nucleoprotein complex within the promoter consisting of serum response factor (SRF), p62TCF/Elk-1 and the serum response element (SRE). Both SRF and p62TCF/Elk-1 contact the DNA and bind in a cooperative manner to the SRE. In this study, we demonstrate that SRF and Elk-1 interact directly in the absence of the SRE. A 30-amino-acid peptide from Elk-1 (B-box) is both necessary and sufficient to mediate protein-protein contacts with SRF. Moreover, the Elk-1 B-box is necessary to enable SRF-dependent binding of an alternative ETS domain (from the transcription factor PU.1) to the c-fos SRE. Mutations in either the Elk-1 B-box or the C-terminal half of the SRF DNA-binding domain (coreSRF) which show reduced ability to form ternary complexes also show greatly reduced protein-protein interactions in the absence of the SRE. Our results clearly demonstrate that direct protein-protein interactions between the transcription factors Elk-1 and SRF, in addition to DNA contacts, contribute to the formation of a ternary complex on the c-fos SRE. We discuss the wider applicability of our results in describing specific protein-protein interactions between short well-defined transcription factor domains.

Regulatory squelching

An important function of transcription factors may be to sequester coactivators or corepressors of transcription. In this manner transcription factors could regulate in trans the activity of promoters to which they do not bind. This may be of widespread significance as a mechanism to control cell cycle-dependent and differentiation-specific transcriptional activity within eukaryotic cells. Therefore squelching in vivo may be important than hitherto appreciated.

The transcription factors Elk-1 and serum response factor interact by direct protein-protein contacts mediated by a short region of Elk-1

Transcriptional induction of the c-fos gene in response to epidermal growth factor stimulation is mediated in part by a ternary nucleoprotein complex within the promoter consisting of serum response factor (SRF), p62TCF/Elk-1 and the serum response element (SRE). Both SRF and p62TCF/Elk-1 contact the DNA and bind in a cooperative manner to the SRE. In this study, we demonstrate that SRF and Elk-1 interact directly in the absence of the SRE. A 30-amino-acid peptide from Elk-1 (B-box) is both necessary and sufficient to mediate protein-protein contacts with SRF. Moreover, the Elk-1 B-box is necessary to enable SRF-dependent binding of an alternative ETS domain (from the transcription factor PU.1) to the c-fos SRE. Mutations in either the Elk-1 B-box or the C-terminal half of the SRF DNA-binding domain (coreSRF) which show reduced ability to form ternary complexes also show greatly reduced protein-protein interactions in the absence of the SRE. Our results clearly demonstrate that direct protein-protein interactions between the transcription factors Elk-1 and SRF, in addition to DNA contacts, contribute to the formation of a ternary complex on the c-fos SRE. We discuss the wider applicability of our results in describing specific protein-protein interactions between short well-defined transcription factor domains.

A requirement for extracellular signal-regulated kinase (ERK) function in the activation of AP-1 by Ha-Ras, phorbol 12-myristate 13-acetate, and serum

The role of ERK-1 and ERK-2 in wild-type (wt) Ha-Ras, phorbol 12-myristate 13-acetate (PMA), and serum-induced AP-1 activity was studied. Microinjection of ERK-specific substrate peptides inhibited the induction of AP-1 activity by all three stimuli, whereas a control peptide had no effect. By using eukaryotic expression constructs encoding wt ERK-1 and kinase-deficient mutants of ERKs 1 and 2, it was found that ERK-1 and ERK-2 activities are required for AP-1 activation stimulated by either wt Ha-Ras, PMA, or serum. Overexpression of ERK-1 augmented wt Ha-Ras stimulation of AP-1, while having no effect upon PMA or serum stimulation. Overexpression of either kinase-deficient ERK-1 or kinase-deficient ERK-2 partially inhibited AP-1 activation by wt Ha-Ras but had no effect on PMA or serum-induced activation. Coexpression of both interfering mutants abolished AP-1 induction by wt Ha-Ras, PMA, or serum. We conclude that ERKs are necessary components in the pathway leading to the activation of AP-1 stimulated by these agents.

The Drosophila SRF homolog is expressed in a subset of tracheal cells and maps within a genomic region required for tracheal development

The Drosophila homolog of the vertebrate serum response factor (SRF) was isolated by low stringency hybridization. Nucleotide sequence analysis revealed that the Drosophila SRF homolog (DSRF) codes for a protein that displays 93% sequence identity with human SRF in the MADS domain, the region required for DNA binding, dimerization and interaction with accessory factors. The DSRF gene is expressed during several phases of embryonic development. In the egg, both the RNA and the protein are maternal in origin and slowly decrease in amount during gastrulation. After germ band retraction, high levels of zygotic expression are observed in a distinct subset of peripheral tracheal cells distributed throughout the embryo. Many of these cells are at the tip of tracheal branches and are in direct contact with the target tissues. The DSRF gene was mapped to position 60C on the second chromosome, and overlapping deficiencies which remove the gene were identified. Analysis of tracheal development in embryos carrying these deletions revealed a degeneration of most of the major branches of the tracheal system. Although the initial migration of tracheal cells was not affected in those deficient embryos, many tracheal cells appeared not to maintain their correct position and continued to migrate. Thus, the DSRF gene might play a role in the proper formation and maintenance of the trachea.

Identification of the functional components of the Ras signaling pathway regulating pituitary cell-specific gene expression

Ras, a small GTP-binding protein, is required for functional receptor tyrosine kinase signaling. Ultimately, Ras alters the activity of specific nuclear transcription factors and regulates novel patterns of gene expression. Using a rat prolactin promoter construct in transient transfection experiments, we show that both oncogenic Ras and activated forms of Raf-1 kinase selectively stimulated the cellular rat prolactin promoter in GH4 rat pituitary cells. We also show that the Ras signal is completely blocked by an expression vector encoding a dominant-negative Raf kinase. Additionally, using a molecular genetic approach, we determined that inhibitory forms of p42 mitogen-activated protein kinase and an Ets-2 transcription factor interfere with both the Ras and the Raf activation of the rat prolactin promoter. These findings define a functional requirement for these signaling constituents in the activation of the prolactin gene, a cell-specific gene which marks the lactotroph pituitary cell type. Further, this analysis allowed us to order the components in the Ras signaling pathway as it impinges on regulation of prolactin gene transcription as Ras-->Raf kinase-->mitogen-activated protein kinase-->Ets. In contrast, we show that intact c-Jun expression inhibited the Ras-induced activation of the prolactin promoter, defining it as a negative regulator of this pathway, whereas c-Jun was able to enhance the Ras activation of an AP-1-driven promoter in GH4 cells. These data show that c-Jun is not the nuclear mediator of the Ras signal for the highly specialized, pituitary cell-specific prolactin cellular promoter. Thus, we have defined a model system which provides an ideal paradigm for studying Ras/Raf signaling pathways and their effects on neuroendocrine cell-specific gene regulation.