Variation in the molecular mass of the Ah receptor among vertebrate species and strains of rats

The Ah receptor in eight vertebrate species was characterized by labeling the cytosolic fraction of tissue with the photoaffinity ligand, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, and analysis of the products by denaturing gel electrophoresis. The apparent molecular mass of the dominant labeled peptide showed appreciable species variation: mouse-95 kDa; chicken (embryo)-101 kDa; guinea pig-103 kDa; rabbit-104 kDa; rat-106 kDa; human-106 kDa; monkey-113 kDa, and hamster-124 kDa. Seven inbred strains of rats, had a Ah receptor ligand binding peptide of 106 kDa; however outbred Long-Evans rats were shown to be polymorphic expressing a 101 kDa and/or 106 kDa allelic forms. The notable frequency of structural variation in the Ah receptor is in contrast to the analogous highly conserved steroid hormone receptors.     

Use of [125I]4'-iodoflavone as a tool to characterize ligand-dependent differences in Ah receptor behavior

We have synthesized [(125)I]4'-iodoflavone to study Ah receptor (AhR)-ligand interactions by a class of AhR ligands distinct from the prototypic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This radioligand allows the comparison of AhR-ligand interactions using a ligand that differs in AhR affinity, and yet has the same radiospecific activity as [(125)I]2-iodo-7,8-dibromodibenzo-p-dioxin. Specific binding of [(125)I]4'-iodoflavone with the AhR was detected as a single radioactive peak ( approximately 9.7 S) following density sucrose gradient analysis. Cytosolic extracts from both Hepa 1 and HeLa cells were used as the source of mouse and human AhR, respectively. A approximately 6.7 S form of radioligand-bound Ah receptor was detected in the high salt nuclear extracts of both cell lines. In HeLa cells approximately twofold more [(125)I]4'-iodoflavone-AhR 6 S complex, compared with [(125)I]2-iodo-7,8-dibromodibenzo-p-dioxin, was recovered in nuclear extracts. A comparison of the ability of 4'-iodoflavone and TCDD to cause time-dependent translocation of AhR-yellow fluorescent protein revealed that 4'-iodoflavone was more efficient at enhancing nuclear accumulation of the receptor. These results suggest that [(125)I]4'-iodoflavone is a particularly useful and easily synthesized ligand for studying the AhR.     

Isolation of the protein B23/nucleophosmin from HeLa cell nuclei

Endogenous forms of the protein B23 were for the first time isolated from HeLa cell nuclei and their structural states were analyzed. It was demonstrated that incubation of HeLa cell nuclei in 10 mM Tris-HCl buffer (pH 7.4) led, not only to their swelling, but also to the release of several nuclear proteins, including the protein B23. PAGE of the supernatant fraction allowed nine major stained protein bands to be detected; the bands were identified by MALDI mass spectrometry (matrix-assisted laser desorption and ionization). The proteins in the range of 35–40 kDa were identified as nucleophosmin, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and heterogeneous nuclear ribonucleoprotein (hnRNP) A2/B1. Analysis of the N- and C-terminal amino acid sequences showed the presence of the isoforms B23.1 and B23.2, GAPDH, and the isoform hnRNP B1 and made it possible to describe the C-and N-terminal processing patterns and demonstrate the presence of isoform B23.2 at a protein level.     

Isolation of the protein B23/nucleophosmin from HeLa cell nuclei

Endogenous forms of the protein B23 were for the first time isolated from HeLa cell nuclei and their structural states were analyzed. It was demonstrated that incubation of HeLa cell nuclei in 10 mM Tris-HCl buffer (pH 7.4) led, not only to their swelling, but also to the release of several nuclear proteins, including the protein B23. PAGE of the supernatant fraction allowed nine major stained protein bands to be detected; the bands were identified by MALDI mass spectrometry (matrix-assisted laser desorption and ionization). The proteins in the range of 35-40 kDa were identified as nucleophosmin, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and heterogeneous nuclear ribonucleoprotein (hnRNP) A2/B1. Analysis of the N- and C-terminal amino acid sequences showed the presence of the isoforms B23.1 and B23.2, GAPDH, and the isoform hnRNP B1 and made it possible to describe the C- and N- terminal processing patterns and demonstrate the presence of isoform B23.2 at a protein level.     

Intracellular degradation of bleomycin hydrolase in two Chinese hamster cell lines in relation to their peplomycin susceptibility

The Chinese hamster lung (V79) cell was intrinsically 10-times more resistant to peplomycin, a bleomycin-related antitumor antibiotic, than the Chinese hamster ovary (CHO) cell. This may be associated with the 3-times higher levels of recovery of bleomycin hydrolase activity of the V79 cell. The degradation of bleomycin hydrolase molecules in both V79 and CHO cells was examined using a monoclonal antibody specific for the enzyme. Labelling experiments showed that the bleomycin hydrolase in CHO cells was less stable than the comparable enzyme in V79 cells, and that 48 kDa subunits comprising bleomycin hydrolase (a homohexameric enzyme) molecules were degraded into 31 kDa forms in both cell lines. The 105,000 X g pellet (microsomes) fraction obtained after subcellular fractionation of CHO cells contained both 48 kDa subunit and 31 kDa forms of bleomycin hydrolase, while the 105,000 X g supernatant cytosol fraction yielded only 48 kDa subunit forms of the enzyme. Moreover, bleomycin hydrolase activity of both V79 and CHO cells was almost entirely recovered from the cytosol fraction. These results suggest that degradation of the 48 kDa subunit form of bleomycin hydrolase in these two lines of cultured cells into the 31 kDa form occurs on the plasma membrane or the endoplasmic reticulum, with which the resulting large number of bleomycin hydrolase molecules or degraded forms of the enzyme that have lost enzymatic activity are associated.     

Chemical cross-linking of the cytosolic and nuclear forms of the Ah receptor in hepatoma cell line 1c1c7.

Both cytosolic and high salt nuclear extracts were isolated from Hepa 1c1c7 cells incubated with 2-azido-3[125I]iodo-7,8-dibromo-dibenzo-p-dioxin ([125I]N3Br2DpD). The [125I]N3Br2DpD-labeled cytosolic fraction was subjected to chemical cross-linking with dimethyl pimelimidate and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Chemical cross-linking of the cytosolic form of the AhR revealed monomeric (97 kDa), dimeric (185 kDa), trimeric (281 kDa), and tetrameric (327 kDa) complexes. In a time course of exposure to the cross-linking reagent, the largest form given above became the predominant AhR form observed in the cytosolic extracts. The 327 kDa cytosolic species apparently consists of a 97 kDa AhR, an approximately 88 kDa protein, an approximately 96 kDa protein, and an approximately 46 kDa protein. Nuclear extracts from [125I]N3Br2DpD-labeled Hepa 1c1c7 cells were applied to sucrose density gradients. The 6 S nuclear receptor peak fractions were pooled and subjected to chemical cross-linking. Analysis by SDS-PAGE revealed a monomeric (97 kDa) ligand binding protein and a dimeric (182 kDa) complex. This would suggest that the nuclear 6 S AhR consists of a 97 kDa AhR and an approximately 85 kDa protein. These findings would indicate that the AhR exists in cytosol as a tetrameric species, while in the nucleus the AhR exists as a heterodimer.     

Characterization of a family of nuclear and chromosomal proteins identified by a monoclonal antibody

A monoclonal antibody (3C5) isolated from a mouse immunized with human chromatin stained the nuclei of all cultured cell types tested by indirect immunofluorescence. Experiments with HeLa and PtK1 cells demonstrated striking cell-cycle-related changes in the staining properties of the target antigen. A rapid increase in nuclear fluorescence was seen in prophase, with antigen located between the condensing chromosomes. In metaphase and anaphase cells antigen was present throughout the cytoplasm with the chromosomes apparently unstained. However, isolated metaphase chromosomes showed intense, peripheral staining. In telophase cells immunofluorescent staining was most intense among the decondensing chromosomes and by early G1 staining was predominantly nuclear. Nuclear fluorescence faded as cells progressed through interphase. By protein blotting and immunostaining, 3C5 recognized protein bands with subunit molecular weights of 130, 73, 50, 38, 32 and 22 to 25 kDa. These bands were present in all human and rodent cultured cell types tested. All bands were extracted by 6 M urea or 1% sodium dodecyl sulfate (SDS) but not by Triton X-100. Our results provide evidence against the involvement of a common carbohydrate moiety, in vitro proteolysis or non-specific cross reaction in this multi-banded pattern. The same family of proteins was detected in mitotic and interphase cells, suggesting that the changes in immunofluorescent staining through mitosis are due to changes in antigen accessibility. Subcellular fractionation experiments showed that all major bands were present in the nuclear fraction. Only two (50 and 32 kDa) were detected also in the post-nuclear membrane fraction and none were present in the soluble cytoplasmic fraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biochemical characterization and biosynthesis of the Ki-1 antigen in Hodgkin-derived and virus-transformed human B and T lymphoid cell lines

The Hodgkin-associated Ki-1 antigen was analyzed in different cell lines. In Hodgkin analogous L428 cells, biosynthetically labeled with radioactive amino acids, the Ki-1 antibody precipitated three glycoproteins with 90, 105, and 120 kDa, respectively. Surface-labeling revealed that the two larger components were membrane-associated forms of the Ki-1 antigen, although the 90-kDa molecule was shown in pulse-chase experiments to be the precursor of the 105- and 120-kDa forms. All three forms of the Ki-1 antigen possess a tunicamycin-sensitive 6-kDa N-linked carbohydrate moiety. O-Linked oligosaccharides could not be detected. Thus, the differences in m.w. are probably not due to glycosylation. The ionophore monensin prevented the appearance of the membrane-associated molecules, which demonstrated that they are assembled between the transcompartment of the Golgi complex and their insertion into the cell membrane. The 90-kDa precursor molecule cannot be generated by disulfide reduction from the two larger forms. After internal labeling with P-32, only the 105- and 120-kDa bands became visible, indicating that the Ki-1 molecule is phosphorylated after its processing into the two larger membrane-associated forms. Analysis of the Ki-1 antigens from other cell lines demonstrated that after external labeling of two other Hodgkin-derived cell lines, six Epstein-Barr virus lymphoblastoid cell lines and one human T leukemia virus I-positive T cell line, both the 105- and the 120-kDa membrane molecules could be detected, regardless of the presence or type of virus integrated.     

N-linked glycosylation and its impact on the electrophoretic mobility and function of the human proton-coupled folate transporter (HsPCFT)

The human proton-coupled folate transporter (HsPCFT, SLC46A1) mediates intestinal absorption of folates and transport of folates into the liver, brain and other tissues. On Western blot, HsPCFT migrates as a broad band (~55 kDa), higher than predicted (~50 kDa) in cell lines. Western blot analysis required that membrane preparations not be incubated in the loading buffer above 50 degrees C to avoid aggregation of the protein. Treatment of membrane fractions from HsPCFT-transfected HeLa cells with peptidyl N-glycanase F, or cells with tunicamycin, resulted in conversion to a ~35 kDa species. Substitution of asparagine residues of two canonical glycosylation sites to glutamine, individually, yielded a ~47 kDa protein; substitution of both sites gave a smaller (~35 kDa) protein. Single mutants retained full transport activity; the double mutant retained a majority of activity. Transport function and molecular size were unchanged when the double mutant was hemagglutinin (HA) tagged at either the NH(2) or COOH terminus and probed with an anti-HA antibody excluding degradation of the deglycosylated protein. Wild-type or deglycosylated HsPCFT HA, tagged at amino or carboxyl termini, could only be visualized on the plasma membrane when HeLa cells were first permeabilized, consistent with the intracellular location of these domains.     

N-linked glycosylation and its impact on the electrophoretic mobility and function of the human proton-coupled folate transporter (HsPCFT)

The human proton-coupled folate transporter (HsPCFT, SLC46A1) mediates intestinal absorption of folates and transport of folates into the liver, brain and other tissues. On Western blot, HsPCFT migrates as a broad band (~ 55 kDa), higher than predicted (~ 50 kDa) in cell lines. Western blot analysis required that membrane preparations not be incubated in the loading buffer above 50 °C to avoid aggregation of the protein. Treatment of membrane fractions from HsPCFT-transfected HeLa cells with peptidyl N-glycanase F, or cells with tunicamycin, resulted in conversion to a ~ 35 kDa species. Substitution of asparagine residues of two canonical glycosylation sites to glutamine, individually, yielded a ~ 47 kDa protein; substitution of both sites gave a smaller (~ 35 kDa) protein. Single mutants retained full transport activity; the double mutant retained a majority of activity. Transport function and molecular size were unchanged when the double mutant was hemagglutinin (HA) tagged at either the NH₂ or COOH terminus and probed with an anti-HA antibody excluding degradation of the deglycosylated protein. Wild-type or deglycosylated HsPCFT HA, tagged at amino or carboxyl termini, could only be visualized on the plasma membrane when HeLa cells were first permeabilized, consistent with the intracellular location of these domains.     

Characterization of a functionally expressed dipeptidyl aminopeptidase III from Drosophila melanogaster.

A Drosophila melanogaster cDNA clone (GH01916) encoding a putative 723-residue long (82 kDa) protein (CG 7415) and displaying 50% identity with mammalian cytosolic dipeptidyl aminopeptidase (DPP) III was functionally expressed in Schneider S2 cells. Immunocytochemical studies using anti-(rat liver DPP III) Ig indicated the expression of this putative DPP III at the outer cell membrane and into the cytosol of transfected cells. Two protein bands (82 and 86 kDa) were immunologically detected after PAGE and Western blot of cytosol or membrane prepared from transfected cells. Western blot analysis of partially purified D. melanogaster DPP III confirmed the overexpression of these two protein bands into the cytosol and on the membranes of transfected cells. Despite the identification of six potential glycosylation sites, PAGE showed that these protein bands were not shifted after deglycosylation experiments. The partially purified enzyme hydrolysed the insect myotropic neuropeptide proctolin (Arg-Tyr-Leu-Pro-Thr) at the Tyr-Leu bond (Km approximately 4 micro m). In addition, low concentration of the specific DPP III inhibitor tynorphin prevented proctolin degradation (IC50 = 0.62 +/- 0.15 micro m). These results constitute the first characterization of an evolutionarily conserved insect DPP III that is expressed as a cytosolic and a membrane peptidase involved in proctolin degradation.     

Insulin modulates PC-1 processing and recruitment in cultured human cells

We evaluated whether insulin signaling modulates plasma cell glycoprotein (PC-1) plasma membrane recruitment, posttranslational processing, and gene expression in human cultured cell lines. Insulin induced a fourfold increase (P < 0.01) of membrane PC-1 expression by rapid and sensitive mechanism(s). This effect was reduced (P < 0.05-0.01) by inhibition of phosphatidylinositol 3-kinase (200 nmol/l wortmannin) and S6 kinase (50 nmol/l rapamycin) activities and intracellular trafficking (50 micromol/l monensin) and was not accompanied by PC-1 gene expression changes. Moreover, at Western blot, insulin elicited the appearance, in both plasma membrane and cytosol, of a PC-1-related 146-kDa band (in addition to bands of 163, 117, 106, and 97 kDa observed also in absence of insulin) that was sensitive to endoglycosidase H. Finally, inhibition of PC-1 translocation to plasma membrane, by wortmannin pretreatment, increases insulin-stimulated receptor autophosphorylation. Our data indicate that insulin stimulates PC-1 posttranslational processing and translocation to the plasma membrane, which in turn impairs insulin receptor signaling. Bidirectional cross talk between insulin and PC-1, therefore, takes place, which may be part of the hormone self-desensitization mechanism.     

Characterization of a 100-kilodalton binding protein for the six serotypes of coxsackie B viruses.

Viral infection of host cells primarily depends on binding of the virus to a specific cell surface protein. In order to characterize the binding protein for group B coxsackieviruses (CVB), detergent-solubilized membrane proteins of different cell lines were tested in virus overlay protein-binding assays. A prominent virus-binding protein with a molecular mass of 100 kDa was detected in various CVB-permissive human and monkey cell lines but was not detected in nonpermissive cell lines. The specificity of CVB binding to the 100-kDa protein on permissive human cells was substantiated by binding of all six serotypes of CVB and by competition experiments. In contrast, poliovirus and Sendai virus did not bind to the 100-kDa CVB-specific protein. A fraction of HeLa membrane proteins enriched in the range of 100 kDa showed functional activity by transforming infectious CVB (160S) into A-particles (135S). In order to purify this CVB-binding protein, solubilized membrane proteins from HeLa cells were separated by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by elution of the 100-kDa protein. Amino acid sequence analysis of tryptic fragments of the CVB-binding protein indicated that this 100-kDa CVB-specific protein is a cell surface protein related to nucleolin. These results were confirmed by immunoprecipitations of the CVB-binding protein with nucleolin-specific antibodies, suggesting that a nucleolin-related membrane protein acts as a specific binding protein for the six serotypes of CVB.     

Factors from Trypanosoma cruzi Interacting with AP-1 Sequences

Interaction between factors from Trypanosoma cruzi extracts and AP-1 sequences was studied by electrophoretic mobility shift assays. Using a double-stranded probe carrying the AP-1 sequence from the SV40 promoter, three specific complexes designated A, B, and C were detected. Complexes A and C were formed when using single-stranded probes. The relative amount of complex B, specific for double-stranded DNA, increased as a function of probe length. Complexes were stabilized by cross-linking with UVC irradiation and resolved on denaturing SDS-PAGE. Complex A generated bands of 60- and 39 kDa; complex B produced two bands of 46- and 43 kDa; and complex C generated one band of 43 kDa. The AP-1 binding activity was much higher in purified nuclear preparations than in soluble fractions, and was detected in crude extracts from the three forms of the parasite. The binding signal, however, was much stronger in amastigote and trypomastigote than in the epimastigote forms. Specific binding was increased by oxidative stress. Antibodies raised against peptides corresponding to conserved domains of mammalian c-Jun and c-Fos detected bands of 40- and 60 kDa, respectively, in a nuclear epimastigote preparation.     

Nuclear localisation of endogenous SUMO-1-modified PDGF-C in human thyroid tissue and cell lines

We investigated post-translational modification and subcellular localisation of endogenous platelet-derived growth factor-C (PDGF-C) in human thyroid papillary carcinomas (PTC), non-neoplastic thyroid tissues, and a selection of cultured cell lines. PDGF-C expressed nuclear localisation in 95% of all tested cell types in culture and in 10% of the thyrocytes from both PTC and non-neoplastic tissue. The cell lines expressed two forms of full-length PDGF-C, approximately 39 and approximately 55 kDa, in cell membrane and cytosol, while the approximately 55 kDa form dominated in the nucleus where it was partly chromatin-associated. The approximately 55 kDa form was post-translationally modified by SUMO-1. The putative PDGF-C SUMOylation site is the surface exposed (314)lysine part of a positively charged loop ((312)RPKTGVRGLHK(322)) with characteristics of a nuclear localisation signal. The tissue thyrocytes expressed a non-SUMOylated approximately 43 kDa and the 55 kDa PDGF-C. The SUMO-1 modified approximately 55 kDa PDGF-C expression was low in PTC where the approximately 43 kDa PDGF-C dominated. This is in contrast to non-neoplastic tissue and cultured cells where the SUMOylated approximately 55 kDa PDGF-C was strongly expressed. Our data provide novel evidence for nuclear localisation of PDGF-C, post-translational modification by SUMOylation and the expression of a novel form of PDGF-C in human papillary thyroid carcinomas.     

Detection of glucocorticoid-receptor complex oligomers in nuclear extracts from cells exposed to hormone at physiological temperature.

When HeLa cells were incubated with tritiated dexamethasone mesylate at 2 degrees C, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cytosolic and nuclear extracts revealed the presence of two monomeric receptor complex forms with estimated molecular masses of about 98 and 87 kDa. If cells were subjected to crosslinking with glutaraldehyde, a third form consisting of a 250 kDa oligomer was also detected. When HeLa cells were treated with dexamethasone mesylate at 37 degrees C, and were subjected to crosslinking, electrophoresis of cytosolic glucocorticoid-receptor complexes was drastically reduced, whereas their levels in nuclear extracts were not appreciably altered.     

S-(4-azidophenacyl)[35S]glutathione photoaffinity labeling of rat liver plasma membrane-associated proteins

A method for the synthesis of the glutathione conjugate S-(4-azidophenacyl)[35S]glutathione is described. The compound was used for photoaffinity labeling of proteins present in canalicular membrane vesicles (CMV), sinusoidal membrane vesicles (SMV), mitochondria and microsomes from rat liver. Most of the radioactivity introduced by photoaffinity labeling of CMV appeared in the 25-29 kDa range. Further labeled proteins were observed in bands at 37, 105 and about 120 kDa. 79% of the 25-29 kDa associated radioactivity was recovered in the supernatant after extensive revesiculation (washing) of the vesicles, together with the 37 kDa protein. CMV and SMV contained glutathione S-transferase (GST) activity which in CMV was decreased by 75% by washing. Photolabeling of a mixture of purified basic GST subunits from rat liver resulted in a band pattern at 25-29 kDa similar to that in the membrane preparations. Isoelectric focusing of the CMV indicated the presence of basic soluble GST subunits. S-Hexylglutathione-Sepharose affinity chromatography showed reversible binding of photolabeled proteins at 25-29 kDa. Difference photoaffinity labeling with GSSG, S-hexylglutathione, taurocholate and phenylmethylsulfonyl fluoride decreased the radioactivity bound by GST, but not that introduced into the 105 kDa protein band present in CMV. It is concluded that membrane-associated basic GST isoenzymes are present in standard membrane vesicle preparations. In the cell, the function may be transport of GST-bound compounds across the membrane and protection of the membranes against electrophiles.     

Purification of T47D human progesterone receptor and immunochemical characterization with monoclonal antibodies

In order to obtain steroid-independent probes for human progesterone receptor (PR), the A [88-93 kilodalton (kDa)] and B (109-119 kDa) forms of PR from T47D human breast cancer cells were partially purified and used to generate a series of 14 monoclonal antibodies. Initially, unoccupied PR was isolated from cytosol extracts by steroid affinity chromatography, followed by chromatography on diethylaminoethyl Bio-Gel. The partially pure (3-15%) PR consisted of two steroid-binding components that migrated at 89 kDa and 109 kDa in reducing sodium dodecyl sulfate gels after being photoaffinity labeled with the synthetic progestin [3H]R5020. Two unique monoclonal antibodies to PR were derived from a male Lewis rat immunized with this material. One of these antibodies (JU601) was coupled to Sepharose 4B and used to purify T47D nuclear PR for additional immunizations. Highly purified (30-70%) PR migrated as 93 kDa and 119 kDa progestin-binding proteins in sodium dodecyl sulfate gels. In all, thirteen monoclonal antibodies were obtained that recognized epitopes shared by both receptor forms. One mouse immunoglobulin G (KC146) was completely specific for the larger B form. Interestingly, the epitope for this antibody was present on all PRs tested, including the B form of PR from chicken oviduct, whereas nine other antibodies recognized only human PR and the remaining four cross reacted with rabbit PR. With the exception of the JU145 and JU601 rat immunoglobulin Ms, all antibodies appeared to be completely specific for the A or B forms of PR. Each recognized the cytosol and nuclear forms of occupied as well as unoccupied PR. Although the relationship between B and A was not established, it is clear that an amino-terminal region of B is not present in A, and that a significant portion of A and B are either identical or very similar in amino acid sequence.     

The homeodomain protein, Pit-1/GHF-1, is capable of binding to and activating cell-specific elements of both the growth hormone and prolactin gene promoters

Studies were conducted to determine whether the trans-acting protein Pit-1/GHF-1 can bind to and activate promoter elements in both the GH and PRL genes that are necessary for cell-specific expression. Four pituitary cell lines that differentially express the endogenous GH and PRL genes were examined for their ability to activate GH and PRL promoter constructs containing sequences necessary for cell-specific expression (CSEs). Plasmids containing one CSE, -96 PRL and -104 GH, were similarly expressed in each of the four cell lines. Of the plasmids containing two CSEs, -173 PRL was always activated to a greater extent than -145 GH, with this relative activation being stronger in GC and GH1 cells than in 235-1 and GH4C1 cells. Protein-DNA binding assays were used to show that the GH and PRL CSEs specifically bound two highly abundant nuclear proteins (31 and 33 kDa). The two proteins were present at similar levels in all four pituitary cell lines and were recognized by a Pit-1/GHF-1 antibody. In contrast, HeLa and Rat2 cells did not activate transfected GH or PRL plasmids and did not contain nuclear proteins that specifically bound to the GH and PRL CSEs. However, cotransfection of these cells with the expression vector RSV-Pit-1/GHF-1 resulted in the activation of -173 PRL and -145 GH (PRL greater than GH). HeLa cells transfected with RSV-Pit-1/GHF-1 also contained 31- and 33-kDa nuclear proteins that bound to the GH and PRL CSEs. These results show that Pit-1/GHF-1 is present at levels in pituitary cell lines that are sufficient to activate the minimal elements in both the GH and PRL promoters necessary for cell-specific expression of these genes.     

Nuclear matrix as acceptor for cAMP-binding proteins

Using [³²P]-8-N₃-cAMP, a photoaffinity analog of cAMP, we have established that nuclear binding of cAMP is preferentially localized in the “nuclear matrix”. Two major radioactive bands corresponded to proteins of M_r 40 K and 50 K, and three minor bands to proteins of M_r 55, 150 and 200 K. Even though the molecular weight of the major nuclear binding proteins in the matrix are similar to those of the cytosolic cAMP binding proteins, the characteristics of the binding reaction in the nucleus were markedly different from those in the cytosol.