Biomedical subjects

D A Gillespie

Publications and source records attributed to D A Gillespie.

At least 19 recordsLinked to original sources

PubMed2000

v-Jun sensitizes cells to apoptosis by a mechanism involving mitochondrial cytochrome C release.

v-Jun shares the ability of the Myc, E1A, and E2F oncogenes to both sustain cell cycle progression and promote apoptosis in the absence of mitogenic stimulation. To gain an insight into the mechanism of apoptosis sensitization, we examined the possible involvement of key regulatory proteins previously implicated in oncogene-induced cell death during v-Jun-induced apoptosis triggered by serum withdrawal. We observed that ectopic expression of the anti-apoptotic Bcl-2 protein, or of two downstream effectors of growth factor signalling, v-PI 3-Kinase and v-Src, partially or completely suppressed apoptosis. Apoptosis was also observed in the presence of serum growth factors when endogenous PI3K activity was blocked using the synthetic inhibitor LY294002, further suggesting an important role for PI3-K in cell survival. Cytochrome C was released into the cytosol of apoptotic v-Jun expressing cells, and this release was inhibited by Bcl-2, suggesting an important role for mitochondrial dysfunction in v-Jun induced apoptosis. In contrast, inhibition of Fas signalling using dominant negative FADD did not inhibit apoptosis, nor was there any evidence for accumulation or activation of p53 in v-Jun transformed cells. Consistent with this latter observation, inhibition of p53 function by HPV16 E6 protein had no effect on v-Jun induced cell death. Taken together, these results suggest that mitochondrial dysfunction is an important component of the mechanism through which v-Jun sensitizes cells to apoptosis, but that the apoptotic signals elicited by v-Jun upstream of the mitochondria do not depend on increased levels of p53 activity or Fas signalling.

Adaptor Proteins, Signal Transducing↗

PubMed2000

Transient deactivation of ERK signalling is sufficient for stable entry into G0 in primary avian fibroblasts.

Re-entry into the cell cycle from quiescence requires the activation of mitogen-activated protein (MAP) kinases of the extracellular-signal-regulated kinase (ERK) family [1,2]. The relationship between ERK and cell-cycle control is, however, complex, as ERK activation can also lead to terminal differentiation [3] or a senescence-like growth arrest [4]. Here, we report that reversible cell-cycle exit induced by serum withdrawal in primary avian fibroblasts is associated with rapid deactivation of ERK, but ERK activity is subsequently regenerated and sustained at high levels in fully quiescent (G0) cells. As in proliferating cells, ERK activation during G0 required the MAPkinase kinase MEK and was partially dependent on cell adhesion. Active, phosphorylated ERK was concentrated in the nucleus in cycling cells, but was largely confined to the cytoplasm during G0. This was unexpected, as activatory phosphorylation mediated by MEK is thought to play an important role in promoting nuclear translocation [5,6]. These results indicate that transient deactivation of ERK signalling can be sufficient for stable cell-cycle exit, and that MEK-mediated phosphorylation is not sufficient for nuclear translocation of active ERK in G0. Cytoplasmic sequestration may prevent active ERK from accessing critical nuclear cell-cycle targets, thus allowing quiescent or post-mitotic cells to retain ERK activity for other physiological functions.

3T3 Cells↗

PubMed2000

Inhibition of adipocyte differentiation by cMyc is not accompanied by alterations in cell cycle control.

Using a preadipocyte cell line constitutively expressing cMyc, we set out to determine if the ability of cMyc to inhibit adipogenic differentiation was functionally distinct from its role in cell cycle progression and transformation. We now report that in this system differentiation control and cellular transformation are separable functions of cMyc. Furthermore, the Myc-induced inhibition of adipocyte differentiation appears to be mediated via suppression of C/EBP-alpha and p21 gene expression late in the adipogenic differentiation programme, without deregulation of either cell cycle control or gene expression during the early stages of the process.

3T3 Cells↗

PubMed2000

Inhibition of the terminal stages of adipocyte differentiation by cMyc.

The nuclear oncoprotein Myc is a pivotal regulator of several important biological processes, including cellular proliferation, differentiation, and apoptosis. Deregulated Myc expression is incompatible with terminal differentiation in a variety of cell types, including adipocytes. To understand how Myc inhibits adipogenesis, we analyzed the effect of Myc on the expression of genes characteristic of distinct phases of the hormonally induced adipogenic differentiation program in 3T3-L1 preadipocytes. We show that the early regulators, C/EBPbeta and C/EBPdelta, are induced normally in response to hormone in 3T3-L1 preadipocytes constitutively expressing Myc, but that expression of the downstream regulators, C/EBPalpha and PPARgamma2, and later markers of differentiation is suppressed. These data demonstrate that Myc specifically inhibits the terminal stages of the adipogenic program and suggest that Myc may act by blocking C/EBPbeta- and C/EBPdelta-directed activation of C/EBPalpha and PPARgamma2 expression, although the precise molecular mechanism is not understood. Surprisingly, a serum component(s) could override the Myc-induced differentiation block, suggesting that the ability of a cell to undergo terminal differentiation is governed by the action of both positive and negative factors. Since differentiation and proliferation are mutually exclusive events, this has important implications since it may be possible to force malignant cells along a differentiation pathway, thereby curbing their proliferative potential.

3T3 Cells↗

PubMed2000

v-Jun overrides the mitogen dependence of S-phase entry by deregulating retinoblastoma protein phosphorylation and E2F-pocket protein interactions as a consequence of enhanced cyclin E-cdk2 catalytic activity.

v-Jun accelerates G(1) progression and shares the capacity of the Myc, E2F, and E1A oncoproteins to sustain S-phase entry in the absence of mitogens; however, how it does so is unknown. To gain insight into the mechanism, we investigated how v-Jun affects mitogen-dependent processes which control the G(1)/S transition. We show that v-Jun enables cells to express cyclin A and cyclin A-cdk2 kinase activity in the absence of growth factors and that deregulation of cdk2 is required for S-phase entry. Cyclin A expression is repressed in quiescent cells by E2F acting in conjunction with its pocket protein partners Rb, p107, and p130; however, v-Jun overrides this control, causing phosphorylated Rb and proliferation-specific E2F-p107 complexes to persist after mitogen withdrawal. Dephosphorylation of Rb and destruction of cyclin A nevertheless occur normally at mitosis, indicating that v-Jun enables cells to rephosphorylate Rb and reaccumulate cyclin A without exogenous mitogenic stimulation each time the mitotic "clock" is reset. D-cyclin-cdk activity is required for Rb phosphorylation in v-Jun-transformed cells, since ectopic expression of the cdk4- and cdk6-specific inhibitor p16(INK4A) inhibits both DNA synthesis and cell proliferation. Despite this, v-Jun does not stimulate D-cyclin-cdk activity but does induce a marked deregulation of cyclin E-cdk2. In particular, hormonal activation of a conditional v-Jun-estrogen receptor fusion protein in quiescent, growth factor-deprived cells stimulates cyclin E-cdk2 activity and triggers Rb phosphorylation and DNA synthesis. Thus, v-Jun overrides the mitogen dependence of S-phase entry by deregulating Rb phosphorylation, E2F-pocket protein interactions, and ultimately cyclin A-cdk2 activity. This is the first report, however, that cyclin E-cdk2, rather than D-cyclin-cdk, is likely to be the critical Rb kinase target of v-Jun.

Animals↗

PubMed1998

Analysis of the interaction between c-Jun and c-Jun N-terminal kinase in vivo.

Regulation of c-Jun transcriptional activity is believed to depend on a physical interaction with c-Jun N-terminal kinase (JNK) that facilitates signal-regulated phosphorylation of multiple regulatory phosphoacceptor sites within the activation domain. Here we have investigated the structural requirements and consequences of regulatory phosphorylation for the interaction between c-Jun and JNK in vivo. We show that binding of JNK to c-Jun in vivo does not require JNK catalytic activity or the presence of the potential phosphoacceptor sites within c-Jun and that JNK retains the capacity to bind to a pseudo-phosphorylated mutant of c-Jun where these sites are replaced by phospho-mimetic aspartic acid residues. The c-Jun delta region docking site is essential for interaction with JNK in vivo but is not sufficient, because a c-Jun mutant that retains this region but that lacks the C-terminal DNA-binding domain fails to interact. Experiments using purified recombinant c-Jun and JNK proteins show that the c-Jun DNA-binding domain harbors an auxiliary interaction domain that has the potential to bind to JNK independently. Our results suggest that JNK can be tethered passively to c-Jun in situ through multiple interacting regions and, when activated, can stimulate c-Jun phosphorylation without necessarily dissociating from its substrate. Auxiliary interactions mediated by the DNA-binding domain could play a role in targeting JNK preferentially to c-Jun in specific homo- or heterodimeric complexes.

Amino Acid Substitution↗

PubMed1998

Insulin-stimulated expression of c-fos, fra1 and c-jun accompanies the activation of the activator protein-1 (AP-1) transcriptional complex.

The activator protein-1 (AP-1) transcriptional complex is made up of members of the Fos (c-Fos, FosB, Fra1, Fra2) and Jun (c-Jun, JunB, JunD) families and is stimulated by insulin in several cell types. The mechanism by which insulin activates this complex is not well understood but it is dependent on the activation of the Erk1 and Erk2 isoforms of mitogen-activated protein kinases. In the current study we show that the AP-1 complex isolated from insulin-stimulated cells contained c-Fos, Fra1, c-Jun and JunB. The activation of the AP-1 complex by insulin was accompanied by (i) a transient increase in c-fos expression, and the transactivation of the ternary complex factors Elk1 and Sap1a, in an Erk1/Erk2-dependent fashion; (ii) a substantial increase in the expression of Fra1 protein and mRNA, which was preceded by a transient decrease in its electrophoretic mobility upon SDS/PAGE, indicative of phosphorylation; and (iii) a sustained increase in c-jun expression without increasing c-Jun phosphorylation on serines 63 and 73 or activation of the stress-activated kinase JNK/SAPK. In conclusion, insulin appears to stimulate the activity of the AP-1 complex primarily through a change in the abundance of the components of this complex, although there may be an additional role for Fra1 phosphorylation.

Animals↗

PubMed1998

An oncogenic mutation uncouples the v-Jun oncoprotein from positive regulation by the SAPK/JNK pathway in vivo.

Stimulation of c-Jun transcriptional activity via phosphorylation mediated by the stress-activated or c-Jun amino-terminal (SAPK/JNK) subgroup of mitogen-activated protein kinases (MAP kinases) is thought to depend on a kinase-docking site (the delta region) within the amino-terminal activation domain, which is deleted from the oncogenic derivative, v-Jun [1] [2] [3]. This mutation markedly enhances v-Jun oncogenicity [4] [5]; however, its transcriptional consequences have not been resolved. In part, this reflects uncertainty as to whether binding of SAPK/JNK inhibits c-Jun function directly [6] [7] or, alternatively, serves to facilitate and maintain the specificity of positive regulatory phosphorylation [8]. Using a two-hybrid approach, we show that SAPK/JNK stimulates c-Jun transactivation in yeast and that this depends on both catalytic activity and physical interaction between the kinase and its substrate. Furthermore, c-Jun is active when tethered to DNA via SAPK/JNK, demonstrating that kinase binding does not preclude transactivation. Taken together, these results suggest that SAPK/JNK acts primarily as a positive regulator of c-Jun transactivation in situ, and that loss of the docking site physically uncouples v-Jun from this control. This loss-of-function model accounts for the deficit of v-Jun regulatory phosphorylation and repression of TPA response element (TRE)-dependent transcription observed in v-Jun-transformed cells and predicts that an important property of the oncoprotein is to antagonise SAPK/JNK-dependent gene expression.

Animals↗

PubMed1998

v-Jun represses c-jun proto-oncogene expression in vivo through a 12-O-tetradecanoylphorbol-13-acetate-responsive element in the proximal gene promoter.

c-jun proto-oncogene expression is extinguished in cells transformed by v-Jun; however, the mechanistic basis of this phenomenon has not been elucidated. c-jun mRNA levels are greatly reduced in v-Jun-transformed cells, and we show that this reduction is associated with a similar decrease in the rate of c-jun transcription. Transcriptional down-regulation was also evident in functional assays in which the c-jun gene promoter was approximately 10-fold less active in v-Jun-transformed cells than it was in normal cells. This reduction was largely attributable to a conserved 12-O-tetradecanoylphorbol-13-acetate-responsive element (TRE)-like motif at position -72 (the proximal junTRE) that was essential for efficient basal expression in normal cells but that conferred little, if any, detectable transcriptional activity in v-Jun-transformed cells. DNA-binding analysis showed that this element was recognized by a mixture of c-Jun/Fra and cyclic AMP-responsive element-binding protein/activating transcription factor-like complexes in normal cells but that v-Jun/Fra heterodimers predominated in v-Jun-transformed cells. Furthermore, ectopic expression of v-Jun repressed c-jun promoter activity in normal cells through the proximal junTRE. Thus, the deficit in transcription mediated by the junTRE correlates with and is most likely attributable to binding of v-Jun to this element in vivo. We also find that the c-jun promoter is refractory to induction via the stress-activated protein kinase/c-jun NH2-terminal kinase pathway in v-Jun-transformed cells, suggesting that v-Jun interferes with signal-regulated gene expression. Therefore, c-jun is an example of a cellular gene, the transcription of which is regulated negatively by v-Jun in vivo.

Activating Transcription Factors↗

PubMed1997

Transformation by v-Jun prevents cell cycle exit and promotes apoptosis in the absence of serum growth factors.

To gain insight into the mechanism of action of the v-Jun oncoprotein, we compared the growth and cell cycle behavior of normal and v-Jun-transformed fibroblasts. We show that v-Jun induces marked alterations in cell cycle regulation in both the presence and absence of serum growth factors. During asynchronous growth, v-Jun-transformed fibroblasts divide more rapidly than their normal counterparts, owing to a reduction in the length of the G1 phase of the cell cycle. When deprived of serum mitogens, normal fibroblasts exit the cycle and enter a reversible state of quiescence (G0). In contrast, v-Jun-transformed fibroblasts continue to cycle and maintain increased levels of retinoblastoma tumor suppressor protein phosphorylation and elevated expression of cell cycle-dependent markers such as cyclin A, cyclin-dependent protein kinase 2 (CDK2), and CDC2. v-Jun-transformed fibroblasts nevertheless remain wholly dependent on growth factors for cell multiplication, because cell cycle progression in the absence of serum is accompanied by high rates of apoptotic cell death. We conclude that v-Jun shares the capacity of the Myc, E1A, and E2F oncoproteins to promote both cell cycle progression and apoptosis under conditions of mitogen depletion.

Adenovirus E1A Proteins↗

PubMed1996

Human ribosomal RNA variants from a single individual and their expression in different tissues.

We have investigated the extent of sequence variation in human ribosomal RNA (rRNA) genes and the expression of specific rRNA gene variants in different tissues of an individual. Focusing on the fifth variable region (V5; nt 2065-2244) of the 28S rRNA gene, we find that sequence differences between rRNA genes of a single individual are characterized by differences in number of repeats of simple sequences at four specific sites. These data support and extend previous findings which show similar V5 sequence variation in rRNA genes from a group of individuals. We performed experiments to determine if there is differential gene expression within the rRNA multigene family. From the analysis of data of six variant V5 probes protected from RNase digestion by rRNAs isolated from different tissues of the individual, we conclude that each variant rRNA is present in a similar proportion in these tissues, whereas the actual contributions of variants differ, their relative proportion is maintained from tissue to tissue in an individual. We favor the explanation of a gene dosage effect over that of a regulated gene effect to account for this pattern of rRNA gene expression. In addition, computer generated secondary structure models of each V5 clone structure predict the same three helix structure with the regions of sequence variation contained in one stem-loop structure.

Animals↗

PubMed1996

The v-Jun oncoprotein replaces p39 c-Jun as the predominant AP-1 constituent in ASV17-transformed fibroblasts: implications for SAPK/JNK-mediated signal transduction.

We have investigated the expression of Jun family proteins and composition of AP-1 in chicken embryo fibroblasts before and after transformation by the v-Jun oncoprotein of ASV17. We show that p39 c-Jun is the predominant Jun family protein expressed in normal fibroblasts, and that heterodimers of c-Jun and Fos-related partners (Fra's) account for the majority of the AP-1 DNA binding activity. Unexpectedly, because ASV17-transformed fibroblasts do not express p39 c-Jun, v-Jun replaces c-Jun as the predominant AP-1 constituent in association with similar or identical Fra's. This substitution has little effect on the overall level of TRE-specific DNA binding activity, however it results in a profound reduction in TRE-dependent transcriptional activity and a striking defect in signal-regulated phosphorylation of the Jun component of AP-1; whilst agonists of SAPK/JNK kinases trigger transient N-terminal phosphorylation of c-Jun in normal fibroblasts, no corresponding modification of v-Jun occurs in ASV17-transformed cells. Because SAPK/JNK-mediated phosphorylation is thought to regulate c-Jun transcriptional activity and thereby cellular gene expression in response to extracellular signals, we propose that subversion of this signal transduction process by v-Jun is likely to contribute to oncogenesis by ASV17.

3T3 Cells↗

PubMed1995

Stress-activated protein kinases bind directly to the delta domain of c-Jun in resting cells: implications for repression of c-Jun function.

The transactivating function of the c-Jun proto-oncogene component of the AP-1 transcription factor is acutely regulated by a wide variety of cellular signals via modulation of phosphorylation of two serines (63 and 73). The viral oncoprotein, v-Jun, while containing homologous serines, is not phosphorylated in cells. A novel family of stress-activated protein kinases (SAPKs), also termed Jun N-terminal domain kinases (JNKs), are responsible for mediating S63/73 phosphorylation in response to a variety of cellular stimuli including tumor necrosis factor-alpha, heat stress and u.v. light. The p54 alpha 1, alpha 2, p54 beta and p46 beta SAPKs are shown to bind directly to c-Jun but not to v-Jun, with an absolute requirement for c-Jun amino acids 31-47, a region deleted in v-Jun. Inactive SAPKs tightly bind c-Jun in resting cells and may be a manifestation of the 'delta' inhibitor, a previously described repressor of c-Jun function.

Calcium-Calmodulin-Dependent Protein Kinases↗

PubMed1995

Differential regulation of AP-1 and novel TRE-specific DNA-binding complexes during differentiation of oligodendrocyte-type-2-astrocyte (O-2A) progenitor cells.

AP-1 is an ubiquitous transcription factor which is composed of the Jun and Fos proto-oncogene proteins and is thought to play a role in both cell proliferation and differentiation. We have used an immortal, bipotential oligodendrocyte-type-2 astrocyte progenitor cell line (O-2A/c-myc) which can differentiate into oligodendrocytes or type-2 astrocytes in vitro, to investigate whether AP-1 DNA-binding activity fluctuates during glial cell differentiation. Unexpectedly, DNA-mobility shift assays using a TRE-containing oligonucleotide derived from the promoter of the glial-specific gene, glial fibrillary acidic protein (GFAP/AP-1), revealed that O-2A/c-myc progenitor cells were devoid of conventional AP-1 DNA-binding complexes. O-2A/c-myc cells did however contain several novel GFAP/AP-1-specific DNA-binding complexes, which we have termed APprog. APprog complexes recognise the TRE consensus motif present in the GFAP/AP-1 oligonucleotide together with adjacent 3' sequences but do not contain c-Jun or any other known Jun-related proteins. When O-2A/c-myc cells underwent terminal differentiation APprog complexes were lost and conventional AP-1 DNA-binding activity became evident, particularly in astrocytes. These changes appear to be closely linked to the differentiation process since they did not occur in a derivative of the O-2A/c-myc cell line that contains an activated v-ras oncogene and which fails to differentiate under appropriate culture conditions. The inverse regulation of conventional AP-1 and APprog complexes within the O-2A lineage suggests that these factors may play a role in the regulation of glial cell differentiation or glial cell-specific gene expression.

Animals↗

PubMed1995

Mitogenesis by v-Src: fluctuations throughout G1 of classical immediate early AP-1 and mitogen-activated protein kinase responses that parallel the need for the oncoprotein.

Activation of the tyrosine kinase of a temperature-sensitive mutant v-Src oncoprotein in quiescent Rat-1 cells leads to passage through the cell cycle. Temperature shift experiments show that v-Src is needed to leave G0, to pass a relatively stable G1 "pause" point, and to pass a later G1 point committing cells to S phase. Classic immediate early responses that activate both AP-1 DNA binding and mitogen-activated protein (MAP) kinase are induced at G0 exit, but unexpectedly they rise again in mid-G1 and before the onset of S phase, fluctuations that parallel the need for v-Src. An estrogen-inducible mutant c-Raf-1 renders these cells susceptible to mitogenic stimulation by beta-estradiol, without v-Src activity, but greatly inhibits the ability of v-Src to induce DNA synthesis and MAP kinase, probably because v-Src physically associates with inactive c-Raf-1 at permissive but not restrictive temperature. This implicates c-Raf-1 association with enzymically active v-Src and consequent activation of the MAP kinase pathway in v-Src mitogenesis. Furthermore, temperature shift experiments indicate that the mid-G1 peak of MAP kinase activity is associated with cells reaching the G1 pause point, while the pre-S phase peak is needed for DNA synthesis. In contrast, cell transformation by v-Src does not require enhanced MAP kinase activity at any stage of the cell cycle.

Animals↗

PubMed1994

c-Myc inhibits myogenic differentiation and myoD expression by a mechanism which can be dissociated from cell transformation.

The c-Myc oncoprotein is a basic-helix-loop-helix-leucine zipper (b-HLH-LZ) transcription factor involved in regulating cell proliferation and differentiation. We have used retrovirus-mediated gene transfer to investigate the effect of ectopic c-Myc expression on the spontaneous differentiation of primary quail myoblasts in vitro. Unlike normal myoblasts, c-Myc-expressing myoblasts are unable to form myotubes or express muscle-specific genes, such as myosin, and show severely reduced expression of the myogenic regulatory factors myoD, myogenin, and myf5. The c-Myc leucine zipper (LZ) motif is essential for the differentiation block since myoblasts expressing a mutant with a partial deletion of this region, c-Myc delta 7, differentiate and express myoD family regulators and muscle-specific genes normally. Remarkably, c-Myc delta 7, like wild-type c-Myc, retains the capacity to transform the growth phenotype of myoblasts, and associates with the b-HLH-LZ Myc partner protein Max in transformed cells. We conclude that the block to myogenic differentiation induced by c-Myc can be dissociated from cell transformation per se, and that this attribute correlates more closely with down-regulation of myoD family gene expression. These findings are discussed in the light of current models of Myc function.

Amino Acid Sequence↗

PubMed1994

Transcriptional activation by the v-Jun oncoprotein is independent of positive regulatory phosphorylation.

Growth factors, phorbol esters, and oncogenes such as ras, src, and sis are believed to stimulate c-Jun transcriptional activation by inducing increased phosphorylation at two serine residues (S63 and S73) within the N-terminal transactivation domain. Although S63 and S73 are conserved in the mutant v-Jun oncoprotein, they are not phosphorylated by two enzymes which target the corresponding residues in c-Jun in vitro; namely a partially purified c-Jun kinase from TPA-stimulated U937 cells and purified p54 mitogen activated protein (MAP) kinase. In addition, v-Jun activates transcription more strongly than c-Jun when fused to the Gal4 DNA-binding domain, and transcriptional activation by Gal4-v-Jun is unaffected when S63, S73, or both, are replaced with non-phosphorylatable alanine residues, amino acid substitutions which severely impair transcriptional activation by Gal4-c-Jun. The novel biochemical and transcriptional properties of v-Jun result from deletion of a 27 amino acid segment, termed delta, which is important for transforming activity. On the basis of these results we propose that unlike c-Jun, v-Jun transcriptional activation is independent of positive regulatory phosphorylation and that this may contribute to oncogenesis by v-Jun.

Animals↗

PubMed1993

Transcription activation by Myc and Max: flanking sequences target activation to a subset of CACGTG motifs in vivo.

The Myc oncoprotein has been implicated in control of cell growth, division and differentiation. Although Myc contains a bHLH-LZ motif, it fails to bind DNA alone but can do so by forming heterodimers with an unrelated bHLH-LZ protein, Max. Max homodimers and Myc-Max heterodimers share the ability to bind CACGTG or CATGTG elements. Current models, based on experimentally induced overexpression of Myc and Max in mammalian cells, propose that Max-Max homodimers repress while Myc-Max heterodimers activate transcription through CACGTG binding sites. The interpretation of the results using mammalian cells is complicated by the presence of numerous unrelated CACGTG binding transcription activators and the existence of two alternative Max dimerization partners, Mad and Mxi-1. Thus, the mechanism whereby overexpression of Max leads to transcriptional repression remains to be established. Using a yeast system we show that Max homodimers have the potential to activate transcription through CACGTG motifs. Activation by Max requires DNA binding and amino acids outside the bHLH-LZ domain but is reduced compared with activation by Myc-Max heterodimers. Moreover, transcriptional activation by Myc-Max heterodimers, but not Max-Max homodimers, is strongly inhibited in vivo by specific sequences flanking the core CACGTG binding motif, presumably reflecting reduced DNA binding affinity. These results suggest a mechanism for directing the Myc-Max complex to a specific subset of CACGTG-containing target genes.

Amino Acid Sequence↗