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

Isolation, sequence analysis and characterization of a cDNA encoding human chaperonin 10

A full-length cDNA clone encoding chaperonin 10 (cpn10) from a HeLa cell cDNA library was isolated. The cDNA is 538 bp in length, contains an ATG codon and a putative polyadenylation signal, and specifies a protein of 102 amino acids. Immunoprecipitation experiment showed that this human cpn10 has an apparent molecular mass of 11 kDa in sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE).

Phosphorylation of E2F-1 modulates its interaction with the retinoblastoma gene product and the adenoviral E4 19 kDa protein

The transcription factor E2F is regulated through its cyclical interaction with a spectrum of cellular proteins. One such protein is the product of the retinoblastoma gene (Rb); association of E2F with Rb inhibits its transactivation potential. However, in adenovirus-infected cells, E2F is complexed to the 19 kDa product of the adenovirus E4 gene. We have studied the interaction of E2F-1 with the Rb and adenovirus E4 proteins and show that phosphorylation of E2F-1 on serine residues 332 and 337 prevented its interaction with Rb but was a prerequisite for interaction with E4. These residues were phosphorylated in vivo and by p34cdc2 kinase in vitro. Upon stimulation of serum-starved cells, phosphorylation was induced in the late G1 phase of the cell cycle. These observations suggest that phosphorylation of E2F-1 is important in the regulation of its activity during the cell cycle and during infection of cells by adenovirus.

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.

The two-hybrid system: an assay for protein-protein interactions

The two-hybrid system is a yeast-based genetic assay for detecting protein-protein interactions. It can be used to identify proteins that bind to a protein of interest, or to delineate domains or residues critical for an interaction. Variations on this methodology have been developed to clone genes that encode DNA-binding proteins, to identify peptides that bind to a protein and, potentially, to screen for drugs.

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.

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.

Fimbriata controls flower development by mediating between meristem and organ identity genes

Two major classes of genes directing flower development have so far been described: early activated genes regulating meristem identity and later acting genes controlling organ identity. Here, we show that the fimbriata (fim) gene acts between these two classes in a sequence of gene activation. The fim gene, originally described in 1930, was cloned by transposon tagging from Antirrhinum majus and encodes a product with no detectable homology to other proteins. Mutations in fim result in partial homeotic transformations of floral organs and in reduced determinacy of the meristem. Expression and function of fim depends on the activity of meristem identity genes, and fim in turn controls the spatial and temporal expression of organ identity genes. The pattern of fim expression defines a new domain of the floral meristem that changes with time in a complementary manner to those of the meristem identity gene floricaula and the organ identity gene plena.

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.

Inhibition of p53-mediated transactivation by E6 of type 1, but not type 5, 8, or 47, human papillomavirus of cutaneous origin

Transient transfection experiments indicated (i) that E6 protein of non-cancer-associated cutaneous human papillomavirus type 1 (HPV-1) can inhibit p53-mediated transcriptional transactivation in both p53-deficient human cells (H358) and normal rat cells (3Y1), but those of cancer-associated cutaneous HPV-5, -8, and -47 cannot do so in either H358 or 3Y1 cells, and (ii) that E6 proteins of HPV-16 and -18 can inhibit the p53 function in H358 cells but not in 3Y1 cells.

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.

YY1 is involved in the regulation of the bi-directional promoter of the Surf-1 and Surf-2 genes

The Surfeit locus is an unusual cluster of at least 6 housekeeping genes whose organisation is conserved between birds and mammals. We have previously shown that the divergently transcribed Surf-1 and Surf-2 genes are separated by a bi-directional, TATA-less promoter. In mouse, the Surf-1/Surf-2 promoter contains three important factor binding sites: Su1, Su2, and Su3. These sites are conserved between the mouse and human Surf-1/Surf-2 promoters, bind nuclear factors in vitro, and are required for accurate and efficient expression of Surf-1 and Surf-2 in vivo. Using gel retardation assays, methylation interference experiments, and specific antibodies we demonstrate that the Su1 binding factor is the initiator protein YY1. Over-expression of YY1 results in a major stimulation of transcription in the Surf-1 direction and a minor stimulation of transcription in the Surf-2 direction.

Rch1, a protein that specifically interacts with the RAG-1 recombination-activating protein

RAG1 and RAG2 are lymphoid-specific genes that together induce V(D)J recombinase activity in a variety of nonlymphoid cell types. While no other lymphoid-specific factors are required to induce recombination, other factors with more widespread expression patterns have been implicated in the reaction. However, none of these factors have been cloned, and their relationship to the RAG proteins is unclear. Using the yeast two-hybrid assay, we have identified RCH1, a gene encoding a protein of molecular weight 58,000 that interacts specifically with RAG-1. The predicted Rch1 protein sequence is 47% identical to yeast SRP1, a protein associated with the nuclear envelope. A truncated form of Rch1, which retains the ability to interact with RAG-1, reduces V(D)J recombination activity in HeLa cells.

Mediation of suppression of c-fos transcription in rasT24-transformed rat cells by a cis-acting repressor element

Prolonged expression of activated ras mutants resulted in both neoplastic transformation and suppression of serum-induced c-fos expression in Rat1 fibroblasts. Expression of other serum-inducible genes, including c-jun and beta-actin, was not suppressed in ras-transformed Rat1 cells, indicating that these effects are specific for c-fos and that growth-factor signal transduction pathways remain essentially intact. Run-on transcription studies indicated that c-fos transcription was blocked at the level of initiation in these cells. Transient transfection studies using 360 bp from the wild-type c-fos promoter as well as a series of mutated c-fos promoter fragments linked to the chloramphenicol acetyltransferase gene indicated that repression of c-fos was mediated by approximately 49 bp immediately upstream of the dyad symmetry element (DSE). Deletion of this region, referred to as the upstream repressor region (URR), restored serum inducibility to the c-fos promoter in ras-transformed cells. In contrast, suppression of c-fos transcription was not affected by either deletion of 240 bp between the DSE and the TATA element or by base-substitution mutations that inactive the ternary complex factor and fos-AP-1-like binding sites. In addition, in vitro competition studies indicated that ras-transformed cells express one or more repressor factors that interact with as-yet-unidentified elements within the c-fos promoter (possibly the URR) and block serum induction of c-fos. These findings suggest that prolonged expression of activated ras results in the activation of one or more as-yet-unidentified proteins that suppress transcription of the c-fos gene by interacting with the URR.

ERP, a new member of the ets transcription factor/oncoprotein family: cloning, characterization, and differential expression during B-lymphocyte development

The ets gene family encodes a group of proteins which function as transcription factors under physiological conditions and, if aberrantly expressed, can cause cellular transformation. We have recently identified two regulatory elements in the murine immunoglobulin heavy-chain (IgH) enhancer, pi and microB, which exhibit striking similarity to binding sites for ets-related proteins. To identify ets-related transcriptional regulators expressed in pre-B lymphocytes that may interact with either the pi or the microB site, we have used a PCR approach with degenerate oligonucleotides encoding conserved sequences in all members of the ets family. We have cloned the gene for a new ets-related transcription factor, ERP (ets-related protein), from the murine pre-B cell line BASC 6C2 and from mouse lung tissue. The ERP protein contains a region of high homology with the ETS DNA-binding domain common to all members of the ets transcription factor/oncoprotein family. Three additional smaller regions show homology to the ELK-1 and SAP-1 genes, a subgroup of the ets gene family that interacts with the serum response factor. Full-length ERP expresses only negligible DNA-binding activity by itself. Removal of the carboxy terminus enables ERP to interact with a variety of ets-binding sites including the E74 site, the IgH enhancer pi site, and the lck promoter ets site, suggesting a carboxy-terminal negative regulatory domain. At least three ERP-related transcripts are expressed in a variety of tissues. However, within the B-cell lineage, ERP is highly expressed primarily at early stages of B-lymphocyte development, and expression declines drastically upon B-cell maturation, correlating with the enhancer activity of the IgH pi site. These data suggest that ERP might play a role in B-cell development and in IgH gene regulation.

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.

DNA-binding and transcriptional activation properties of the EWS-FLI-1 fusion protein resulting from the t(11;22) translocation in Ewing sarcoma

The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, FLI-1, and ERG, in several human tumors. The most frequent occurrence of this situation results from the t(11;22)(q24;q12) chromosome translocation specific for Ewing sarcoma (ES) and related tumors which joins EWS sequences to the 3' half of FLI-1, which encodes a member of the Ets family of transcriptional regulators. We show here that this chimeric gene encodes an EWS-FLI-1 nuclear protein which binds DNA with the same sequence specificity as the wild-type parental FLI-1 protein. We further show that EWS-FLI-1 is an efficient sequence-specific transcriptional activator of model promoters containing FLI-1 (Ets)-binding sites, a property which is strictly dependent on the presence of its EWS domain. Comparison of the properties of the N-terminal activation domain of FLI-1 to those of the EWS domain of the fusion protein indicates that EWS-FLI-1 has altered transcriptional activation properties compared with FLI-1. These results suggest that EWS-FLI-1 contributes to the transformed phenotype of ES tumor cells by inducing the deregulated and/or unscheduled activation of genes normally responsive to FLI-1 or to other close members of the Ets family. ES and related tumors are characterized by an elevated level of c-myc expression. We show that EWS-FLI-1 is a transactivator of the c-myc promoter, suggesting that upregulation of c-myc expression is under control of EWS-FLI-1.

ERP, a new member of the ets transcription factor/oncoprotein family: cloning, characterization, and differential expression during B-lymphocyte development

The ets gene family encodes a group of proteins which function as transcription factors under physiological conditions and, if aberrantly expressed, can cause cellular transformation. We have recently identified two regulatory elements in the murine immunoglobulin heavy-chain (IgH) enhancer, pi and microB, which exhibit striking similarity to binding sites for ets-related proteins. To identify ets-related transcriptional regulators expressed in pre-B lymphocytes that may interact with either the pi or the microB site, we have used a PCR approach with degenerate oligonucleotides encoding conserved sequences in all members of the ets family. We have cloned the gene for a new ets-related transcription factor, ERP (ets-related protein), from the murine pre-B cell line BASC 6C2 and from mouse lung tissue. The ERP protein contains a region of high homology with the ETS DNA-binding domain common to all members of the ets transcription factor/oncoprotein family. Three additional smaller regions show homology to the ELK-1 and SAP-1 genes, a subgroup of the ets gene family that interacts with the serum response factor. Full-length ERP expresses only negligible DNA-binding activity by itself. Removal of the carboxy terminus enables ERP to interact with a variety of ets-binding sites including the E74 site, the IgH enhancer pi site, and the lck promoter ets site, suggesting a carboxy-terminal negative regulatory domain. At least three ERP-related transcripts are expressed in a variety of tissues. However, within the B-cell lineage, ERP is highly expressed primarily at early stages of B-lymphocyte development, and expression declines drastically upon B-cell maturation, correlating with the enhancer activity of the IgH pi site. These data suggest that ERP might play a role in B-cell development and in IgH gene regulation.

DNA-binding and transcriptional activation properties of the EWS-FLI-1 fusion protein resulting from the t(11;22) translocation in Ewing sarcoma

The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, FLI-1, and ERG, in several human tumors. The most frequent occurrence of this situation results from the t(11;22)(q24;q12) chromosome translocation specific for Ewing sarcoma (ES) and related tumors which joins EWS sequences to the 3' half of FLI-1, which encodes a member of the Ets family of transcriptional regulators. We show here that this chimeric gene encodes an EWS-FLI-1 nuclear protein which binds DNA with the same sequence specificity as the wild-type parental FLI-1 protein. We further show that EWS-FLI-1 is an efficient sequence-specific transcriptional activator of model promoters containing FLI-1 (Ets)-binding sites, a property which is strictly dependent on the presence of its EWS domain. Comparison of the properties of the N-terminal activation domain of FLI-1 to those of the EWS domain of the fusion protein indicates that EWS-FLI-1 has altered transcriptional activation properties compared with FLI-1. These results suggest that EWS-FLI-1 contributes to the transformed phenotype of ES tumor cells by inducing the deregulated and/or unscheduled activation of genes normally responsive to FLI-1 or to other close members of the Ets family. ES and related tumors are characterized by an elevated level of c-myc expression. We show that EWS-FLI-1 is a transactivator of the c-myc promoter, suggesting that upregulation of c-myc expression is under control of EWS-FLI-1.

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.

The human beta 2 integrin CD18 promoter consists of two inverted Ets cis elements

To define the minimal promoter responsible for expression of CD18 in myeloid and lymphoid cells, we generated 5' and 3' deletion constructs of a segment extending 785 bp upstream and 19 bp downstream of a major transcription start site and determined their effects on driving expression of the luciferase reporter gene in transfected hematopoietic cell lines. A region extending from nucleotides (nt) -302 to +19 was sufficient for cell-restricted and phorbol ester-inducible expression. DNase I footprinting of this region revealed two adjacent protected segments extending from nt -81 to -68 (box A) and -55 to -41 (box B). When a construct of 47 nt in length containing box A and box B and lacking other 3' or 5' elements was cloned into a promoterless vector, it conferred tissue-specific and phorbol ester-inducible expression. Gel retardation revealed that the protein components of two major protein-DNA complexes that form on both box A and box B and are required for transcriptional activation are members of the Ets oncoprotein family; one is related to the GA-binding protein (GABP), and the other is related to PU.1/Spi-1. The minimal CD18 promoter, lacking TATA, CAAT, and initiator elements and consisting primarily of Ets repeats, may exemplify an emerging class of promoters with which the concerted binding of Ets factors is necessary and sufficient to mediate transcriptional activation through direct recruitment of the basal transcription machinery.

pct1+, which encodes a new DNA-binding partner of p85cdc10, is required for meiosis in the fission yeast Schizosaccharomyces pombe

The transcriptional activation of genes at late G1 is an important regulatory step in the commitment to a new cell division cycle. In Schizosaccharomyces pombe, this regulation is mediated by MCB elements that serve as binding sites for the MBF/DSC-1 complex. The cdc10(+)-encoded protein is a component of this complex. We report the cloning of a new gene, pct1+, encoding a 73-kD protein that interacts with p85cdc10 to form an MCB-binding heteromer. Pct1+ is related to, but distinct from, the res1+/sct1+ gene that also encodes a p85cdc10 partner. p73pct1 has centrally located ankyrin repeats and a putative amino-terminal DNA-binding domain that has extensive sequence similarity to the DNA-binding domains of the Saccharomyces cerevisiae SWI4 and MBP1 proteins. The p73pct1/p85cdc10 complex binds both in vitro and in vivo to MCB but not SCB or E2F sites. Overexpression of pct1+ is sufficient to rescue the growth of the cdc10-129 temperature-sensitive mutant at the restrictive temperature, although it is unable to rescue a cdc10 null mutation. A deletion of pct1+ is not lethal but does result in a severe meiotic defect. Our results indicate that there are two cdc10-containing heteromeric complexes that bind to MCB elements and play differential roles in mitotic division and meiosis.

The human beta 2 integrin CD18 promoter consists of two inverted Ets cis elements

To define the minimal promoter responsible for expression of CD18 in myeloid and lymphoid cells, we generated 5' and 3' deletion constructs of a segment extending 785 bp upstream and 19 bp downstream of a major transcription start site and determined their effects on driving expression of the luciferase reporter gene in transfected hematopoietic cell lines. A region extending from nucleotides (nt) -302 to +19 was sufficient for cell-restricted and phorbol ester-inducible expression. DNase I footprinting of this region revealed two adjacent protected segments extending from nt -81 to -68 (box A) and -55 to -41 (box B). When a construct of 47 nt in length containing box A and box B and lacking other 3' or 5' elements was cloned into a promoterless vector, it conferred tissue-specific and phorbol ester-inducible expression. Gel retardation revealed that the protein components of two major protein-DNA complexes that form on both box A and box B and are required for transcriptional activation are members of the Ets oncoprotein family; one is related to the GA-binding protein (GABP), and the other is related to PU.1/Spi-1. The minimal CD18 promoter, lacking TATA, CAAT, and initiator elements and consisting primarily of Ets repeats, may exemplify an emerging class of promoters with which the concerted binding of Ets factors is necessary and sufficient to mediate transcriptional activation through direct recruitment of the basal transcription machinery.

A Mef2 gene that generates a muscle-specific isoform via alternative mRNA splicing

Members of the myocyte-specific enhancer-binding factor 2 (MEF2) family of transcription factors bind a conserved A/T-rich sequence in the control regions of numerous muscle-specific genes. Mammalian MEF2 proteins have been shown previously to be encoded by three genes, Mef2, xMef2, and Mef2c, each of which gives rise to multiple alternatively spliced transcripts. We describe the cloning of a new member of the MEF2 family from mice, termed MEF2D, which shares extensive homology with other MEF2 proteins but is the product of a separate gene. MEF2D binds to and activates transcription through the MEF2 site and forms heterodimers with other members of the MEF2 family. Deletion mutations show that the carboxyl terminus of MEF2D is required for efficient transactivation. MEF2D transcripts are widely expressed, but alternative splicing of MEF2D transcripts gives rise to a muscle-specific isoform which is induced during myoblast differentiation. The mouse Mef2, Mef2c, and Mef2d genes map to chromosomes 7, 13, and 3, respectively. The complexity of the MEF2 family of regulatory proteins provides the potential for fine-tuning of transcriptional responses as a consequence of combinatorial interactions among multiple MEF2 isoforms encoded by the four Mef2 genes.

A Mef2 gene that generates a muscle-specific isoform via alternative mRNA splicing

Members of the myocyte-specific enhancer-binding factor 2 (MEF2) family of transcription factors bind a conserved A/T-rich sequence in the control regions of numerous muscle-specific genes. Mammalian MEF2 proteins have been shown previously to be encoded by three genes, Mef2, xMef2, and Mef2c, each of which gives rise to multiple alternatively spliced transcripts. We describe the cloning of a new member of the MEF2 family from mice, termed MEF2D, which shares extensive homology with other MEF2 proteins but is the product of a separate gene. MEF2D binds to and activates transcription through the MEF2 site and forms heterodimers with other members of the MEF2 family. Deletion mutations show that the carboxyl terminus of MEF2D is required for efficient transactivation. MEF2D transcripts are widely expressed, but alternative splicing of MEF2D transcripts gives rise to a muscle-specific isoform which is induced during myoblast differentiation. The mouse Mef2, Mef2c, and Mef2d genes map to chromosomes 7, 13, and 3, respectively. The complexity of the MEF2 family of regulatory proteins provides the potential for fine-tuning of transcriptional responses as a consequence of combinatorial interactions among multiple MEF2 isoforms encoded by the four Mef2 genes.

One of two Ets-binding sites in the cytokeratin EndoA enhancer is essential for enhancer activity and binds to Ets-2 related proteins

Expression of the mouse cytokeratin EndoA gene is restricted in endodermal and epithelial cells, and is regulated by an enhancer that is located 1 kilobase (kb) 3' downstream from the gene. The enhancer consists of six direct repeats, of which each contains two predicted Ets binding sites (EBS1 and EBS2) containing GGAA as a core. Mutation analysis showed that EBS1 is essential for the enhancer activity and additional effects of EBS2, suggesting that some Ets-related proteins bind and activate the enhancer through EBS1. We also showed that Ets-2 mRNA is expressed in PYS-2 cells and that Ets-2 protein produced by E. coli interacts with EBS1 but not with EBS2. Using co-transfection assays, we showed that Ets-2 can trans-activate the enhancer in PYS-2 cells. Mutations that impair Ets-2 binding abolished the activity of the EndoA enhancer. The results obtained from the binding competition assay using an Ets-2 specific antibody, however, suggest that EBS1 binds to an Ets protein which is distinct from Ets-2. These data show that Ets-2 related protein binds and activates the EndoA enhancer in a sequence-specific fashion.

Ternary complex factors: growth factor regulated transcriptional activators

Members of a family of Ets domain proteins, the ternary complex factors (TCFs), are recruited to the c-fos serum response element by interaction with the serum response factor. Recent findings indicate that phosphorylation of TCFs occurs in response to activation of the MAP kinase pathway, and that regulation of TCF activity is an important mechanism by which the serum response element responds to growth factor signals.

Phosphatidylinositol 3-kinase activation is mediated by high-affinity interactions between distinct domains within the p110 and p85 subunits

Domains of interaction between the p85 and p110 subunits of phosphatidylinositol 3-kinase (PI 3-kinase) were studied with the yeast two-hybrid expression system. A gene fusion between the GAL4 transactivation domain and p85 activated transcription from a GAL1-lacZ reporter gene when complemented with a gene fusion between the GAL4 DNA binding domain and p110. To define subdomains responsible for this interaction, a series of p85 deletion mutants were analyzed. A 192-amino-acid inter-SH2 (IS) fragment (residues 429 to 621) was the smallest determinant identified that specifically associated with p110. In analogous experiments, the subdomain within p110 responsible for interaction with p85 was localized to an EcoRI fragment encoding the amino-terminal 127 residues. Expression of these two subdomains [p85(IS) with p110RI] resulted in 100-fold greater reporter activity than that obtained with full-length p85 and p110. Although the p85(IS) domain conferred a strong interaction with the p110 catalytic subunit, this region was not sufficient to impart phosphotyrosine peptide stimulation of PI 3-kinase activity. In contrast, coexpression of the p110 subunit with full-length p85 or with constructs containing the IS sequences flanked by both SH2 domains of p85 [p85(n/cSH2)] or either of the individual SH2 domains [p85(nSH2+IS) or p85(IS+cSH2)] resulted in PI 3-kinase activity that was activated by a phosphotyrosine peptide. These data suggest that phosphotyrosine peptide binding to either SH2 domain generates an intramolecular signal propagated through the IS region to allosterically activate p110.

The macrophage transcription factor PU.1 directs tissue-specific expression of the macrophage colony-stimulating factor receptor

The macrophage colony-stimulating factor (M-CSF) receptor is expressed in a tissue-specific fashion from two distinct promoters in monocytes/macrophages and the placenta. In order to further understand the transcription factors which play a role in the commitment of multipotential progenitors to the monocyte/macrophage lineage, we have initiated an investigation of the factors which activate the M-CSF receptor very early during the monocyte differentiation process. Here we demonstrate that the human monocytic M-CSF receptor promoter directs reporter gene activity in a tissue-specific fashion. Since one of the few transcription factors which have been implicated in the regulation of monocyte genes is the macrophage- and B-cell-specific PU.1 transcription factor, we investigated whether PU.1 binds and activates the M-CSF receptor promoter. Here we demonstrate that both in vitro-translated PU.1 and PU.1 from nuclear extracts bind to a specific site in the M-CSF receptor promoter just upstream from the major transcription initiation site. Mutations in this site which eliminate PU.1 binding decrease M-CSF receptor promoter activity significantly in macrophage cell lines only. Furthermore, PU.1 transactivates the M-CSF receptor promoter in nonmacrophage cells. These results suggest that PU.1 plays a major role in macrophage gene regulation and development by directing the expression of a receptor for a key macrophage growth factor.

Developmental stage-specific regulation of atrial natriuretic factor gene transcription in cardiac cells

Cardiac myocytes undergo a major genetic switch within the first week of postnatal development, when cell division ceases terminally and many cardiac genes are either activated or silenced. We have developed stage-specific cardiocyte cultures to analyze transcriptional control of the rat atrial natriuretic factor (ANF) gene to identify the mechanisms underlying tissue-specific and developmental regulation of this gene in the heart. The first 700 bp of ANF flanking sequences was sufficient for cardiac muscle- and stage-specific expression in both atrial and ventricular myocytes, and a cardiac muscle-specific enhancer was localized between -136 and -700 bp. Deletion of this enhancer markedly reduced promoter activity in cardiac myocytes and derepressed ANF promoter activity in nonexpressing cells. Two distinct domains of the enhancer appeared to contribute differentially to cardiac specificity depending on the differentiation stage of the myocytes. DNase I footprinting of the enhancer domain active in differentiated cells revealed four putative regulatory elements including an A+T-rich region and a CArG element. Deletion mutagenesis and promoter reconstitution assays revealed an important role for the CArG-containing element exclusively in cardiac cells, where its activity was switched on in differentiated myocytes. Transcriptional activity of the ANF-CArG box correlated with the presence of a cardiac- and stage-specific DNA-binding complex which was not recognized by the c-fos serum response element. Thus, the use of this in vitro model system representing stage-specific cardiac development unraveled the presence of different regulatory mechanisms for transcription of the ANF gene during cardiac differentiation and may be useful for studying the regulatory pathways of other genes that undergo switching during cardiac myogenesis.

Cooperative binding of Ets-1 and core binding factor to DNA

Two phorbol ester-inducible elements (beta E2 and beta E3) within the human T-cell receptor beta gene enhancer each contain consensus binding sites for the Ets and core binding factor (CBF) transcription factor families. Recombinant Ets-1 and purified CBF bound individually to beta E2 and beta E3, in which the Ets and core sites are directly adjacent. In this report, we show that CBF and Ets-1 bind together to beta E2 and beta E3 and that Ets-1-CBF-DNA complexes are favored over the binding of either protein alone to beta E2. Formation of Ets-1-CBF-DNA complexes increased the affinity of Ets-1-DNA interactions and decreased the rate of dissociation of CBF from DNA. Ets-1-CBF-DNA complexes were not observed when either the Ets or core site was mutated. The spatial requirements for the cooperative interaction of Ets-1 and CBF were analyzed by oligonucleotide mutagenesis and binding site selection experiments. Core and Ets sites were coselected, and there appeared to be little constraint on the relative orientation and spacing of the two sites. These results demonstrate that CBF and Ets-1 form a high-affinity DNA-binding complex when both of their cognate sites are present and that the relative spacing and orientation of the two sites are unimportant. Ets and core sites are found in several T-cell-specific enhancers, suggesting that this interaction is of general importance in T-cell-specific transcription.

The Ewing's sarcoma EWS/FLI-1 fusion gene encodes a more potent transcriptional activator and is a more powerful transforming gene than FLI-1

EWS/FLI-1 is a chimeric protein formed by a tumor-specific 11;22 translocation found in both Ewing's sarcoma and primitive neuroectodermal tumor of childhood. EWS/FLI-1 has been shown to be a potent transforming gene, suggesting that it plays an important role in the genesis of these human tumors. We now demonstrate that EWS/FLI-1 has the characteristics of an aberrant transcription factor. Subcellular fractionation experiments localized the EWS/FLI-1 protein to the nucleus of primitive neuroectodermal tumor cells. EWS/FLI-1 specifically bound in vitro an ets-2 consensus sequence similarly to normal FLI-1. When coupled to a GAL4 DNA-binding domain, the amino-terminal EWS/FLI-1 region was a much more potent transcriptional activator than the corresponding amino-terminal domain of FLI-1. Finally, EWS/FLI-1 efficiently transformed NIH 3T3 cells, but FLI-1 did not. These data suggest that EWS/FLI-1, functioning as a transcription factor, leads to a phenotype dramatically different from that of cells expressing FLI-1. EWS/FLI-1 could disrupt normal growth and differentiation either by more efficiently activating FLI-1 target genes or by inappropriately modulating genes normally not responsive to FLI-1.

Phosphatidylinositol 3-kinase activation is mediated by high-affinity interactions between distinct domains within the p110 and p85 subunits

Domains of interaction between the p85 and p110 subunits of phosphatidylinositol 3-kinase (PI 3-kinase) were studied with the yeast two-hybrid expression system. A gene fusion between the GAL4 transactivation domain and p85 activated transcription from a GAL1-lacZ reporter gene when complemented with a gene fusion between the GAL4 DNA binding domain and p110. To define subdomains responsible for this interaction, a series of p85 deletion mutants were analyzed. A 192-amino-acid inter-SH2 (IS) fragment (residues 429 to 621) was the smallest determinant identified that specifically associated with p110. In analogous experiments, the subdomain within p110 responsible for interaction with p85 was localized to an EcoRI fragment encoding the amino-terminal 127 residues. Expression of these two subdomains [p85(IS) with p110RI] resulted in 100-fold greater reporter activity than that obtained with full-length p85 and p110. Although the p85(IS) domain conferred a strong interaction with the p110 catalytic subunit, this region was not sufficient to impart phosphotyrosine peptide stimulation of PI 3-kinase activity. In contrast, coexpression of the p110 subunit with full-length p85 or with constructs containing the IS sequences flanked by both SH2 domains of p85 [p85(n/cSH2)] or either of the individual SH2 domains [p85(nSH2+IS) or p85(IS+cSH2)] resulted in PI 3-kinase activity that was activated by a phosphotyrosine peptide. These data suggest that phosphotyrosine peptide binding to either SH2 domain generates an intramolecular signal propagated through the IS region to allosterically activate p110.

The macrophage transcription factor PU.1 directs tissue-specific expression of the macrophage colony-stimulating factor receptor

The macrophage colony-stimulating factor (M-CSF) receptor is expressed in a tissue-specific fashion from two distinct promoters in monocytes/macrophages and the placenta. In order to further understand the transcription factors which play a role in the commitment of multipotential progenitors to the monocyte/macrophage lineage, we have initiated an investigation of the factors which activate the M-CSF receptor very early during the monocyte differentiation process. Here we demonstrate that the human monocytic M-CSF receptor promoter directs reporter gene activity in a tissue-specific fashion. Since one of the few transcription factors which have been implicated in the regulation of monocyte genes is the macrophage- and B-cell-specific PU.1 transcription factor, we investigated whether PU.1 binds and activates the M-CSF receptor promoter. Here we demonstrate that both in vitro-translated PU.1 and PU.1 from nuclear extracts bind to a specific site in the M-CSF receptor promoter just upstream from the major transcription initiation site. Mutations in this site which eliminate PU.1 binding decrease M-CSF receptor promoter activity significantly in macrophage cell lines only. Furthermore, PU.1 transactivates the M-CSF receptor promoter in nonmacrophage cells. These results suggest that PU.1 plays a major role in macrophage gene regulation and development by directing the expression of a receptor for a key macrophage growth factor.

Developmental stage-specific regulation of atrial natriuretic factor gene transcription in cardiac cells

Cardiac myocytes undergo a major genetic switch within the first week of postnatal development, when cell division ceases terminally and many cardiac genes are either activated or silenced. We have developed stage-specific cardiocyte cultures to analyze transcriptional control of the rat atrial natriuretic factor (ANF) gene to identify the mechanisms underlying tissue-specific and developmental regulation of this gene in the heart. The first 700 bp of ANF flanking sequences was sufficient for cardiac muscle- and stage-specific expression in both atrial and ventricular myocytes, and a cardiac muscle-specific enhancer was localized between -136 and -700 bp. Deletion of this enhancer markedly reduced promoter activity in cardiac myocytes and derepressed ANF promoter activity in nonexpressing cells. Two distinct domains of the enhancer appeared to contribute differentially to cardiac specificity depending on the differentiation stage of the myocytes. DNase I footprinting of the enhancer domain active in differentiated cells revealed four putative regulatory elements including an A+T-rich region and a CArG element. Deletion mutagenesis and promoter reconstitution assays revealed an important role for the CArG-containing element exclusively in cardiac cells, where its activity was switched on in differentiated myocytes. Transcriptional activity of the ANF-CArG box correlated with the presence of a cardiac- and stage-specific DNA-binding complex which was not recognized by the c-fos serum response element. Thus, the use of this in vitro model system representing stage-specific cardiac development unraveled the presence of different regulatory mechanisms for transcription of the ANF gene during cardiac differentiation and may be useful for studying the regulatory pathways of other genes that undergo switching during cardiac myogenesis.

Cooperative binding of Ets-1 and core binding factor to DNA

Two phorbol ester-inducible elements (beta E2 and beta E3) within the human T-cell receptor beta gene enhancer each contain consensus binding sites for the Ets and core binding factor (CBF) transcription factor families. Recombinant Ets-1 and purified CBF bound individually to beta E2 and beta E3, in which the Ets and core sites are directly adjacent. In this report, we show that CBF and Ets-1 bind together to beta E2 and beta E3 and that Ets-1-CBF-DNA complexes are favored over the binding of either protein alone to beta E2. Formation of Ets-1-CBF-DNA complexes increased the affinity of Ets-1-DNA interactions and decreased the rate of dissociation of CBF from DNA. Ets-1-CBF-DNA complexes were not observed when either the Ets or core site was mutated. The spatial requirements for the cooperative interaction of Ets-1 and CBF were analyzed by oligonucleotide mutagenesis and binding site selection experiments. Core and Ets sites were coselected, and there appeared to be little constraint on the relative orientation and spacing of the two sites. These results demonstrate that CBF and Ets-1 form a high-affinity DNA-binding complex when both of their cognate sites are present and that the relative spacing and orientation of the two sites are unimportant. Ets and core sites are found in several T-cell-specific enhancers, suggesting that this interaction is of general importance in T-cell-specific transcription.

A functional Ets DNA-binding domain is required to maintain multipotency of hematopoietic progenitors transformed by Myb-Ets

Earlier work demonstrated that the Myb-Ets fusion protein of E26 avian leukemia virus induces the proliferation of multipotent hematopoietic progenitors (MEPs). These progenitors differentiate spontaneously at low frequencies along the erythroid lineage, and following the introduction of kinase/ras-type oncogenes or treatment with TPA, they are induced to differentiate along the myelomonocytic and eosinophilic lineages. Here, we show that the ts1.1 mutant of E26 encodes an Ets DNA-binding domain that is both defective and thermolabile for binding of specific DNA sequences. Correlating with this, ts1.1 MEP colonies transformed at the permissive temperature exhibit elevated levels of erythroid cells and eosinophils, whereas at the nonpermissive temperature they are induced to differentiate along the erythroid and myelomonocytic lineages and, to a lesser extent, along the eosinophil lineage. Induction of the former two lineages cannot be separated by pulse shift experiments and is essentially completed 2.5 days after temperature shift. Our results indicate that the Ets portion of the Myb-Ets fusion protein inhibits the lineage commitment of multipotent hematopoietic progenitors, probably via binding to regulatory DNA sequences of specific target genes.