Pan: a transcriptional regulator that binds chymotrypsin, insulin, and AP-4 enhancer motifs

Two proteins act as the IUF1 insulin gene enhancer binding factor

IUF1 is a pancreatic beta cell-specific factor which binds to the sequence 5'-CPyCTAATG-3' (CT box) within the human insulin gene enhancer. Here we show that IUF1 is composed of 2 binding activities that can be separated by DEAE ion exchange chromatography. South Western blot analysis indicates that these distinct binding activities have apparent molecular weights of 115 kDa and 46 kDa.

Transactivation of pancreas-specific gene sequences in somatic cell hybrids

Enhancer/promoter elements from two pancreas-specific genes, those encoding amylase and elastase, were ligated to the bacterial GPT gene. The resulting construct can be used to select for expression of gene products which activate these pancreas-specific promoters in hybrid cells. The selectable GPT construct was stably transferred into several cell lines either directly or by cotransfection with pSV2Neo. GPT was expressed when transferred to pancreatic cell lines but not when transferred to GPT-fibroblast (L) cells or hepatoma cells. When the transformed L cells and hepatoma cells were fused with pancreatic cell lines, GPT was activated in the hybrid cells. Endogenous pancreas-specific genes from the L-cell and hepatoma parents were also activated in the hybrids. In addition, a pancreas-specific nuclear protein, PTF1, was produced in pancreatic and hybrid cells, correlating with GPT expression. The transformed L cells and hepatoma cells thus contained a nonexpressed construct which could be activated in trans by factors present in pancreatic cells. The hepatoma hybrid also continued to produce albumin, demonstrating the coexpression of liver and pancreas-specific genes in the hybrid-cell population. Cell lines carrying the amylase/elastase/GPT construct may be useful as a selection system for cloning of pancreatic transcription activators.

Transactivation of pancreas-specific gene sequences in somatic cell hybrids

Enhancer/promoter elements from two pancreas-specific genes, those encoding amylase and elastase, were ligated to the bacterial GPT gene. The resulting construct can be used to select for expression of gene products which activate these pancreas-specific promoters in hybrid cells. The selectable GPT construct was stably transferred into several cell lines either directly or by cotransfection with pSV2Neo. GPT was expressed when transferred to pancreatic cell lines but not when transferred to GPT-fibroblast (L) cells or hepatoma cells. When the transformed L cells and hepatoma cells were fused with pancreatic cell lines, GPT was activated in the hybrid cells. Endogenous pancreas-specific genes from the L-cell and hepatoma parents were also activated in the hybrids. In addition, a pancreas-specific nuclear protein, PTF1, was produced in pancreatic and hybrid cells, correlating with GPT expression. The transformed L cells and hepatoma cells thus contained a nonexpressed construct which could be activated in trans by factors present in pancreatic cells. The hepatoma hybrid also continued to produce albumin, demonstrating the coexpression of liver and pancreas-specific genes in the hybrid-cell population. Cell lines carrying the amylase/elastase/GPT construct may be useful as a selection system for cloning of pancreatic transcription activators.

A pancreatic exocrine cell factor and AP4 bind overlapping sites in the amylase 2A enhancer

A factor found in pancreatic exocrine cell lines and pancreatic nuclei binds selectively to the alpha-amylase 2A transcriptional enhancer. Pancreatic exocrine cell extracts protect asymmetrically an unusually large, 35 base pair region from DNase I digestion in vitro, suggesting the involvement of a multimeric DNA binding complex. We show that this region of the enhancer contains a major affinity recognition sequence for the HeLa transcription factor AP4. A 4 base pair mutation in the enhancer sequence shown previously to abolish activity in vivo [Boulet, A. M., Erwin, C. R., & Rutter, W. J. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 3599-3603] abolishes AP4 binding in vitro and weakens but does not eliminate the binding of adjacent enhancer factors. Further, sequences similar to the AP4 binding site are found within a consensus sequence of most pancreatic exocrine genes (Boulet et al., 1986). We have identified three AP4 binding sites in the pancreatic elastase gene: one occurs in the consensus sequence of the enhancer. Thus, protein(s) with the binding selectivity of AP4 may play a role in the expression of the pancreatic exocrine gene family.

Functional activity of myogenic HLH proteins requires hetero-oligomerization with E12/E47-like proteins in vivo

In this report we provide four lines of evidence indicating that E12/E47-like proteins interact in vivo with the myogenic HLH proteins MyoD and myogenin. First, cotransfection of MyoD and E47 in COS cells indicates that these factors synergistically enhance transcription of a reporter gene containing an oligomerized MyoD-binding site. Second, mobility-shift assays of muscle cell nuclear extracts, "double shifted" with specific antisera, have identified complexes binding to the MEF1 site that contain either MyoD or myogenin in association with E12/E47-like proteins. Third, association with E47 alters the phosphorylation state of MyoD. Fourth, C3H10T1/2 cells expressing antisense E2A transcripts contain low levels of E2A gene products and display less terminal muscle differentiation when infected with retroviral MyoD or when challenged to differentiate with 5-azacytidine treatment. In addition we demonstrate that MyoD, in conjunction with E12/E47-like proteins, is functioning as a regulatory nodal point for activation of several other downstream muscle regulators.

Distribution and characterization of helix-loop-helix enhancer-binding proteins from pancreatic beta cells and lymphocytes

Transcription of a number of mammalian genes is controlled in part by closely-related DNA elements sharing a CAxxTG consensus sequence (E boxes). In this report, we survey cell extracts from a variety of mammalian cell lineages for ability to bind to the E box denoted IEB1/kappa E1, which plays an important role in expression of both insulin and immunoglobulin kappa genes. Insulin enhancer factor 1 (IEF1), a binding activity previously identified in beta cells, was also present in pituitary endocrine cells but absent in 7 other mammalian cell lines tested. A distinct binding activity, lymphoid enhancer factor 1 (LEF1), was observed in several lymphoid cell lines, but was absent from all nonlymphoid cells tested. IEF1 and LEF1 were distinct according to electrophoretic mobility, and DNA binding specificity. As previously reported, both beta cell and lymphoid cell factors are recognized by antibodies to helix-loop-helix (HLH) proteins, indicating that they may contain functional helix-loop-helix dimerization domains. To directly demonstrate this, we showed that the binding factors are able to interact in vitro with the HLH domain of a characterized HLH protein. These results support the notion that HLH proteins play a key role in cell-specific transcriptional regulation in cells from endocrine and lymphocyte lineages.

Inefficient homooligomerization contributes to the dependence of myogenin on E2A products for efficient DNA binding

Myogenin is a muscle-specific transcription factor that can activate myogenesis; it belongs to a family of transcription factors that share homology within a basic region and an adjacent helix-loop-helix (HLH) motif. Although myogenin alone binds DNA inefficiently, in the presence of the widely expressed HLH proteins E12 and E47 (encoded by the E2A gene), it forms heterooligomers that bind with high affinity to a DNA sequence known as a kappa E-2 site. In contrast, E47 and to a lesser extent E12 are both able to bind the kappa E-2 site relatively efficiently as homooligomers. To define the relative contributions of the basic regions of myogenin and E12 to DNA binding and muscle-specific gene activation, we created chimeras of the two proteins by swapping their basic regions. We showed that myogenin's weak affinity for the kappa E-2 site is attributable to inefficient homooligomerization and that the myogenin basic domain alone can mediate high-affinity DNA binding when placed in E12. Within a heterooligomeric complex, two basic regions were required to form a high-affinity DNA-binding domain. Basic-domain mutants of myogenin or E2A gene products that cannot bind DNA retained the ability to oligomerize and could abolish DNA binding of the wild-type proteins in vitro. These myogenin and E2A mutants also acted as trans-dominant inhibitors of muscle-specific gene activation in vivo. These findings support the notion that muscle-specific gene activation requires oligomerization between myogenin and E2A gene products and that E2A gene products play an important role in myogenesis by enhancing the DNA-binding activity of myogenin, as well as other myogenic HLH proteins.

Inefficient homooligomerization contributes to the dependence of myogenin on E2A products for efficient DNA binding

Myogenin is a muscle-specific transcription factor that can activate myogenesis; it belongs to a family of transcription factors that share homology within a basic region and an adjacent helix-loop-helix (HLH) motif. Although myogenin alone binds DNA inefficiently, in the presence of the widely expressed HLH proteins E12 and E47 (encoded by the E2A gene), it forms heterooligomers that bind with high affinity to a DNA sequence known as a kappa E-2 site. In contrast, E47 and to a lesser extent E12 are both able to bind the kappa E-2 site relatively efficiently as homooligomers. To define the relative contributions of the basic regions of myogenin and E12 to DNA binding and muscle-specific gene activation, we created chimeras of the two proteins by swapping their basic regions. We showed that myogenin's weak affinity for the kappa E-2 site is attributable to inefficient homooligomerization and that the myogenin basic domain alone can mediate high-affinity DNA binding when placed in E12. Within a heterooligomeric complex, two basic regions were required to form a high-affinity DNA-binding domain. Basic-domain mutants of myogenin or E2A gene products that cannot bind DNA retained the ability to oligomerize and could abolish DNA binding of the wild-type proteins in vitro. These myogenin and E2A mutants also acted as trans-dominant inhibitors of muscle-specific gene activation in vivo. These findings support the notion that muscle-specific gene activation requires oligomerization between myogenin and E2A gene products and that E2A gene products play an important role in myogenesis by enhancing the DNA-binding activity of myogenin, as well as other myogenic HLH proteins.

B-cell- and myocyte-specific E2-box-binding factors contain E12/E47-like subunits

Recent studies have identified a family of DNA-binding proteins that share a common DNA-binding and dimerization domain with the potential to form a helix-loop-helix (HLH) structure. Various HLH proteins can form heterodimers that bind to a common DNA sequence, termed the E2-box. We demonstrate here that E2-box-binding B-cell- and myocyte-specific nuclear factors contain subunits which are identical or closely related to ubiquitously expressed (E12/E47) HLH proteins. These biochemical function for E12/E47-like molecules in mammalian differentiation, similar to the genetically defined function of daughterless in Drosophila development.

B-cell- and myocyte-specific E2-box-binding factors contain E12/E47-like subunits

Recent studies have identified a family of DNA-binding proteins that share a common DNA-binding and dimerization domain with the potential to form a helix-loop-helix (HLH) structure. Various HLH proteins can form heterodimers that bind to a common DNA sequence, termed the E2-box. We demonstrate here that E2-box-binding B-cell- and myocyte-specific nuclear factors contain subunits which are identical or closely related to ubiquitously expressed (E12/E47) HLH proteins. These biochemical function for E12/E47-like molecules in mammalian differentiation, similar to the genetically defined function of daughterless in Drosophila development.

An inhibitory domain of E12 transcription factor prevents DNA binding in E12 homodimers but not in E12 heterodimers

The kappa E2 sequence binding proteins, E12 and E47, are generated by alternative splicing of the E2A gene, giving closely related basic and helix-loop-helix structures crucial for DNA binding and dimerization. Measurements of dimerization constants and binding strengths to the optimal DNA sequence (the kappa E2 site or its near relatives) showed that E47 homodimers and MyoD heterodimers with E12 or E47 dimerized and bound avidly, but E12 homodimerized efficiently and bound to DNA poorly; MyoD homodimerized poorly and bound strongly. An inhibitory domain N-terminal to the basic region of E12 prevents E12 homodimers but not E12/MyoD heterodimers from binding to DNA. Thus, E47 binds to DNA both as a heterodimer with MyoD and as a homodimer, while E12 and MyoD bind to DNA efficiently only as heterodimers.

Sequence of a HeLa cDNA provides the DNA binding domain and carboxy terminus of HE47: a human helix-loop-helix protein related to the enhancer binding factor E47

In order to identify cDNA encoding regulators of gene expression, a HeLa lambda gt11 expression library was screened with a DNA segment containing multiple copies of a sequence spanning the AP4 site in the simian virus 40 control region. We identified a partial cDNA encoding HE47, a sequence-specific DNA binding protein. The HeLa clone overlaps with a previously reported human B-cell partial cDNA encoding E47. The HeLa cDNA includes the HE47 DNA binding domain, its carboxy terminus, and the 3' untranslated region of its mRNA.