Expression of active hormone and DNA-binding domains of the chicken progesterone receptor in E. coli.

Bacterially-expressed fusion proteins containing the DNA-(region C) or hormone-binding (region E) domains of the chicken progesterone receptor (cPR) fused to the C terminus of Escherichia coli beta-galactosidase were analysed for the specificity of interaction with natural and synthetic hormone-responsive elements (HREs) and progestins, respectively. The purified fusion protein containing the progestin-binding domain bound progesterone with an apparent Kd of 1.0-1.5 nM and was specifically photocross-linked with the synthetic progestin R5020 in crude bacterial lysates. Labelling of intact bacterial cells with [3H]R5020 revealed that the majority, if not all, of the bacterially produced hormone-binding domain was active. No differences in the binding to a synthetic palindromic glucocorticoid/progestin-responsive element (GRE/PRE) were found when the bacterially produced cPR DNA-binding domain was compared in methylation interference assays with the full-length chicken progesterone receptor form A expressed in eukaryotic cells. The study of dissociation kinetics, however, revealed differences in the half-life of the complexes formed between the palindromic GRE/PRE and either the receptor form A or the fusion protein containing the cPR DNA-binding domain. DNase I protection experiments demonstrated that the bacterially produced region C of the cPR generated specific 'footprints' on the mouse mammary tumour virus long terminal repeat (MMTV-LTR) which were nearly identical to those previously reported for the rat glucocorticoid receptor.     

A frameshift mutation destabilizes androgen receptor messenger RNA in the Tfm mouse.

A composite mouse androgen receptor DNA sequence was obtained by amplifying genomic DNA or cDNA using the polymerase chain reaction. The open reading frame was 2,697 basepairs, encoding a polypeptide of 899 amino acids (98,204 mol wt). Amino acid sequence comparisons indicated that the mouse androgen receptor (AR) is 97% homologous with rat AR and 83% with human AR. The amino acid sequences of the three receptors are identical within the DNA- and steroid-binding domains. Northern blot analysis revealed the predominant mouse AR mRNA to be 10 kilobases (kb). A 1.7-kb mRNA species was detected in mouse kidney using a cDNA probe containing only 5' untranslated AR sequence. Lack of hybridization with AR-coding sequence probes suggested that the 1.7-kb mRNA was not a truncated form of AR mRNA. Sequencing of genomic DNA isolated from testicular feminized (Tfm) mice revealed a single base deletion in the N-terminal domain, resulting in a frameshift mutation. Cycloheximide treatment caused a dramatic increase in AR mRNA in kidneys of Tfm mice, but not wild-type mice, suggesting that the Tfm mutation results in an unstable AR mRNA.     

Artificial mosaic protein containing antigenic epitopes of hepatitis E virus.

A synthetic gene encoding an artificial polypeptide composed of antigenic epitopes of the hepatitis E virus (HEV) proteins was constructed from short oligodeoxyribonucleotides by using PCR. The polypeptide comprises a mosaic of three antigenically active dominant regions from the protein encoded by open reading frame 2 (ORF2), one antigenically active region from the protein encoded by ORF3 of the Burmese HEV strain, and one antigenically active region from the protein encoded by ORF3 of the Mexican HEV strain. The mosaic protein was expressed in Escherichia coli as a chimera with glutathione S-transferase or beta-galactosidase. Guinea pig sera containing antibodies to the corresponding HEV synthetic peptides were used to demonstrate by Western immunoblot analysis and enzyme immunoassay the presence and accessibility of all HEV-specific antigenic epitopes introduced into the mosaic protein. Both the glutathione S-transferase and beta-galactosidase hybrid proteins were analyzed by using a panel of human anti-HEV-positive and -negative sera. The data obtained strongly indicate a diagnostic potential for the mosaic protein.     

Partial apolipoprotein E-beta-galactosidase fusion protein expressed in Escherichia coli retains binding activity to the LDL(B/E) receptor

A partial rat apo E-beta-galactosidase fusion protein was produced in Escherichia coli Y1089 infected with recombinant lambda GT11 obtained by immunoscreening of a rat liver cDNA library with an anti-rat LDL antiserum. Partial cDNA overlapped the apo E mRNA sequence coding for apo E binding domain towards the LDL(B/E) receptor up to codon for Arg-139. Fusion protein specifically bound to human fibroblasts. The high-affinity component exhibited a Kd of 5 x 10(-8) M and 4.1 x 10(5) sites per cell. Fusion protein binding to fibroblasts was mediated by their apo E moiety and not by beta-galactosidase since: (1) specific binding of fusion protein was competed out by human LDL; (2) beta-galactosidase did not compete with fusion protein binding; and (3) human fibroblasts from a patient with familial hypercholesterolemia, deficient in LDL(B/E) receptor, bound fusion protein 10-times lower than control fibroblasts. It was demonstrated that partial fusion protein retained the functional activity of the native apo E. However, compared to full-length native or engineered apo E, fusion protein was able to bind fibroblasts without being complexed with phospholipids. Fusion proteins might be a useful tool for studying the functional efficiency of the LDL(B/E) receptor and for mapping residues and domains involved in the binding process.     

Association of the 90-kDa heat shock protein does not affect the ligand-binding ability of androgen receptor

An N-terminal truncated androgen receptor with putative DNA- and ligand- binding domains (AR438) and that with a ligand-binding domain (AR612) were expressed under control of the T7 promoter in E. coli or translated in vitro with rabbit reticulocyte lysate, and their ligand-binding properties and the interaction with HSP90 were investigated. Bacterially expressed AR438 and AR612 bound a synthetic androgen, [³H]R1881, with apparent dissociation constant of 2.6 ± 0.2 and 3.1 ± 0.7 nM, respectively, values which are comparable to those of androgen receptor in target tissues. The recombinant androgen receptors sedimented at the 4–5 S region irrespective of the presence of 10 mM tungstate, indicating that the receptor exists free from HtpG, which is the bacterial homolog of eukaryotic HSP90. The apparent dissociation constant of truncated androgen receptors translated in vitro was 0.1 nM for AR438 and 0.2 nM for AR612. Sedimentation coefficients of in vitro translated molecules were converted from 7–8 S in the presence of tungstate to 3 S in the absence of tungstate. Both AR438 and AR612 translated in vitro were retained by anti-rat HSP90 antibody-protein A Sepharose. Exposure to 0.3 M NaCl in the presence of ligand caused dissociation of AR438 and AR612 from HSP90, and concomitantly, the DNA-cellulose binding ability of AR438 was enhanced. Thus, we conclude that the androgen receptor associates with HSP90 through the ligand-binding domain and that this association prevents the interaction of the androgen receptor with DNA. However, HSP90 seems to have little effect on the ligand-binding characteristics of the androgen receptor.     

The androgen receptor (AR) in syndromes of androgen insensitivity and in prostate cancer

The actions of androgens, principally testosterone and 5alpha-dihydrotestosterone, are mediated by a specific receptor protein, the androgen receptor (AR), which is encoded by a single-copy gene located on the human X-chromosome. This receptor protein is a prototypical member of the nuclear receptor family and modulates a range of processes during embryogenesis and in the adult. During embryogenesis, normal AR function is critical to the development of the male phenotype and defects of the AR cause a range of phenotypic abnormalities of male sexual development. Complete loss of AR function has been traced to a number of distinct types of genetic events, including abnormalities of mRNA splicing, the introduction of premature termination codons, and amino acid substitution mutations. An interesting subset of mutations is that in which the AR is completely undetectable using sensitive immunoassays. In all instances, these functional abnormalities are associated with a phenotype of complete androgen insensitivity (complete testicular feminization). By contrast, partial defects of AR function are almost invariably caused by amino acid substitutions within the DNA- and hormone-binding domains of the receptor protein. Such partial defects of receptor function may be caused by changes in either receptor function or receptor abundance.The alterations of AR function and expression that have been characterized in clinical prostatic cancers and in prostate cancer cell lines differ in several important respects. A number of studies have documented the emergence of considerable heterogeneity of AR expression at early stages in the development of prostate cancer. Despite these early changes of AR expression, a substantial body of information suggests that the AR is expressed in advanced forms of prostate cancer, in some cases as the result of amplification events. While infrequent in localized tumors, mutations of the AR have been identified in a number of advanced prostatic cancers and in some instances appear to alter the ligand specificity of the AR. Finally, it appears that other signaling pathways can act to influence AR function.     

Specific phosphorylation of the acidic central region of the N-myc protein by casein kinase II.

The central region of the N-myc protein has a characteristic amino acid sequence EDTLSDSDDEDD, which is very similar to those of particular domains of adenovirus E1A, human papilloma virus E7, Simian virus 40 large T, c-myc and L-myc proteins. Domains of these three viral oncoproteins have recently been shown to be specific binding sites for the tumor-suppressor gene retinoblastoma protein. We have noted that the sequence of serine followed by a cluster of acidic amino acids is exactly the same as that of a typical substrate of casein kinase II (CKII). Therefore, we investigated whether these nuclear oncoproteins are phosphorylated by CKII. For this purpose, we fused the beta-galactosidase and N-myc genes including this domain and expressed it in Escherichia coli cells. Several mutant N-myc genes, containing single amino acid substitutions in this domain, were also used to produce fused proteins. Strong phosphorylation by CKII was detected with the fused protein of wild-type N-myc. However, no phosphorylation of beta-galactosidase itself was observed and the phosphorylations of fused mutant proteins were low. Another fused N-myc protein containing most of the C-terminal region downstream of this acidic region was not phosphorylated by CKII. Analysis of phosphorylation sites in synthetic peptides of this acidic region identified the major sites phosphorylated by CKII as Ser261 and Ser263. On two-dimensional tryptic mapping of phosphorylated N-myc proteins, major spots of in vitro-labeled and in-vivo-labeled N-myc proteins were detected in the same positions. These results suggest that two serine residues of the acidic central region of the N-myc protein are phosphorylated by CKII in vivo as well as in vitro. The functional significance of this acidic domain is discussed.     

Engineering of Escherichia coli beta-galactosidase for solvent display of a functional scFv antibody fragment

Protein engineering allows the generation of hybrid polypeptides with functional domains from different origins and therefore exhibiting new biological properties. We have explored several permissive sites in Escherichia coli beta-galactosidase to generate functional hybrid enzymes displaying a mouse scFv antibody fragment. When this segment was placed at the amino-terminus of the enzyme, the whole fusion protein was stable, maintained its specific activity and interacted specifically with the target antigen, a main antigenic determinant of foot-and-mouth disease virus. In addition, the antigen-targeted enzyme was enzymatically active when bound to the antigen and therefore useful as a reagent in single-step immunoassays. These results prove the flexibility of E. coli beta-galactosidase as a carrier for large-sized functional domains with binding properties and prompt the further exploration of the biotechnological applicability of the scFv enzyme targeting principle for diagnosis or other biomedical applications involving antigen tagging.     

The human androgen receptor: Structure/function relationship in normal and pathological situations

Discrete functions have been attributed to precise regions of the human androgen receptor (hAR) by expression of deletion mutants in COS and HeLa cells. A large C-terminal domain constitutes the hormone-binding region and a central basis, cysteine-rich domain is responsible for DNA binding. In addition, separate domains responsible for transactivation and nuclear translocation have been identified. In LNCaP cells (a prostate tumor cell line) the hAR is a heterogeneous protein which is synthesized as a single 110 kDa protein, but becomes rapidly phosphorylated to a 112 kDa protein. Metabolic labeling experiments using radioactive orthophosphate also indicated that the hAR is a phosphoprotein. Structural analysis of the AR gene in LNCaP cells and in 46, XY-individuals displaying androgen insensitivity (AIS) has revealed several different point mutations. In LNCaP cells the mutation affects both binding specificity and transactivation by different steroids. In a person with complete AIS a point mutation was identified in the splice donor site of intron 4, which prevents normal splicing and activates a cryptic splice donor site in exon 4. The consequence is a functionally inactive AR protein due to an in-frame deletion in the steroid-binding domain. In two unrelated individuals with complete AIS, two different single nucleotide alterations in codon 686 (Asp) were found. Both mutations resulted in functionally inactive ARs due to rapidly dissociating hormone-AR complexes. It is concluded that the hAR is a heterogeneous phosphoprotein in which functional errors have a dramatic impact on phenotype and fertility of 46, XY-individuals.     

Evidence for the involvement of type II domains in collagen binding by 72 kDa type IV procollagenase

The fibronectin-related region of the 72 kDa type IV procollagenase has been expressed in E. coli as a beta-galactosidase fusion product. The fragment containing the three type II units of the protein was found to have affinity for denatured collagen, suggesting that these domains may be responsible for the collagen-affinity of type IV collagenase. We have also shown that segment Ala-Ala-His-Glu of type IV collagenase (residues 372-375), which is similar to a fibronectin-segment previously implicated in collagen-binding, is not essential for binding activity.