extradenticle, a regulator of homeotic gene activity, is a homolog of the homeobox-containing human proto-oncogene pbx1

Genetic evidence for the subdivision of the arthropod limb into coxopodite and telopodite

Arthropod appendages are thought to have evolved as outgrowths from the body wall of a limbless ancestor. Snodgrass, in his Principles of Insect Morphology (1935), proposed that, during evolution, expansion of the body wall would originate the base of the appendages, or coxopodite, upon which the most distal elements that represent the true outer limb, or telopodite, would develop. The homeobox gene Distal-less (Dll), which is required in the Drosophila appendages for development of distal regions, has been proposed to promote formation of telopodite structures above the evolutionary ground-state of non-limb or body wall. Here, we present evidence that another homeobox gene, extradenticle (exd), which is required for appropriate development of the trunk and the proximal parts of the appendages, represents a coxopodite gene. We show that exd function is eliminated from the distal precursors in the developing limb and remains restricted to proximal precursors throughout development. This elimination is important because, when ectopically expressed, exd prevents distal development and gives rise to truncated appendages lacking distal elements. Moreover, the maintenance of exd expression during larval stages, contrary to Dll, does not require the hedgehog (hh) signaling pathway, suggesting that the proximal regions of the appendages develop independently of hh function. Finally, we show that in the crustacean Artemia, exd and Dll are expressed in comparable patterns as in Drosophila, suggesting a conserved genetic mechanism subdividing the arthropod limb.

Paralogous Hox genes: function and regulation

The Hox homeobox gene family plays a pivotal role in regulating patterning and axial morphogenesis in vertebrates. Molecular characterization of the four Hox clusters has shown that they are evolutionarily related with respect to sequence, organization, and expression, suggesting they arose by duplication and divergence. Transgenic analysis has clearly demonstrated the functional roles of individual genes in a broad range of embryonic tissues, and in compound mutants has addressed the issues of cooperativity and redundancy. There is an emerging picture of the cis-regulatory elements underlying Hox expression, and for the 3' members of the clusters there is a considerable degree of conservation between paralogous genes with respect to their functional roles and regulatory control.

Nuclear import of the homeodomain protein extradenticle in response to Wg and Dpp signalling

In Drosophila, Decapentaplegic (Dpp) and Wingless (Wg) are two secreted signalling proteins of the transforming growth factor (TGF)-beta and Wnt families, respectively. Although both are often required during development, only a few downstream components of these signalling pathways have been described. Here we present evidence that in the embryonic midgut both signalling pathways control the subcellular localization of the homeodomain protein encoded by the extradenticle (exd) gene. Exd protein is predominantly nuclear in endoderm cells close to Dpp-and Wg-secreting cells of the visceral mesoderm, but is in the cytoplasm in more distant endoderm cells. Both dpp and wg are required for the nuclear localization of Exd in the endoderm, whereas ectopic expression of dpp and wg expands the domain of nuclear Exd. Furthermore, the nuclear import of Exd correlates with the transcription of an exd-dependent reporter gene in the endoderm. Thus one mechanism by which extracellular signals might control pattern is by directing the graded nuclear localization of homeodomain proteins such as Exd that directly control the expression of target genes.

Transcriptional control of Hox genes in the vertebrate nervous system

The identification, in transgenic mice, of Hox gene DNA regulatory elements that can recapitulate certain aspects of the endogenous gene expression pattern has proceeded with great success. Perfect reproduction of the correct expression pattern, however, is uncommon, even when large genomic fragments spanning neighboring genes are analyzed, suggesting that important regulatory regions may be located at large distances from the genes they control or that their specific context may be important. Four classes of transcriptional regulators have been identified recently that have been shown to directly regulate Hox gene expression in the murine nervous system: retinoic acid receptors, Krox20, the Pbx/exd family, and the Hox genes themselves.

The Drosophila teashirt homeotic protein is a DNA-binding protein and modulo, a HOM-C regulated modifier of variegation, is a likely candidate for being a direct target gene

The Drosophila teashirt (tsh) gene has an homeotic function which, in combination with HOM-C genes, determines thoracic and abdominal (trunk) identities. Analysis of TSH protein distribution during embryogenesis using a specific polyclonal antibody shows that it is nuclear. The protein is present with regional modulation in several tissues within the trunk, suggesting additional tsh functions to those already studied. We identified a candidate tsh target shared with some HOM-C genes, the modifier of variegation gene modulo (mod). The TSH zinc-finger protein recognizes in vitro two specific sites within a 5' control element of the mod gene which responds in vivo to tsh activity. TSH is therefore a DNA binding protein and might directly control mod expression.

Extra specificity from extradenticle: the partnership between HOX and PBX/EXD homeodomain proteins

For many DNA-binding transcription factors it is often difficult to reconcile their highly specific in vivo functions with their less specific in vitro DNA-binding properties. Cooperative DNA binding with cofactors often provides part of the answer to this paradox and recent studies have demonstrated this to be the case for the homeotic complex (HOX) family of transcription factors. However, the unique problem posed by these highly related and developmentally important transcription factors requires additional twists to the standard solution, which are beginning to become apparent from the characterization of the HOX cofactors encoded by the extradenticle and PBX genes.

The divergent homeobox gene PBX1 is expressed in the postnatal subventricular zone and interneurons of the olfactory bulb

In the mammalian brain, an important phase of neurogenesis occurs postnatally in the subventricular zone (SVZ). This region consists of a heterogeneous population of cells, some mitotically active, others postmitotic. A subset of mitotically active SVZ precursor cells gives rise to a population of neurons that migrates over a long distance to their final destination, the olfactory bulb. Other SVZ precursor cells continue to proliferate or undergo cell death. The combination of genes that regulates proliferation and cell fate determination of SVZ precursor cells remains to be identified. We have used the rat homolog of the human homeobox gene PBX1 in Northern analysis and in situ hybridization studies to determine the temporal and regional localization of PBX1 expression during embryonic and postnatal rat brain development. PBX1 is expressed embryonically in the telencephalon. In addition, it is expressed at high levels postnatally in the SVZ, in the migratory pathway to the olfactory bulb, and in the layers of the olfactory bulb that are the targets of these migratory neurons. Combining in situ hybridization for PBX1 with immunostaining for markers of cell proliferation (PCNA), postmitotic neurons (class III beta-tubulin), and glia (GFAP), we show that SVZ proliferating cells and their neuronal progeny express rat PBX1 mRNA, whereas glial cells do not express detectable levels of PBX1. The expression of PBX1 in SVZ precursor cells and postmitotic neurons suggests a role for PBX1 in the generation of olfactory bulb interneurons and in mammalian neurogenesis.

Pbx-1 Hox heterodimers bind DNA on inseparable half-sites that permit intrinsic DNA binding specificity of the Hox partner at nucleotides 3' to a TAAT motif

Heterodimers between the Pbx/Exd and Hox/HOM-C classes of homeodomain proteins bind regulatory elements in tissue-specific and developmentally regulated genes. In this work, we characterize the half-site bound by both Pbx1 and Hox proteins on a prototypic element (TGATTAAT) and determine how the orientation of the Hox protein contributes to the DNA binding specificity of Pbx-Hox heterodimers. We demonstrate that the Hox protein binds the 3' TAAT sequence as its recognition core and exhibits sequence-specific binding at positions 3' to the TAAT core. Unfavored sequences at this position, such as two cytosines, abrogate binding to the element. The upstream Pbx1 core sequence, TGAT, must immediately juxtapose the Hox core. This geometry maintains the preference of Hox/HOM-C proteins for a T base at position -1, as T represents the fourth position of the Pbx1 core, and suggests that this T base is bound by both Pbx1 and Hox proteins, Pbx1 binding in the major grove and the Hox protein binding in the minor grove. Pbx1 also exhibits base selectivity 5' to its TGAT recognition sequence.

The yeast alpha2 and Mcm1 proteins interact through a region similar to a motif found in homeodomain proteins of higher eukaryotes

Homeodomain proteins are transcriptional regulatory factors that, in general, bind DNA with relatively low sequence specificity and affinity. One mechanism homeodomain proteins use to increase their biological specificity is through interactions with other DNA-binding proteins. We have examined how the yeast (Saccharomyces cerevisiae) homeodomain protein alpha2 specifically interacts with Mcm1, a MADS box protein, to bind DNA specifically and repress transcription. A patch of predominantly hydrophobic residues within a region preceding the homeodomain of alpha2 has been identified that specifies direct interaction with Mcm1 in the absence of DNA. This hydrophobic patch is required for cooperative DNA binding with Mcm1 in vitro and for transcriptional repression in vivo. We have also found that a conserved motif, termed YPWM, frequently found in homeodomain proteins of insects and mammals, partially functions in place of the patch in alpha2 to interact with Mcm1. These findings suggest that homeodomain proteins from diverse organisms may use analogous interaction motifs to associate with other proteins to achieve high levels of DNA binding affinity and specificity.

Stage, tissue, and cell specific distribution of alternative Ultrabithorax mRNAs and protein isoforms in the Drosophila embryo

The homeotic gene Ultrabithorax encodes a family of six homeoproteins translated from alternatively spliced mRNAs. The structures of these UBX isoforms have been conserved among anciently diverged Drosoph-ila species and functional distinctions between some isoforms have been reported that suggest subtle but important roles in Ubx action. We present a detailed analysis of the expression patterns of Ubx mRNAs and proteins during embryogenesis, using isoform-specific monoclonal antibodies and synthetic oligonucleotide probes. These patterns are remarkably complex, each mRNA and corresponding protein isoform being expressed in a partially overlapping but distinct stage and tissue-specific pattern. The complexity is greatest in the central nervous system, where different isoforms predominate during successive developmental stages and where their relative proportions differ from one metamere to another and even among individual neurons within a given metamere. The distributions of UBX isoforms are consistent with those functional distinctions that have been described; they also suggest that different isoforms may be specialized or optimized to control different aspects of central nervous system development. The close correspondence between the mRNA and protein patterns indicates that the mRNAs do not differ strongly in translatability, despite the abundance of rare codons in the optional exons. There is a delay between the detection of particular splicing events in the nucleus and the detection of the 3' end of the message or the appearance of the corresponding mRNAs and proteins in the cytoplasm. This delay is consistent with the size of the Ubx introns and indicates a cotranscriptional mechanism of splicing.

Mesoderm-specific expression of the divergent homeobox gene Hlx during murine embryogenesis

We have determined the expression pattern of the divergent homeobox gene Hlx during post-implantation mouse development, utilizing in situ hybridization. Expression was mesoderm-specific and occurred in a complex tissue distribution. Transcripts were first detected at 9.5 days post coitum (p.c.) in splanchnic mesoderm of the midgut and hindgut region, then during organogenesis, prominently in mesenchyme of the developing liver, gall bladder, and intestines, as well as their mesenteric tissues. In the foregut, lung mesenchyme became positive from 10.5 days p.c. Hlx transcripts were also detected in a subset of skeletal myogenic cells: those within branchial arches from 9.5 days p.c. and within limb buds from 12 days p.c. Hlx was not expressed in myogenic cells which are derived from the myotome and populate the trunk. However, from 10 days p.c., expression was seen in a region of the sclerotome immediately adjacent to the myotome and corresponding to precursors of the ribs and vertebral neural arches. In the anterior-posterior aspect of the developing sclerotome, Hlx expression was out of register with original segmental boundaries (intersomitic fissures), a pattern consistent with a classical hypothesis that the developing vertebral column undergoes resegmentation. Hlx expression was also observed in vibrissae, pericardium, snout mesenchyme, and meningeal epithelium. Overall, expression of Hlx in only a subset of individual lineage progenitors and at know sites of inductive tissue interactions, suggests that the gene regulates local patterning or growth through cell:cell signalling at those embryonic sites.

Pbx modulation of Hox homeodomain amino-terminal arms establishes different DNA-binding specificities across the Hox locus

Pbx cofactors are implicated to play important roles in modulating the DNA-binding properties of heterologous homeodomain proteins, including class I Hox proteins. To assess how Pbx proteins influence Hox DNA-binding specificity, we used a binding-site selection approach to determine high-affinity target sites recognized by various Pbx-Hox homeoprotein complexes. Pbx-Hox heterodimers preferred to bind a bipartite sequence 5'-ATGATTNATNN-3' consisting of two adjacent half sites in which the Pbx component of the heterodimer contacted the 5' half (ATGAT) and the Hox component contacted the more variable 3' half (TNATNN). Binding sites matching the consensus were also obtained for Pbx1 complexed with HoxA10, which lacks a hexapeptide but requires a conserved tryptophan-containing motif for cooperativity with Pbx. Interactions with Pbx were found to play an essential role in modulating Hox homeodomain amino-terminal arm contact with DNA in the core of the Hox half site such that heterodimers of different compositions could distinguish single nucleotide alterations in the Hox half site both in vitro and in cellular assays measuring transactivation. When complexed with Pbx, Hox proteins B1 through B9 and A10 showed stepwise differences in their preferences for nucleotides in the Hox half site core (TTAT to TGAT, 5' to 3') that correlated with the locations of their respective genes in the Hox cluster. These observations demonstrate previously undetected DNA-binding specificity for the amino-terminal arm of the Hox homeodomain and suggest that different binding activities of Pbx-Hox complexes are at least part of the position-specific activities of the Hox genes.

Structural determinants within Pbx1 that mediate cooperative DNA binding with pentapeptide-containing Hox proteins: proposal for a model of a Pbx1-Hox-DNA complex

Genetic studies have identified a family of divergent homeodomain proteins, including the human protooncoprotein Pbx1 and its drosophila homolog extradenticle (Exd), which function as cofactors with a subset of Hox and HOM-C proteins, and are essential for specific target gene expression. Pbx1/Exd binds DNA elements cooperatively with a large subset of Hox/HOM-C proteins containing a conserved pentapeptide motif, usually YPWMR, located just N terminally to their homeodomains. The pentapeptide is essential for cooperative DNA binding with Pbx1. In this study, we identify structural determinants of Pbx1 that are required for cooperative DNA binding with the pentapeptide-containing Hox protein HoxA5. We demonstrate that the homeodomain of Pbx1 contains a surface that binds the pentapeptide motif and that the Pbx1 homeodomain is sufficient for cooperative DNA binding with a Hox protein. A sequence immediately C terminal to the Pbx1 homeodomain, which is highly conserved in Pbx2 and Pbx3 and predicted to form an alpha-helix, enhances monomeric DNA binding by Pbx1 and also contributes to maximal cooperativity with Hox proteins. Binding studies with chimeric HoxA5-Pbx1 fusion proteins suggest that the homeodomains of Pbx1 and HoxA5 are docked on the representative element, TTGATTGAT, in tandem, with Pbx1 recognizing the 5' TTGAT core motif and the Hox protein recognizing the 3' TGAT core. The proposed binding orientation permits Hox proteins to exhibit further binding specificity on the basis of the identity of the four residues 3' to their core binding motif.

Hox homeodomain proteins exhibit selective complex stabilities with Pbx and DNA

Eight of the nine homeobox genes of the Hoxb locus encode proteins which contain a conserved hexapeptide motif upstream from the homeodomain. All eight proteins (Hoxb-1-Hoxb-8) bind to a target oligonucleotide in the presence of Pbx1a under conditions where minimal or no binding is detected for the Hox or Pbx1a proteins alone. The stabilities of the Hox-Pbx1a-DNA complexes vary >100-fold, with the proteins from the middle of the locus (Hoxb-5 and Hoxb-6) forming very stable complexes, while Hoxb-4, Hoxb-7 and Hoxb-8 form complexes of intermediate stability and proteins at the 3'-side of the locus (Hoxb-1-Hoxb-3) form complexes which are very unstable. Although Hox-b proteins containing longer linker sequences between the hexapeptide and homeodomains formed unstable complexes, shortening the linker did not confer complex stability. Homeodomain swapping experiments revealed that this motif does not independently determine complex stability. Naturally occurring variations within the hexapeptides of specific Hox proteins also do not explain complex stability differences. However, two core amino acids (tryptophan and methionine) which are absolutely conserved within the hexapeptide domains appear to be required for complex formation. Removal of N- and C-terminal flanking regions did not influence complex stability and the members of paralog group 4 (Hoxa-4, b-4, c-4 and d-4), which share highly conserved hexapeptides, linkers and homeodomains but different flanking regions, form complexes of similar stability. These data suggest that the structural features of Hox proteins which determine Hox-Pbx1a-DNA complex stability reside within the precise structural relationships between the homeodomain, hexapeptide and linker regions.

Selective repression of transcriptional activators by Pbx1 does not require the homeodomain

PBX1 is a homeobox-containing gene identified as the chromosome 1 participant of the t(1;19) chromosomal translocation of childhood pre-B-cell acute lymphoblastic leukemia. This translocation produces a fusion gene encoding the chimeric oncoprotein E2A-Pbx1, which can induce both acute myeloid and T-lymphoid leukemia in mice. The binding of Pbx1 to DNA is weak; however, both Pbx1 and E2A-Pbx1 exhibit tight binding to specific DNA motifs in conjunction with certain other homeodomain proteins, and E2A-Pbx1 activates transcription through these motifs, whereas Pbx1 does not. In this report, we investigate potential transcriptional functions of Pbx1, using transient expression assays. While no segments of Pbx1 activated transcription, an internal domain of Pbx1 repressed transcription induced by the activation domain of Sp1, but not by the activation domains of VP16 or p53. This Pbx1 domain, which lies upstream of the homeodomain and is highly conserved among Pbx proteins, is thus predicted to bind a specific transcription factor. Surprisingly, the repression activity of Pbx1 did not require homeodomain-dependent DNA binding. Thus, Pbx1 may be able to alter gene transcription by both DNA-binding-dependent and DNA-binding-independent mechanisms.

The pancreatic islet factor STF-1 binds cooperatively with Pbx to a regulatory element in the somatostatin promoter: importance of the FPWMK motif and of the homeodomain

A number of homeodomain proteins have been shown to regulate cellular development by stimulating the transcription of specific target genes. In contrast to their distinct activities in vivo, however, most homeodomain proteins bind indiscriminately to potential target sites in vitro, suggesting the involvement of cofactors which specify target site selection. One such cofactor, termed extradenticle, has been shown to influence segmental morphogenesis in Drosophila melanogaster by binding cooperatively with certain homeodomain proteins to target regulatory elements. Here we demonstrate that STF-1, an orphan homeodomain protein required for pancreatic development in mammals, binds cooperatively to DNA with Pbx, the mammalian homolog of extradenticle. Cooperative binding with Pbx requires a pentapeptide motif (FPWMK) which is well conserved among a large subset of homeodomain proteins. The FPMWK motif is not sufficient to confer Pbx cooperativity on other homeodomain proteins, however; the N-terminal arm of the STF-1 homeodomain is also essential. As cooperative binding with Pbx occurs on only a subset of potential STF-1 target sites, our results suggest that Pbx may specify target gene selection in the developing pancreas by forming heterodimeric complexes with STF-1.

extradenticle determines segmental identities throughout Drosophila development

extradenticle (exd) and the homeotic selector proteins together establish segmental identities by coordinately regulating the expression of downstream target genes. The inappropriate expression of these targets in exd mutant embryos results in homeotic transformations and aberrant morphogenesis. Here we examine the role of exd in adult development by using genetic mosaics and a hypomorphic exd allele caused by a point mutation in the homeodomain. exd continues to be essential for the specification of segmental identities, consistent with a continuing requirement for exd as cofactor of the homeotic selector proteins. Loss of exd results in the homeotic transformation of abdominal segments to an A5 or A6 segmental identity, the antenna and arista to leg, and the head capsule to dorsal thorax or notum. Proximal leg structures are particularly sensitive to the loss of exd, although exd does not affect the allocation of proximal positional values of the leg imaginal disc. Using heat-shocks to induce expression of a hsp70-exd fusion gene, we show that, in contrast to the homeotic selector genes, ubiquitously high levels of exd expression do not cause pattern abnormalities or segmental transformations.

Crystal structure of the MATa1/MAT alpha 2 homeodomain heterodimer bound to DNA

The Saccharomyces cerevisiae MATa1 and MAT alpha 2 homeodomain proteins, which play a role in determining yeast cell type, form a heterodimer that binds DNA and represses transcription in a cell type-specific manner. Whereas the alpha 2 and a1 proteins on their own have only modest affinity for DNA, the a1/alpha 2 heterodimer binds DNA with high specificity and affinity. The three-dimensional crystal structure of the a1/alpha 2 homeodomain heterodimer bound to DNA was determined at a resolution of 2.5 A. The a1 and alpha 2 homeodomains bind in a head-to-tail orientation, with heterodimer contacts mediated by a 16-residue tail located carboxyl-terminal to the alpha 2 homeodomain. This tail becomes ordered in the presence of a1, part of it forming a short amphipathic helix that packs against the a1 homeodomain between helices 1 and 2. A pronounced 60 degree bend is induced in the DNA, which makes possible protein-protein and protein-DNA contacts that could not take place in a straight DNA fragment. Complex formation mediated by flexible protein-recognition peptides attached to stably folded DNA binding domains may prove to be a general feature of the architecture of other classes of eukaryotic transcriptional regulators.

Intestinal epithelial cell differentiation: new insights from mice, flies and nematodes

Decisions commonly made during development that affect proliferation, cell fate specification, differentiation, migration, and death are made repeatedly in the mouse small intestinal epithelium throughout adulthood. The results of these decisions are a stratification of proliferation, differentiation, and death along the mouse small intestine's crypt/villus axis. Recent genetic studies in Caenorhabditis elegans and Drosophila melanogaster have identified factors involved in determining cell fate and differentiation in gut endoderm. The stem cell hierarchy of the adult mouse intestinal epithelium makes it ideally suited for using chimeric animals to examine the functions of homologs of these lower eukaryotic (and other) proteins.

The specificity of homeotic gene function

How transcription factors achieve their in vivo specificities is a fundamental question in biology. For the Homeotic Complex (HOM/Hox) family of homeoproteins, specificity in vivo is likely to be in part determined by subtle differences in the DNA binding properties inherent in these proteins. Some of these differences in DNA binding are due to sequence differences in the N-terminal arms of HOM/Hox homeodomains. Evidence also exists to suggest that cofactors can modify HOM/Hox function by cooperative DNA binding interactions. The Drosophila homeoprotein extradenticle (exd) is likely to be one such cofactor. In HOM/Hox proteins, both the conserved 'YPWM' peptide motif and the homeodomain are important for interacting with exd. Although exd provides part of the answer as to how specificity is achieved, there may be additional cofactors and mechanisms that have yet to be identified.

Meis1, a PBX1-related homeobox gene involved in myeloid leukemia in BXH-2 mice

Leukemia results from the accumulation of multiple genetic alterations that disrupt the control mechanisms of normal growth and differentiation. The use of inbred mouse strains that develop leukemia has greatly facilitated the identification of genes that contribute to the neoplastic transformation of hematopoietic cells. BXH-2 mice develop myeloid leukemia as a result of the expression of an ecotropic murine leukemia virus that acts as an insertional mutagen to alter the expression of cellular proto-oncogenes. We report the isolation of a new locus, Meis1, that serves as a site of viral integration in 15% of the tumors arising in BXH-2 mice. Meis1 was mapped to a distinct location on proximal mouse chromosome 11, suggesting that it represents a novel locus. Analysis of somatic cell hybrids segregating human chromosomes allowed localization of MEIS1 to human chromosome 2p23-p12, in a region known to contain translocations found in human leukemias. Northern (RNA) blot analysis demonstrated that a Meis1 probe detected a 3.8-kb mRNA present in all BXH-2 tumors, whereas tumors containing integrations at the Meis1 locus expressed an additional truncated transcript. A Meis1 cDNA clone that encoded a novel member of the homeobox gene family was identified. The homeodomain of Meis1 is most closely related to those of the PBX/exd family of homeobox protein-encoding genes, suggesting that Meis1 functions in a similar fashion by cooperative binding to a distinct subset of HOX proteins. Collectively, these results indicate that altered expression of the homeobox gene Meis1 may be one of the events that lead to tumor formation in BXH-2 mice.

The hexapeptide LFPWMR in Hoxb-8 is required for cooperative DNA binding with Pbx1 and Pbx2 proteins

The Hox gene products are DNA-binding proteins, containing a homeodomain, which function as a class of master control proteins establishing the body plan in organisms as diverse as Drosophila and vertebrates. Hox proteins have recently been shown to bind cooperatively to DNA with another class of homeodomain proteins that include extradenticle, Pbx1, and Pbx2. Hox gene products contain a highly conserved hexapeptide connected by a linker of variable length to the homeodomain. We show that the hexapeptide and the linker region are required for cooperativity with Pbx1 and Pbx2 proteins. Many of the conserved residues present in the Hoxb-8 hexapeptide are required to modulate the DNA binding of the Pbx proteins. Position of the hexapeptide relative to the homeodomain is important. Although deletions of two and four residues of the linker peptide still show cooperative DNA binding, removal of all six linker residues strongly reduces cooperativity. In addition, an insertion of 10 residues within the linker peptide significantly lowers cooperative DNA binding. These results show that the hexapeptide and the position of the hexapeptide relative to the homeodomain are important determinants to allow cooperative DNA binding involving Hox and Pbx gene products.

How the Hox gene Ultrabithorax specifies two different segments: the significance of spatial and temporal regulation within metameres

In Drosophila, the Hox gene Ultrabithorax (Ubx) specifies the development of two different metameres--parasegment 5, which is entirely thoracic, and parasegment 6, which includes most of the first abdominal segment. Here we investigate how a single Hox gene can specify two such different morphologies. We show that, in the early embryo, cells respond similarly to UBX protein in both parasegments. The differences between parasegments 5 and 6 can be explained by the different spatial and temporal pattern of UBX protein expression in these two metameres. We find no evidence for multiple threshold responses to different levels of UBX protein. We examine in particular the role of Ubx in limb development. We show that UBX protein will repress limb primordia before 7 hours, when Ubx is expressed in the abdomen, but not later, when UBX is first expressed in the T3 limb primordium. The regulation of one downstream target of UBX, the Distalless gene, provides a model for this transition at the molecular level.

Homeotic response elements are tightly linked to tissue-specific elements in a transcriptional enhancer of the teashirt gene

Along the anterior-posterior axis of animal embryos, the choice of cell fates, and the organization of morphogenesis, is regulated by transcription factors encoded by clustered homeotic or ‘Hox’ genes. Hox genes function in both epidermis and internal tissues by regulating the transcription of target genes in a position- and tissue-specific manner. Hox proteins can have distinct targets in different tissues; the mechanisms underlying tissue and homeotic protein specificity are unknown. Light may be shed by studying the organization of target gene enhancers. In flies, one of the target genes is teashirt (tsh), which encodes a zinc finger protein. tsh itself is a homeotic gene that controls trunk versus head development. We identified a tsh gene enhancer that is differentially activated by Hox proteins in epidermis and mesoderm. Sites where Antennapedia (Antp) and Ultrabithorax (Ubx) proteins bind in vitro were mapped within evolutionarily conserved sequences. Although Antp and Ubx bind to identical sites in vitro, Antp activates the tsh enhancer only in epidermis while Ubx activates the tsh enhancer in both epidermis and in somatic mesoderm. We show that the DNA elements driving tissue-specific transcriptional activation by Antp and Ubx are separable. Next to the homeotic protein-binding sites are extensive conserved sequences likely to control tissue activation by different homeodomain proteins. We propose that local interactions between homeotic proteins and other factors effect activation of targets in proper cell types.

Cooperative interactions between HOX and PBX proteins mediated by a conserved peptide motif

Homeoprotein products of the Hox/HOM gene family pattern the animal embryo through the transcriptional regulation of target genes. We have previously shown that the labial group protein HOXA-1 has intrinsically weak DNA-binding activity due to residues in the N-terminal arm of its homeodomain (M. L. Phelan, R. Sadoul, and M. S. Featherstone, Mol. Cell. Biol. 14:5066-5075, 1994). This observation, among others, suggests that HOX and HOM proteins require cofactors for stable interactions with DNA. We have demonstrated that a putative HOX cofactor, PBX1A, participates in cooperative DNA binding with HOXA-1 and the Deformed group protein HOXD-4. Three Abdominal-B class HOX proteins failed to cooperate with PBX1A. We mapped the interacting domain of HOXD-4 to the YPWMK pentapeptide motif, a conserved sequence found N terminal to the homeodomain of HOXA-1 and many other homeoproteins but absent from the Abdominal-B class. The naturally occurring fusion of the transcriptional activation domain of E2A with PBX1 creates an oncoprotein implicated in human pre-B-cell leukemias (M. P. Kamps, C. Murre, X.-H. Sun, and D. Baltimore, Cell 60:547-555, 1990; J. Nourse, J. D. Mellentin, N. Galili, J. Wilkinson, E. Starbridge, S. D. Smith, and M. L. Cleary, Cell 60:535-545, 1990). A pentapeptide mutation that abolished cooperative interaction with PBX1A in vitro also abrogated synergistic transcriptional activation with the E2A/PBX oncoprotein. The direct contact of PBX family members by the HOX pentapeptide is likely to play an important role in developmental and oncogenic processes.

Control of Drosophila adult pattern by extradenticle

The homeobox gene extradenticle (exd) acts as a cofactor of the homeotic genes in the specification of larval patterns during embryogenesis. To study its role in adult patterns, we have generated clones of mutant exd- cells and examined their effect on the different body parts. In some regions, exd- clones exhibit homeotic transformations similar to those produced by known homeotic mutations such as Ultrabithorax (Ubx), labial (lab), spineless-aristapedia (ssa) or Antennapedia (Antp). In other regions, the lack of exd causes novel homeotic transformations producing ectopic eyes and legs. Moreover, exd is also required for functions normally not associated with homeosis, such as the maintenance of the dorsoventral pattern, the specification of subpatterns in adult appendages or the arrangement of bristles in the mesonotum and genitalia. Our findings indicate that exd is critically involved in adult morphogenesis, not only in the homeotic function but also in several other developmental processes.

Both Pbx1 and E2A-Pbx1 bind the DNA motif ATCAATCAA cooperatively with the products of multiple murine Hox genes, some of which are themselves oncogenes

E2A-PBX1 is the oncogene produced at the t(1;19) chromosomal breakpoint of pediatric pre-B-cell leukemia. Expression of E2A-Pbx1 induces fibroblast transformation and myeloid and T-cell leukemia in mice and arrests differentiation of granulocyte macrophage colony-stimulating factor-dependent myeloblasts in cultured marrow. Recently, the Drosophila melanogaster protein Exd, which is highly related to Pbx1, was shown to bind DNA cooperatively with the Drosophila homeodomain proteins Ubx and Abd-A. Here, we demonstrate that the normal Pbx1 homeodomain protein, as well as its oncogenic derivative, E2A-Pbx1, binds the DNA sequence ATCAATCAA cooperatively with the murine Hox-A5, Hox-B7, Hox-B8, and Hox-C8 homeodomain proteins, which are themselves known oncoproteins, as well as with the Hox-D4 homeodomain protein. Cooperative binding to ATCAATCAA required the homeodomain-dependent DNA-binding activities of both Pbx1 and the Hox partner. In cotransfection assays, Hox-B8 suppressed transactivation by E2A-Pbx1. These results suggest that (i) Pbx1 may participate in the normal regulation of Hox target gene transcription in vivo and therein contribute to aspects of anterior-posterior patterning and structural development in vertebrates, (ii) that E2A-Pbx1 could abrogate normal differentiation by altering the transcriptional regulation of Hox target genes in conjunction with Hox proteins, and (iii) that the oncogenic mechanism of certain Hox proteins may require their physical interaction with Pbx1 as a cooperating, DNA-binding partner.

Hox gene products modulate the DNA binding activity of Pbx1 and Pbx2

A new family of homeodomain proteins has recently been identified that includes extradenticle, ceh-20 and three mammalian proteins Pbx1, Pbx2 and Pbx3. We show here that two members of this family, Pbx1 and Pbx2 bind cooperatively to DNA with both Hoxb-7 and Hoxb-8. Engrailed-2 modulates the DNA binding activity of the Pbx proteins to a different target site. E2A-Pbx1, a chimeric Pbx1 gene product involved in pre-B acute lymphoblastoid leukemia, has retained its ability to interact with the Hox proteins. These data show that vertebrate Hox and Pbx gene products have the ability to bind cooperatively to DNA.

Segmental expression of Hoxb-1 is controlled by a highly conserved autoregulatory loop dependent upon exd/pbx

Comparison of Hoxb-1 regulatory regions from different vertebrates identified three related sequence motifs critical for rhombomere 4 (r4) expression in the hindbrain. Functional analysis in transgenic mice and Drosophila embryos demonstrated that the conserved elements are involved in a positive autoregulatory loop dependent on labial (lab) family members. Binding of Hoxb-1 to these elements in vitro requires cofactors, and the motifs closely resemble the consensus binding site for pbx1, a homolog of the Drosophila extradenticle (exd) homoedomain protein. In vitro exd/pbx serves as a Hoxb-1 cofactor in cooperative binding and in Drosophila expression mediated by the r4 enhancer is dependent on both lab and exd. This provides in vivo and in vitro evidence that r4 expression involves direct autoregulation dependent on cooperative interactions of Hoxb-1 with exd/pbx proteins as cofactors.

Localization of Pbx1 transcripts in developing rat embryos

Recently, a new family of homeodomain proteins has emerged, that includes extradenticle, ceh-20, Pbx1, Pbx2 and Pbx3. The Pbx family has been shown to modulate the biological activities of the Hox proteins. We demonstrate here by in situ hybridization that Pbx1 transcripts are present in many embryonic tissues. Highest levels of Pbx1 expression in the developing embryo, from 12 to 20 days post coitum, are found in neuronal tissues, including brain, spinal cord and ganglia. In addition, Pbx1 transcripts are also detectable in the gut, lung, olfactory epithelium and kidney. The expression pattern of Pbx1 overlaps with that of many of the Hox gene products and is consistent with them acting in parallel to regulate common target genes.

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

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

Pbx proteins display hexapeptide-dependent cooperative DNA binding with a subset of Hox proteins

The human proto-oncogene PBX1 codes for a homolog of Drosophila extradenticle, a divergent homeo domain protein that modulates the developmental and DNA-binding specificity of select HOM proteins. We demonstrate that wild-type Pbx proteins and chimeric E2a-Pbx1 oncoproteins cooperatively bind a consensus DNA probe with HoxB4, B6, and B7 of the Antennapedia class of Hox/HOM proteins. Specificity of Hox-Pbx interactions was suggested by the inability of Pbx proteins to cooperatively bind the synthetic DNA target with HoxA10 or Drosophila even-skipped. Site-directed mutagenesis showed that the hexapeptide motif (IYPWMK) upstream of the Hox homeo domain was essential for HoxB6 and B7 to cooperatively bind DNA with Pbx proteins. Engraftment of the HoxB7 hexapeptide onto HoxA10 endowed it with robust cooperative properties, demonstrating a functional role for the highly conserved hexapeptide element as one of the molecular determinants delimiting Hox-Pbx cooperativity. The Pbx homeo domain was necessary but not sufficient for cooperativity, which required conserved amino acids carboxy-terminal of the homeo domain. These findings demonstrate that interactions between Hox and Pbx proteins modulate their DNA-binding properties, suggesting that Pbx and Hox proteins act in parallel as heterotypic complexes to regulate expression of specific subordinate genes.

Transcriptional repression by Msx-1 does not require homeodomain DNA-binding sites

This study investigates the transcriptional properties of Msx-1, a murine homeodomain protein which has been proposed to play a key role in regulating the differentiation and/or proliferation state of specific cell populations during embryogenesis. We show, using basal and activated transcription templates, that Msx-1 is a potent repressor of transcription and can function through both TATA-containing and TATA-less promoters. Moreover, repression in vivo and in vitro occurs in the absence of DNA-binding sites for the Msx-1 homeodomain. Utilizing a series of truncated Msx-1 polypeptides, we show that multiple regions of Msx-1 contribute to repression, and these are rich in alanine, glycine, and proline residues. When fused to a heterologous DNA-binding domain, both N- and C-terminal regions of Msx-1 retain repressor function, which is dependent upon the presence of the heterologous DNA-binding site. Moreover, a polypeptide consisting of the full-length Msx-1 fused to a heterologous DNA-binding domain is a more potent repressor than either the N- or C-terminal regions alone, and this fusion retains the ability to repress transcription in the absence of the heterologous DNA site. We further show that Msx-1 represses transcription in vitro in a purified reconstituted assay system and interacts with protein complexes composed of TBP and TFIIA (DA) and TBP, TFIIA, and TFIIB (DAB) in gel retardation assays, suggesting that the mechanism of repression is mediated through interaction(s) with a component(s) of the core transcription complex. We speculate that the repressor function of Msx-1 is critical for its proposed role in embryogenesis as a regulator of cellular differentiation.

Extradenticle protein is a selective cofactor for the Drosophila homeotics: role of the homeodomain and YPWM amino acid motif in the interaction

The Drosophila homeotic selector (HOM) genes encode a family of DNA binding transcription factors that specify developmental fates of different body segments by differentially regulating the activity of downstream target genes. A central question is how the HOM proteins achieve their developmental specificity despite the very similar DNA binding specificities of isolated HOM proteins in vitro. Specificity could be achieved by differential interactions with protein cofactors. The extradenticle gene might encode such a cofactor since it interacts genetically in parallel with Ultrabithorax, abdominal-A, and perhaps other HOM genes. By using a yeast two-hybrid system, we demonstrate selective interaction of the extradenticle homeodomain protein with certain Ultrabithorax and abdominal-A proteins but not with an Antennapedia protein or a more distant homeodomain protein. Strong interaction with Ultrabithorax proteins requires only the Ultrabithorax homeodomain and a 15-residue N-terminal extension that includes Tyr-Pro-Trp-Met (YPWM), a tetrapeptide motif found near the homeodomain in most HOM proteins and their mammalian Hox counterparts. The size and sequence of the region between the YPWM element and the homeodomain differ among Ultrabithorax isoforms, and this variable region appears to affect the interaction detected in the assay.

Homeodomain proteins. Cooperating to be different

The product of the Drosophila extradenticle gene interacts cooperatively with homeodomain proteins encoded by homeotic selector genes, and may account in part for their distinct regulatory properties.

Transformation properties of the E2a-Pbx1 chimeric oncoprotein: fusion with E2a is essential, but the Pbx1 homeodomain is dispensable

The t(1;19) chromosomal translocation in acute lymphoblastic leukemias creates chimeric E2a-Pbx1 oncoproteins that can act as DNA-binding activators of transcription. A structural analysis of the functional domains of E2a-Pbx1 showed that portions of both E2a and Pbx1 were essential for transformation of NIH 3T3 cells and transcriptional activation of synthetic reporter genes containing PBX1 consensus binding sites. Hyperexpression of wild-type or experimentally truncated Pbx1 proteins was insufficient for transformation, consistent with their inability to activate transcription. When fused with E2a, the Pbx-related proteins Pbx2 and Pbx3 were also transformation competent, demonstrating that all known members of this highly similar subfamily of homeodomain proteins have latent oncogenic potential. The oncogenic contributions of E2a to the chimeras were localized to transactivation motifs AD1 and AD2, as their mutation significantly impaired transformation. Either the homeodomain or Pbx1 amino acids flanking this region could mediate transformation when fused to E2a. However, the homeodomain was not essential for transformation, since a mutant E2a-Pbx1 protein (E2a-Pbx delta HD) lacking the homeodomain efficiently transformed fibroblasts and induced malignant lymphomas in transgenic mice. Thus, transformation mediated by the chimeric oncoprotein E2a-Pbx1 is absolutely dependent on motifs acquired from E2a but the Pbx1 homeodomain is optional. The latter finding suggests that E2a-Pbx1 may interact with cellular proteins that assist or mediate alterations in gene expression responsible for oncogenesis even in the absence of homeodomain-DNA interactions.

Specification of C/EBP function during Drosophila development by the bZIP basic region

The biologically relevant interactions of a transcription factor are those that are important for function in the organism. Here, a transgenic rescue assay was used to determine which molecular functions of Drosophila CCAAT/enhancer binding protein (C/EBP), a basic region-leucine zipper transcription factor, are required for it to fulfill its essential role during development. Chimeric proteins that contain the Drosophila C/EBP (DmC/EBP) basic region, a heterologous zipper, and a heterologous activation domain could functionally substitute for DmC/EBP. Mammalian C/EBPs were also functional in Drosophila. In contrast, 9 of 25 single amino acid substitutions in the basic region disrupted biological function. Thus, the conserved basic region specifies DmC/EBP activity in the organism.

Transformation properties of the E2a-Pbx1 chimeric oncoprotein: fusion with E2a is essential, but the Pbx1 homeodomain is dispensable

The t(1;19) chromosomal translocation in acute lymphoblastic leukemias creates chimeric E2a-Pbx1 oncoproteins that can act as DNA-binding activators of transcription. A structural analysis of the functional domains of E2a-Pbx1 showed that portions of both E2a and Pbx1 were essential for transformation of NIH 3T3 cells and transcriptional activation of synthetic reporter genes containing PBX1 consensus binding sites. Hyperexpression of wild-type or experimentally truncated Pbx1 proteins was insufficient for transformation, consistent with their inability to activate transcription. When fused with E2a, the Pbx-related proteins Pbx2 and Pbx3 were also transformation competent, demonstrating that all known members of this highly similar subfamily of homeodomain proteins have latent oncogenic potential. The oncogenic contributions of E2a to the chimeras were localized to transactivation motifs AD1 and AD2, as their mutation significantly impaired transformation. Either the homeodomain or Pbx1 amino acids flanking this region could mediate transformation when fused to E2a. However, the homeodomain was not essential for transformation, since a mutant E2a-Pbx1 protein (E2a-Pbx delta HD) lacking the homeodomain efficiently transformed fibroblasts and induced malignant lymphomas in transgenic mice. Thus, transformation mediated by the chimeric oncoprotein E2a-Pbx1 is absolutely dependent on motifs acquired from E2a but the Pbx1 homeodomain is optional. The latter finding suggests that E2a-Pbx1 may interact with cellular proteins that assist or mediate alterations in gene expression responsible for oncogenesis even in the absence of homeodomain-DNA interactions.

Regulation of a decapentaplegic midgut enhancer by homeotic proteins

The clustered homeotic genes encode transcription factors that regulate pattern formation in all animals, conferring cell fates by coordinating the activities of downstream ‘target’ genes. In the Drosophila midgut, the Ultrabithorax (Ubx) protein activates and the abdominalA (abd-A) protein represses transcription of the decapentaplegic (dpp) gene, which encodes a secreted signalling protein of the TGF beta class. We have identified an 813 bp dpp enhancer which is capable of driving expression of a lacZ gene in a correct pattern in the embryonic midgut. The enhancer is activated ectopically in the visceral mesoderm by ubiquitous expression of Ubx or Antennapedia but not by Sex combs reduced protein. Ectopic expression of abd-A represses the enhancer. Deletion analysis reveals regions required for repression and activation. A 419 bp subfragment of the 813 bp fragment also drives reporter gene expression in an appropriate pattern, albeit more weakly. Evolutionary sequence conservation suggests other factors work with homeotic proteins to regulate dpp. A candidate cofactor, the extradenticle protein, binds to the dpp enhancer in close proximity to homeotic protein binding sites. Mutation of either this site or another conserved motif compromises enhancer function. A 45 bp fragment of DNA from within the enhancer correctly responds to both UBX and ABD-A in a largely tissue-specific manner, thus representing the smallest in vivo homeotic response element (HOMRE) identified to date.

Role of the teashirt gene in Drosophila midgut morphogenesis: secreted proteins mediate the action of homeotic genes

Homeotic genes control the development of embryonic structure by coordinating the activities of downstream ‘target’ genes. The identities and functions of target genes must be understood in order to learn how homeotic genes control morphogenesis. Drosophila midgut development is regulated by homeotic genes expressed in the visceral mesoderm, where two of their target genes have been identified. Both encode secreted proteins. The Ultrabithorax (Ubx) homeotic gene activates transcription of the decapentaplegic (dpp) gene, which encodes a TGF beta class protein, while in adjacent mesoderm cells the abdominal-A (abd-A) homeotic gene activates transcription of the wingless (wg) gene, which encodes a Wnt class protein. The homeotic genes Antennapedia (Antp) and Sex combs reduced (Scr) act in more anterior midgut regions. Here we report the identification of another homeotic gene target in the midgut mesoderm, the teashirt (tsh) gene, which encodes a protein with zinc finger motifs. tsh is necessary for proper formation of anterior and central midgut structures. Antp activates tsh in anterior midgut mesoderm. In the central midgut mesoderm Ubx, abd-A, dpp, and wg are required for proper tsh expression. The control of tsh by Ubx and abd-A, and probably also by Antp, is mediated by secreted signaling molecules. By responding to signals as well as localized transcription regulators, the tsh transcription factor is produced in a spatial pattern distinct from any of the homeotic genes.

Design of a "minimAl" homeodomain: the N-terminal arm modulates DNA binding affinity and stabilizes homeodomain structure

This report investigates the sequence specificity requirements for homeodomain structure and DNA binding activity by the design and synthesis of a "minimAl" homeodomain (for minimalist design and alanine scanning mutagenesis) which contains the consensus residues and in which all nonconsensus residues have been replaced with alanine. The murine homeodomain Msx served as the prototype for the minimAl homeodomain, Ala-Msx. We show that Ala-Msx binds to DNA specifically, albeit with lower affinity than Msx. A derivative of the minimAl homeodomain, Ala-Msx(NT), which contains a native rather than an alanine-substituted N-terminal arm, has similar DNA binding affinity as Msx. We show that the native N-terminal arm stabilizes the tertiary structure of the minimAl homeodomain. Although Ala-Msx resembles a molten-globule protein, the structure of Ala-Msx(NT) is similar to Msx. The requirement for an intact N-terminal arm is not unique to the minimAl homeodomain, since the N-terminal arm also promotes high-affinity binding activity and appropriate tertiary structure of Msx. Therefore, the homeodomain "scaffold" consists of consensus residues, which are sufficient for DNA recognition, and nonconsensus residues in the N-terminal arm, which are required for optimal DNA binding affinity and appropriate tertiary structure. MinimAl design provides a powerful strategy to probe homeodomain structure and function. This approach should be of general utility to study the sequence specificity requirements for structure and function of other DNA-binding domains.

extradenticle raises the DNA binding specificity of homeotic selector gene products

Recently, a Drosophila gene has been identified, extradenticle, whose product modulates the morphological consequences of homeotic selector genes. We show here that extradenticle protein raises the DNA binding specificity of Ultrabithorax and abdominal-A but not that of Abdominal-B. We further show that extradenticle modulates the DNA binding activity of engrailed to a different target site. While a region N-terminal of the extradenticle homeodomain is required for Ultrabithorax and abdominal-A cooperativity, engrailed requires a domain C-terminal of the extradenticle homeobox. These studies show directly how the DNA binding specificity of selector gene products can be raised by extradenticle and provides a mechanism, cooperative DNA binding, that allows selector gene products to achieve some of their biological specificity.

The DNA binding specificity of Ultrabithorax is modulated by cooperative interactions with extradenticle, another homeoprotein

The Ultrabithorax (Ubx) and Antennapedia (Antp) genes of Drosophila encode homeodomain proteins that have very similar DNA binding specificities in vitro but specify the development of different segmental patterns in vivo. We describe cooperative interactions between Ubx protein and a divergent homeodomain protein, extradenticle (exd), that selectively increases the affinity of Ubx, but not Antp, for a particular DNA target. We also provide evidence that Ubx and exd bind to neighboring sites on this DNA and interact directly to stabilize the DNA-bound form of Ubx. Thus, the ability of different homeotic genes to specify distinct segmental patterns may depend on cooperative interactions with proteins such as exd that selectively modulate their otherwise similar DNA binding specificities.

Retinoic acid and homeobox gene regulation

Hox genes have been shown to be important regulators of pattern formation in vertebrates. Retinoic acid has been shown to affect the expression of Hox genes in vitro and in vivo, and some of its effects on development correspond to changes in Hox gene expression. The idea that retinoic acid is not simply a powerful pharmocological agent, but rather that it plays an important role in creating the normal expression patterns of Hox genes, is provided by the recent identification of retinoic acid responsive enhancers near Hox genes.

Fusion with E2A converts the Pbx1 homeodomain protein into a constitutive transcriptional activator in human leukemias carrying the t(1;19) translocation

E2A-PBX1 is a chimeric gene formed by the t(1;19)(q23;p13.3) chromosomal translocation of pediatric pre-B-cell leukemia. The E2A-Pbx1 fusion protein contains sequences encoding the transactivation domain of E2A joined to a majority of the Pbx1 protein, which contains a novel homeodomain. Earlier, we found that expression of E2A-Pbx1 causes malignant transformation of NIH 3T3 fibroblasts and induces myeloid leukemia in mice. Here we demonstrate that the homeodomains encoded by PBX1, as well as by the highly related PBX2 and PBX3 genes, bind the DNA sequence ATCAATCAA. E2A-Pbx1 strongly activates transcription in vivo through this motif, while Pbx1 does not. This finding suggests that E2A-Pbx1 transforms cells by constitutively activating transcription of genes regulated by Pbx1 or by other members of the Pbx protein family.