Evidence that the novobiocin-sensitive ATP-binding site of the heat shock protein 90 (hsp90) is necessary for its autophosphorylation

The 90kDa heat shock protein (Hsp90) is one of the most abundant protein and essential for all eukaryotic cells. Many proteins require the interaction with Hsp90 for proper function. Upon heat stress the expression level of Hsp90 is even enhanced. It is assumed, that under these conditions Hsp90 is required to protect other proteins from aggregation. One property of Hsp90 is its ability to undergo autophosphorylation. The N-terminal domain of Hsp90 has been shown to contain an unusual ATP-binding site. A well-known inhibitor of Hsp90 function is geldanamycin binding to the N-terminal ATP-binding site with high affinity. Recently it was shown that Hsp90 possesses a second ATP-binding site in the C-terminal region, which can be competed with novobiocin. Autophosphorylation of Hsp90 was analysed by incubation with gamma(32)P-ATP. Addition of geldanamycin did not interfere with the capability for autophosphorylation, while novobiocin indeed did. These results suggest that the C-terminal ATP-binding site is required for autophosphorylation of Hsp90.

Intracellular localization of the 90 kDA heat shock protein (HSP90alpha) determined by expression of a EGFP-HSP90alpha-fusion protein in unstressed and heat stressed 3T3 cells

Heat shock protein 90 (Hsp90) is an abundant protein and essential for all eukaryotic cells. The expression of Hsp90 is further enhanced after exposure to stress factors, e.g. a heat shock. Many proteins interacting with Hsp90 as well as the various functions for Hsp90 have been described. In this study, an Hsp90alpha fusion protein along with the enhanced green fluorescence protein (EGFP) was expressed under the control of the human cytomegalovirus immediate early promoter. EGFP-Hsp90alpha was mainly localized in the cytoplasm, with only minor amounts inside the nuclei. No EGFP-Hsp90alpha could be detected inside the nucleoli. Following exposure to elevated temperatures, higher amounts of EGFP-Hsp90alpha are inside the nucleus, but not within the nucleoli. As the most remarkable finding under these conditions, an association of EGFP-Hsp90alpha with the nuclear membrane became visible.

Combined use of nonmyelosuppressive nitrosourea analogues with hydroxyurea in the induction of F-cell production in a human erythroleukemic cell line

OBJECTIVE

Although hydroxyurea (HU) has been used clinically to treat patients with sickle cell disease (SCD), not all patients benefit from HU treatment due to its toxicity. The objective of this study was to investigate the effectiveness of the use of two new Hb F-inducing nitrosourea analogues, 2-[3-(2-methyl, 2-nitroso) ureido]-2-deoxy-D-glucopyranose (MNGU) and 2-[3-(2-chloroethyl) ureido]-2-deoxy-D-glucopyranose (CGU), in combination with HU in K562 cells or erythroid progenitors.

MATERIALS AND METHODS

After K562 cells were cultured with different concentrations of HU with CGU or MNGU, aliquots of the cells were obtained to determine the total (benzidine-positive) hemoglobin level, number of F cells, and Hb F level. Erythroid progenitor cells of SCD patients and healthy donors were cultured with the optimal drug concentrations, and the number of BFU-E and Hb F level were determined.

RESULTS

Our results showed that the combined use of HU with CGU or MNGU increased the number of both benzidine-positive normoblasts and F cells in a synergistic manner. Further, a lower concentration of HU was required to induce a significant level of Hb F synthesis when combined with either of the two compounds in comparison with treatment with HU alone. On day 4, the number of benzidine-positive cells was 4.5- to 6.5-fold and the number of F cells was 5.0- to 8.0-fold higher than the respective numbers in the untreated K562 cells. Similarly, a 3.2- to 14.3-fold induction of Hb F was obtained when human erythroid progenitors from SCD patients were treated with the same drug combinations.

CONCLUSION

Based on these results, the use of CGU or MNGU in combination with HU might offer substantial benefits to patients with SCD and other hemoglobinopathies.

A novel function for the 90 kDa heat-shock protein (Hsp90): facilitating nuclear export of 60 S ribosomal subunits

Ribosomal subunits are assembled in the nucleus, and mature 40 S and 60 S subunits are exported stoichiometrically into the cytoplasm. The nuclear export of ribosomal subunits is a unidirectional, saturable and energy-dependent process. An in vitro assay for the nuclear export of 60 S ribosomal subunits involves the use of resealed nuclear envelopes. The export of ribosomal subunits from resealed nuclear envelopes is enhanced by cytoplasmic proteins. Here we present evidence that the export-promoting activity was due to the cytoplasmic 90 kDa heat-shock protein (Hsp90). Isolated, purified Hsp90 vastly enhanced the export of 60 S ribosomal subunits from resealed nuclear envelopes, while inhibition of Hsp90 function, either with the Hsp90-binding drug geldanamycin or with anti-Hsp90 antibodies, resulted in reduced release of 60 S ribosomal subunits. To confirm these findings under in vivo conditions, corresponding experiments were performed with Xenopus oocytes using microinjection techniques; the results obtained confirmed the findings obtained with resealed nuclear envelopes. These findings suggest that Hsp90 facilitates the nuclear export of 60 S ribosomal subunits, probably by chaperoning protein interactions during the export process.

Hydroxyurea-induced oxidative damage of normal and sickle cell hemoglobins in vitro: amelioration by radical scavengers

Hydroxyurea (HU) induces fetal hemoglobin (Hb F) production in patients with sickle cell anemia. The therapeutic dosage of HU used for Hb F induction often elicits myelosuppression, which becomes its major associated complication. We examined the effect of HU on hemoglobin modulation and the role of radical scavengers on these induced changes. In vitro exposure of human blood to various concentrations of HU at predetermined time intervals induced a progressive dose-dependent oxidation (MetHb formation) of both adult (Hb AA) and sickle (Hb SS) hemoglobins. The oxidative effect of HU on Hb SS was 3 times greater than its effect on Hb AA. Similar but less profound changes were observed in H2O2-treated samples. Hb F was, however, observed to be relatively resistant to HU-induced oxidative damage. A substantial protective effect of Hb by alpha-tocopherol, ascorbic acid, and D-mannitol was observed during pretreatment of Hb AA and Hb SS blood samples. Analyses of the hemoglobins and their globin chain components by high-performance liquid chromatography revealed a considerable protective effect by these free radical scavengers. These results indicate that the HU-induced damage of hemoglobin and their component globin chains can be reduced by radical scavengers.

Isolation and quantification of the heat shock protein 90 alpha and beta isoforms from rat liver

Heat shock protein 90 (Hsp90) is an abundant cytosolic protein. In higher eukaryotes two isoforms of Hsp90 exist, Hsp90 alpha and Hsp90 beta. Hsp90 was purified from rat liver and after sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a double band at about 90 kDa. The two bands were separated and identified as the Hsp90 alpha and Hsp90 beta isoforms. There was no entry in the protein databases for the Hsp90 alpha isoform from rat. Furthermore, the ratio of the two Hsp90 isoforms was determined.

Enhancement of hemoglobin and F-cell production by targeting growth inhibition and differentiation of K562 cells with ribonucleotide reductase inhibitors (didox and trimidox) in combination with streptozotocin

Upon appropriate drug treatment, the human erythroleukemic K562 cells have been shown to produce hemoglobin and F-cells. Fetal hemoglobin (Hb F) inhibits the polymerization events of sickle hemoglobin (Hb S), thereby ameliorating the clinical symptoms of sickle cell disease. Ribonucleotide reductase inhibitors (RRIs) have been shown to inhibit the growth of myeloid leukemia cells leading to the production of Hb F upon differentiation. Of the RRIs currently in use, hydroxyurea is the most effective agent for Hb F induction. We have examined the capacity of two novel RRIs, didox (DI) and trimidox (TRI), in combination with streptozotocin (STZ), to induce hemoglobin and F-cell production. The K562 cells were cultured with different concentrations of didox-STZ or trimidox-STZ at a fixed molar ratio of 3:1 and 1:5 for 96 hr, respectively. At pre-determined time intervals, aliquots of cells were obtained and total hemoglobin (benzidine positive) levels, number of F-cells, and Hb F were determined by the differential staining technique, fetal hemoglobin assay kit, and fluorescence cytometry respectively. The effect of combined drug treatment on the growth of K562 cells was examined by isobologram analysis. Our results indicate that a synergistic growth-inhibitory differentiation effect occurred when didox or trimidox was used in combination with STZ on K562 cells. There was an increase in the number of both benzidine-positive normoblasts and F-cells, accompanied by morphologic appearances typical of erythroid maturation. On day 4, the number of benzidine-positive cells showed a 6-9-fold increase and the number of F-cells was between 2.5- and 5.7-fold higher than the respective controls. Based upon these results, treatment with a ribonucleotide reductase inhibitor, such as didox or trimidox, in combination with STZ, might offer an additional promising option in sickle cell disease therapy.

Trimidox-mediated morphological changes during erythroid differentiation is associated with the stimulation of hemoglobin and F-cell production in human K562 cells

Trimidox (3,4,5-trihdroxybenzamidoxime) has been shown to reduce the activity of ribonucleotide reductase with accompanied growth inhibition and differentiation of mammalian cells. Hydroxyurea (HU) is the only ribonucleotide reductase inhibitor in clinical use for the treatment and management of sickle cell anemia, since this compound increases fetal hemoglobin (Hb F) production: a potent inhibitor of sickle hemoglobin (Hb SS) polymerization. However, the main limitations of HU is its lack of potency, myelosuppression and short half life. These studies investigated the effects of trimidox on the induction of hemoglobin and F-cells production in K562 erythroleukemia cells. Our study reveals that trimidox exhibits concentration dependent inhibitory effect on K562 cells with increase in benzidine positive normoblasts and F-cells production as well as morphological changes typical of erythroid differentiation. These findings provide the first evidence that the growth inhibitory differentiation of cells induced by trimidox enhance hemoglobin and F-cells production.

Three different actions of phenylglyoxal on band 3 protein-mediated anion transport across the red blood cell membrane

Phenylglyoxalation of the red blood cell membrane leads to three superimposed effects on band 3 protein-mediated anion equilibrium exchange as measured by means of radiosulfate: (1) a shift of the curve relating transport activity to pH towards lower pH values, possibly in combination with an increase of the maximal transport activity. This is accompanied by effect (2), the abolishment of a chloride-stimulated component of anion transport seen at low pH values. Effect (3) consists of inhibition of anion equilibrium exchange. Effect (1) prevails when phenylglyoxalation is performed at low concentrations of PG and low pH, while effect (3) predominates when exposure to PG is executed at high pH and high concentration of PG. Effect (1) is associated with a decrease of the Ki values for inhibition and binding of the reversibly acting stilbene disulfonates DNDS and DBDS. The inhibition observed as a consequence of effect (3) is linearly related to a decrease of the capacity of band 3 to combine with the stilbene disulfonate DBDS. The results are interpreted on the assumption that PG is capable of reacting with two or possibly three distinct binding sites in band 3. Reaction with one of them leads to effect (1) and, perhaps, to effect (2); reaction with the other to effect (3). The latter is possibly due to modification of Arg 730, which is homologous to Arg 748 in mouse band 3. Site-directed mutagenesis of this arginine residue showed that it is required for band 3-mediated anion transport.

Export of ribosomal subunits from resealed rat liver nuclear envelopes

We have previously described the rat liver resealed nuclear envelope model system for the study of the selective import of nuclear proteins, and the export of poly(A)-containing mRNA [Riedel, N., Bachmann, M., Richter, H. & Fasold, H. (1987) Proc. Natl Acad. Sci. USA 83, 3540-3544]. The vesicles still respond to the importin-ATP signal for the uptake of nuclear-location-sequence (NLS)-carrying proteins. During the preparation of the vesicles and extraction of the chromatin from nuclei in cold hypotonic heparin solution, ribosomal subunits may be introduced into these envelopes, and after resealing remain stably included. Efflux from the resealed nuclear envelopes is effected by a cytoplasmatic protein fraction, and strongly enhanced in the presence of ATP. The heterogeneous nuclear RNP (hnRNP) A1, the components of importin, or GTP showed no influence on this export. The ATP-dependent efflux of mRNA is not affected by these cytoplasmic proteins in this model system.

Cross-linking of nucleic acids to proteins. Modified poly(A) as mRNA for Escherichia coli ribosomes

Poly(adenylic acid) was modified by methylchlorotetrolic ester in a reproducible and defined content of the derivatized bases. The nucleic acid derivative is protein reactive and was coupled to 70S ribosomes from Escherichia coli, in order to identify proteins along the mRNA pathway. The binding of the label becomes specific under the direction of tRNA(Lys) and is then almost exclusively located on the small subunit. The proteins S1, S12, S18 and S21 were labeled, as shown by an antibody assay. The yield of the affinity label was 5.4%, as calculated from the labeled nucleic acid. This compares favourably with the yields from photolabile compounds.

4'-Amino-benzamido-taurocholic acid selectively solubilizes glycosyl-phosphatidylinositol-anchored membrane proteins and improves lipolytic cleavage of their membrane anchors by specific phospholipases

Glycosyl-phosphatidylinositol-anchored membrane proteins (GPI-proteins) are normally identified either by cleavage of the lipid anchor using (glycosyl)phosphatidylinositol-specific phospholipases C or D (GPI-PLs) or by metabolic labeling of the lipid moiety with specific building blocks. Therefore, methods for discrimination between transmembrane proteins and GPI-proteins on the basis of their physicochemical properties are desirable. Here we are presenting a selective extraction method for typical well-characterized mammalian GPI-proteins, e.g., acetylcholine esterase, alkaline phosphatase, 5'-nucleotidase, and lipoprotein lipase, using a derivative of taurocholate. The results were compared to those obtained with well-characterized transmembrane proteins, e.g., insulin receptor and hydroxymethyl glutaryl coenzyme A-reductase, glucose transporters, or aminopeptidase M and several commercially available detergents. With regard to total membrane proteins, it was possible to selectively enrich GPI-proteins up to 8- to 14-fold by using concentrations between 0.1 and 0.3% of 4'-NH2-amino-7 beta-benzamido-taurocholic acid (BATC). In addition, the cleavage specificity and efficiency of (G)PI-PLs were increased in the presence of identical concentrations of BATC compared to commonly used detergents, e.g., Nonidet P-40. Therefore, the present study shows that the use of BATC facilitates the identification of glycosyl-phosphatidylinositol-anchored membrane proteins.

Purification and partial sequence of proteins involved in the cholic acid transport into rat liver hepatocytes

Two proteins, in previous work labeled by affinity markers derived from taurocholic acid, were purified and partially sequenced. Antibodies were raised against purified proteins, and cross-reactions were carefully checked. The influence of these antibodies upon taurocholic acid import into vesicles from rat liver plasma membranes was measured, and showed a distinct inhibition of transport in the case of the 54 kD protein.

[Age-dependent changes in mRNA transport (nucleus-cytoplasm)]

Transport of mRNA from nucleus to cytoplasm is an ATP-dependent process which occurs strictly vectorially. Because the mRNA is structurally bound during transport, mRNA transport is a "solid-state" process consisting of i) mRNA release from the nuclear matrix, ii) mRNA translocation through the nuclear pore, and iii) cytoskeletal binding. We identified and purified the following components involved in the translocation step: i) the nuclear envelope (NE) nucleoside triphosphatase (NTPase) which is stimulated by the 3'poly(A) tail of mRNA, ii) the poly(A)-recognizing mRNA carrier, iii) the NE protein kinase, and iv) the NE phosphatase. In addition, we found that an RNA helicase activity is present in NE, which also may be involved in RNA transport. Our results show that, besides poly(A), also double-stranded RNA structures may modulate RNA export. The amount of mRNA released from nuclei markedly decreases with age. Evidence is presented that this age-dependent change is caused by an impairment of polyadenylation of mRNA, hnRNA processing, release of mRNA from nuclear matrix, and translocations of mRNA from nuclear to cytoplasmic compartment (decrease in activities of NE NTPase, protein kinase, and phosphatase; decrease in poly(A)-binding affinity of mRNA carrier).

Characterization and chemical modification of the Na(+)-dependent bile-acid transport system in brush-border membrane vesicles from rabbit ileum

The Na(+)-dependent uptake system for bile acids in the ileum from rabbit small intestine was characterized using brush-border membrane vesicles. The uptake of [3H]taurocholate into vesicles prepared from the terminal ileum showed an overshoot uptake in the presence of an inwardly-directed Na(+)-gradient ([Na+]out > [Na+]in), in contrast to vesicles prepared from the jejunum. The Na(+)-dependent [3H]taurocholate uptake was cis-inhibited by natural bile acid derivatives, whereas cholephilic organic compounds, such as phalloidin, bromosulphophthalein, bilirubin, indocyanine green or DIDS - all interfering with hepatic bile-acid uptake - did not show a significant inhibitory effect. Photoaffinity labeling of ileal membrane vesicles with 3,3-azo- and 7,7-azo-derivatives of taurocholate resulted in specific labeling of a membrane polypeptide with apparent molecular mass 90 kDa. Bile-acid derivatives inhibiting [3H]taurocholate uptake by ileal vesicles also inhibited labeling of the 90 kDa polypeptide, whereas compounds with no inhibitory effect on ileal bile-acid transport failed to show a significant effect on the labeling of the 90 kDa polypeptide. The involvement of functional amino-acid side-chains in Na(+)-dependent taurocholate uptake was investigated by chemical modification of ileal brush-border membrane vesicles with a variety of group-specific agents. It was found that (vicinal) thiol groups and amino groups are involved in active ileal bile-acid uptake, whereas carboxyl- and hydroxyl-containing amino acids, as well as tyrosine, histidine or arginine are not essential for Na(+)-dependent bile-acid transport activity. The irreversible inhibition of [3H]taurocholate transport by DTNB or NBD-chloride could be partially reversed by thiols like 2-mercaptoethanol or DTT. Furthermore, increasing concentrations of taurocholate during chemical modification with NBD-chloride were able to protect the ileal bile-acid transporter from inactivation. These findings suggest that a membrane polypeptide of apparent M(r) 90,000 is a component of the active Na(+)-dependent bile-acid reabsorption system in the terminal ileum from rabbit small intestine. Vicinal thiol groups and amino groups of the transport system are involved in Na(+)-dependent transport activity, whereas other functional amino acids are not essential for transport activity.

Covalent attachment of ribonucleic acids to proteins

As a prerequisite for the synthesis of affinity labels, we describe methods to couple histones to ribonucleic acids. For the synthesis of these covalent hybrid molecules, we used a population of histones H1, H2A, H2B, H3, and H4 from calf thymus and polyadenylic acid with an average chain length of up to 260-280 bases, representing the size of poly(A)-tails from mature mRNAs. Three methods were investigated. (a) Poly(A) containing an 8-N3-A residue was cross-linked to histones by ultraviolet irradiation. (b) The 3'-end of the polynucleotide was connected to a mononucleotide containing an aliphatic amino group, and the resulting poly(A)-derivative was coupled to histones via derivation with a bromoacetyl group. (c) The 3'-end of the polynucleotide was oxidized with sodium periodate and bound covalently to an amino group of the polypeptide. To demonstrate the RNA content of the hybrid molecule, the poly(A) was removed with RNase T2.

Interleukin-2 and blood brain barrier in cats: pharmacokinetics and tolerance following intrathecal and intravenous administration

Single bolus doses of glycosylated human interleukin-2 (n IL-2) in the range of 2.8 x 10(3) to 2.0 x 10(6) IU/kg were administered to anesthesized cats via the cephalic vein (n = 10) or using suboccipital puncture (n = 8). CSF (cerebrospinal fluid) and blood samples were collected by repeated puncture. The n IL-2 concentration in four cats was determined on the basis of its biologic activity using 3H-thymidine incorporation into human ConA-blasts and by radioimmunoassay. In additional experiments radioactivity was determined in cerebrospinal fluid and serum after intravenous and intrathecal (i.th.) application of 5.8 x 10(3) - 3.2 x 10(3) IU/kg of 14C-acetyl-n IL-2 in regular time intervals. CSF and serum concentration time-profiles show a biexponential decline in the plasma elimination phase with half-lives of 4 min (alpha-phase) and 90 min (beta-phase) after intravenous and 20-120 min (alpha-phase) and 2-16 hours (beta-phase) after intrathecal application. There is a trend towards longer terminal elimination half-lives with increasing doses. Interleukin-2 is able to penetrate the blood brain barrier from the circulation into the cerebrospinal fluid and vice versa. Due to a slow rate of penetration and rapid elimination from blood only traces of n IL-2 (2-8 IU/ml) are detected in CSF after i.v. injection of 2 x 10(6) IU/kg, whereas concentrations between 400 and 1600 IU/ml are maintained in CSF for several hours following i.th. administration of 2-10 x 10(5) IU/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

A strongly basic protein of the MAP2 family copolymerizes with tubulin and induces polymerization

The family of microtubuli-associated proteins of approximately 300 kD molecular weight (MAP2) from porcine brain was fractionated into components of neutral isoelectric point and one polypeptide of strongly basic nature. Both fractions are able to induce the polymerization of purified porcine brain tubulin. In the case of the fractions of an isoelectric point of 7.2, thick and short tubular structures result. Under the influence of the basic protein, extremely long tubules of normal diameter of microtubules are produced. This basic MAP2 copolymerizes with tubulin.

Poly(A) binding proteins located at the inner surface of resealed nuclear envelopes

We have used a photoreactive cross-linking reagent, poly(A/8-N3-A) (a poly(A) of average molecular mass of 100 kDa in which 5-10% of the A residues are replaced by 8-N3-A), to label poly(A) binding proteins of rat liver nuclear envelopes. This reagent was prepared by polymerizing a mixture of ADP and 8-N3-ADP with polynucleotide phosphorylase. The purified poly(A) was labeled in the 5'-position with a 32P group. In nuclear envelopes prepared by a low salt DNase I procedure, the poly(A/8-N3-A) labeled a protein-nucleic acid complex of approximately 270 kDa, which on degradation with RNase U2 or NaOH at pH 10 yielded two polypeptides of approximately 50 and 30 kDa. These photoreaction products were markedly decreased when resealed nuclear envelopes or non-nuclear envelope proteins were irradiated in the presence of poly(A/8-N3-A). The affinity labeling was intensified when resealed vesicles were made leaky by freezing or ultrasonication, suggesting that the poly(A) binding proteins are accessible from the nucleoplasmic but not the cytoplasmic face of the envelope. Moreover binding was specific for poly(A). Alternative reagents, random poly(A/8-N3-A,C,G,U) of about 100 kDa and poly(dA) (molecular mass between 350 and 515 kDa), showed a very low affinity for poly(A) recognition proteins in the low salt DNase I-treated nuclear envelopes; the 270-kDa band was labeled only weakly. The binding site was not protected by poly(A,C,G,U), weakly by poly(dA), and distinctly by poly(A).

Interaction of a nuclear location signal with isolated nuclear envelopes and identification of signal-binding proteins by photoaffinity labeling

The nuclear envelope (NE) separates the two major compartments of eukaryotic cells, the nucleus and the cytoplasm. Recent studies suggest that the uptake of nuclear proteins into the nucleus is initiated by binding of nuclear location signals (NLSs) contained within these proteins to receptors in the NE, followed by translocation through the nuclear pore complex. To examine the binding step without interference from intranuclear events, we have used a system consisting of (i) purified rat liver NEs fixed onto glass slides and (ii) the prototype simian virus 40 large T antigen (SV40 T) NLS conjugated to nonnuclear carrier proteins, and we have visualized the receptor-ligand interaction by indirect immunofluorescence. In this system, incubation of isolated NEs with the wild-type SV40 T NLS conjugate with carrier proteins resulted in binding that was signal sequence-dependent, could be competitively blocked with excess conjugated and unconjugated wild-type peptide, did not require ATP, and was not affected by the transport-inhibiting lectin wheat germ agglutinin. In contrast, only minimal binding was observed with a mutant SV40 T NLS conjugate. These results are consistent with those obtained in other, more complex in vitro systems and suggest that binding of the SV40 T NLS is receptor-mediated. Binding is largely abolished by extraction of the NE with the nonionic detergent Triton X-100, suggesting that the receptor is soluble in detergent. We find in the Triton X-100 supernatant four major NLS-binding proteins with apparent molecular masses of 76, 67, 59, and 58 kDa by photoaffinity labeling with a highly specific crosslinker, azido-NLS. The reduced complexity of the system described here should be useful for the functional study of other potential NLSs for the identification and isolation of their binding sites and for the screening of antibodies raised against these binding sites.

Major proteolytic fragments of the murine band 3 protein as obtained after in situ proteolysis

Proteolytic fragments of murine band 3 were produced by exposure to extracellular chymotrypsin and intracellular trypsin. The ensuing proteolytic fragments were isolated, their N-terminal sequences were determined and their locations in the known amino acid sequence of murine band 3 established. Equivalents of the human 60, 35 and 17 kDa fragments were obtained through the cleavage sites were situated at locations that are not strictly homologous to the corresponding cleavage sites in human band 3, although all of them were near such sites. Exposure of the intact murine red cell to chymotrypsin leads to the formation of two fragments of 67 kDa and 41 kDa, which are equivalent to the 60 kDa and the 35 kDa fragments of the human band 3. Internal trypsin cleaves the chymotryptic 67 kDa fragment while the 41 kDa fragment appears essentially unaffected. The 67 kDa fragment is first degraded to 64 kDa, then further to 22 kDa and finally to 19 kDa. The anion transport inhibitor H2DIDS (4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonate) combines with murine band 3 protein as it does with human band 3. Anion transport is maximally inhibited when 5.10(5) H2DIDS molecules per cell are bound to band 3. As in the human red cell, after exposure to high pH (9.0-9.5) of the H2DIDS-labeled, chymotryptically cleaved band 3 intramolecular cross-linking takes place. This joins the 67 and 41 kDa chymotryptic pieces together to form a peptide of the original molecular mass of band 3 of 108 kDa. If cross-linking is performed after additional tryptic cleavage, the 19 and 22 kDa pieces join together with 41 kDa pieces to form overlapping bands that cover the molecular weight range from 60 to 63 kDa.

Bile acid binding proteins in hepatocellular membranes of newborn and adult rats. Identification of transport proteins with azidobenzamidotauro[14C]cholate ([14C]ABATC)

Neonatal hepatocytes are less active in uptake of bile acids than are mature hepatocytes. This phenomenon has been further investigated by transport studies with azidobenzamidotaurocholate (ABATC). Taurocholate, cholate and the photolabile ABATC were taken up by liver cells of adult rats by a sodium-dependent and by an additional sodium-independent mechanism. In the dark, ABATC inhibited the uptake of taurocholate and cholate. Taurocholate decreased the transport of ABATC in a competitive manner, both in the presence and absence of sodium. In neonatal hepatocytes the Vmax for taurocholate and for ABATC was similar but was lower than in mature liver cells. In contrast, the Km was similar for neonatal and mature hepatocytes. For identification of binding proteins in both kinds of cells ABATC was photolysed after preincubation with isolated hepatocytes. Under our experimental conditions (single ultraviolet flash) about 80% of the azido groups was converted to nitrene. The covalently binding nitrene derivative inhibited bile salt transport irreversibly. Photolabeling of intact hepatocytes or of isolated plasma membranes with ABATC resulted in radioindication of membrane proteins with 67, 60, 54, 50 and 43 kDa in mature plasma membranes but of proteins with masses of 67, 54, 43 and 37 kDa in neonatal basolateral membranes. The 50 kDa protein in largely lacking in membranes of 9-day-old rats. The process of photolabeling itself was sodium-independent when isolated cells were treated with ABATC. In contrast, the degree of labeling of intact hepatocytes was markedly reduced in the absence of sodium and chloride. 100-fold molar excess of taurocholate, benzamidotaurocholate (BATC), phalloidin or cyclosomatostatin protected isolated plasma membranes against coupling of ABATC. Photolabeling of hepatoma cells known to be deficient in bile salt transport did not result in radiomodification of membrane proteins.

The anion-transport inhibitor H2DIDS cross-links hemoglobin interdimerically and enhances oxygen unloading

Human hemoglobin treated with equal concentrations of the anion-transport inhibitor H2DIDS produces a right shift in the oxygen dissociation curve. concomitantly, the Hill coefficient is reduced from n = 2.7 to 2.1. When higher concentrations of H2DIDS are applied (H2DIDS: hemoglobin = 5:0.5 mM), the Hill coefficient decreases further to 1.5 and the oxygen dissociation curve of hemoglobin is shifted slightly to the left of the control. Similar results were also obtained with DIDS instead of H2DIDS. SDS-PAGE shows that H2DIDS cross-links hemoglobin monomers mainly into dimers. Cross-linking is more effective under anaerobic conditions. With tritiated H2DIDS the larger part of the radioactivity is found in the dimer position of hemoglobin. Separation of the alpha and beta units of hemoglobin reacted with tritiated H2DIDS demonstrated a stoichiometry of 2.2 and 2.4 molecules H2DIDS per molecule alpha and beta unit hemoglobin, leading to about 8-9 H2DIDS molecules per native hemoglobin. The right shift produced in the hemoglobin oxygen dissociation curve and the cross-linking of monomers into dimers, especially under anaerobic condition, suggest that H2DIDS can also react with those amino groups of hemoglobin which are involved in 2,3-DPG binding. A comparison of H2DIDS, DIDS and 2,3-DPG at three different concentrations close to the hemoglobin concentration revealed a concentration dependent right shift in the oxygen dissociation curve with the order of potency 2,3-DPG greater than H2DIDS greater than DIDS. The Hill coefficients (n) at the three concentrations of 2,3-DPG demonstrated no changes, but H2DIDS and DIDS reduced in a concentration-dependent manner the cooperativity of hemoglobin. Again, H2DIDS is more potent than DIDS, especially at the low concentration. These anion-transport inhibitors provide novel approaches to the exploration of hemoglobin function.

Artificial dimers of native actin: preparation and properties in biological functions

With the aid of tartryl-bis-epsilon-aminocaprylazide artificial dimers were produced from F actin from rabbit striated muscle. These derivatives will not polymerize by themselves but are able to copolymerize fully with native G actin. By modification of a single side chain per dimer, this copolymerization was completely inhibited. The dimers are able to activate subfragment 1 ATPase of myosin and bind to DNase I with inactivation of the enzyme in the same manner as native G actin. Within the dimer, one ADP is immobilized and will exchange against ATP extremely slowly. The dimers do not bind to the mushroom toxin phalloidin.

Permeability measurements with closed vesicles from rat liver nuclear envelopes

Closed nuclear envelope ghosts in the physiological orientation were prepared from rat liver and nuclei as previously described. Here we report transport measurements of various proteins and ribonucleic acids across the envelope of these vesicles. Histones were accumulated rapidly in the ghosts, in contrast to other, nonnuclear, proteins. Triton X-100 removal of the external nuclear membrane from loaded vesicles, as well as comparative studies with open vesicles, excluded the effects of external adsorption. The exchange rate of histones across the nuclear envelope is strongly depressed in the presence of GTP and GDP. The vesicles contain the translocation mechanism for poly(A)-containing RNA. The translocation of poly(A), messenger RNA, and ribosomal RNA was investigated after entrapment of these nucleic acids during the preparation of vesicles. Our data show that the complete export of only poly(A)-containing RNA from the vesicles is enhanced in the presence of 2 mM ATP. This RNA, as well as poly(A), is transported unidirectionally.

RNA-protein cross-linking in Escherichia coli 30S ribosomal subunits; determination of sites on 16S RNA that are cross-linked to proteins S3, S4, S5, S7, S8, S9, S11, S13, S19 and S21 by treatment with methyl p-azidophenyl acetimidate

RNA-protein cross-links were introduced into E. coli 30S ribosomal subunits by treatment with methyl p-azidophenyl acetimidate. After partial nuclease digestion of the RNA moiety, a number of cross-linked RNA-protein complexes were isolated by a new three-step procedure. Protein and RNA analysis of the individual complexes gave the following results: Proteins S3, S4, S5 and S8 are cross-linked to the 5'-terminal tetranucleotide of 16S RNA. S5 is also cross-linked to the 16S RNA within an oligonucleotide encompassing positions 559-561. Proteins S11, S9, S19 and S7 are cross-linked to 16S RNA within oligonucleotides encompassing positions 702-705, 1130-1131, 1223-1231 and 1238-1240, respectively. Protein S13 is cross-linked to an oligonucleotide encompassing positions 1337-1338, and is also involved in an anomalous cross-link within positions 189-191. Protein S21 is cross-linked to the 3'-terminal dodecanucleotide of the 16S RNA.

Preparation and characterization of nuclear-envelope vesicles from rat liver nuclei

We describe a procedure for the preparation of sealed nuclear-envelope vesicles from rat liver nuclei. These vesicles are strikingly similar in their polypeptide composition when compared with those of nuclear envelopes prepared conventionally using deoxyribonuclease I. Subfractionation analysis by means of extraction with high salt and urea show that the components of the nuclear envelope, e.g. the pore-complex/lamina fraction, are present. The residual DNA content is only 1.5%, and typical preparations consist of about 80% vesicles, with the vesicular character of these envelopes shown by microscopic and biochemical studies. The vesicles can be obtained in high yield, are tight and stable for at least two days and are enriched in a nucleoside triphosphatase thought to be involved in nucleocytoplasmic transport processes. Because the vesicles are largely free of components of the nuclear interior, but retain properties of intact nuclei, we believe that they are a valuable model system to study nucleocytoplasmic transport. Although in transport studies with isolated nuclei interference from intranuclear events has to be considered, the nuclear-envelope vesicles provide the possibility of studying translocation alone. Furthermore, the less complex nature of these vesicles compared with whole nuclei should facilitate investigation of the components involved in the regulation of nuclear transport processes.

Nuclear-envelope vesicles as a model system to study nucleocytoplasmic transport. Specific uptake of nuclear proteins

In the preceding paper [Riedel & Fasold (1987) Biochem. J. 241, 203-212] we have described a procedure for the preparation of nuclear-envelope vesicles (NE vesicles) from rat liver nuclei. These vesicles, which are largely free of components of the nuclear interior, were employed in an assay system in vitro to study protein translocation across the NE. We found that nuclear proteins such as histones, high-mobility-group proteins and acidic chromosomal proteins are specifically taken up and accumulated in the NE vesicles, whereas there is little or no affinity for non-nuclear proteins like immunoglobulin, myoglobin and cytochrome c. The kinetics of histone uptake into the NE vesicles are similar to those obtained for whole rat liver nuclei, and comparative studies with non-vesicular NEs prepared by deoxyribonuclease I-treatment (DNAase-NEs) indicate that the NE of the vesicles affects the uptake kinetics and increases the capacity for nuclear proteins. The uptake of histones into NE vesicles, but not the binding to DNAase-NEs, can be stimulated by GTP and GDP. Furthermore, we found that even very large molecules can be entrapped in the vesicles during their preparation. These results indicate that the NE vesicles might provide a useful system in vitro with which to investigate the structures and mechanisms involved in protein translocation across the NE.

Mediation of anion transport in oocytes of Xenopus laevis by biosynthetically inserted band 3 protein from mouse spleen erythroid cells

mRNA from the spleens of anemic mice was purified by oligo(dT)-cellulose chromatography and fractionated by density gradient centrifugation. After injection into oocytes of Xenopus laevis, two of the four fractions obtained led, after 16 h of incubation at 20 degrees C, to the expression of mouse band 3 protein, as demonstrated by immunoprecipitation with polyclonal antibodies against mouse band 3. Flux measurements showed an approximately 2- to 4-fold increment of 36Cl- uptake, which could be abolished by two different stilbene disulfonates, specific inhibitors of band 3-mediated anion transport in red blood cells.

Coupled Na+-H+ exchange in isolated acinar cells from rat exocrine pancreas

Isolated acinar cells from the rat exocrine pancreas were loaded with 6-carboxyfluorescein diacetate (CFDA), and the intracellular pH (pHi) was estimated from the pH-dependent fluorescence intensity of trapped 6-carboxyfluorescein liberated from CFDA by intracellular esterases. The intracellular fluorescence intensity was calibrated by equilibrating the internal and external pH with nigericin in K+ buffers. In the absence of Na+ (130 mmol/l K+) a pHi of 6.86 +/- 0.04 was found; in its presence (130 mmol/l Na+) a pHi of 7.17. Acute addition of Na+ increased intracellular pH with increasing Na+ concentrations, reaching a maximum at 150 mmol/l with an apparent Km of approximately 40 mmol/l. Of the different cations tested on pHi, such as Li+, K+, Rb+, and Cs+, only Li+ showed an effect on pHi similar to that of Na+. Amiloride dose dependently inhibited both Na+- and Li+-induced alkalinization (apparent Km approximately 10(-5) mol/l). In the presence of ouabain pHi was decreased by 0.2 pH units. Intracellular acidification induced by permeable buffers such as acetic acid-acetate or CO2-HCO3- was dissipated more rapidly in the presence of Na+ compared with K+ or with Na+ and amiloride in the medium. In Li+-preincubated cells intracellular acidification was higher in the absence of Li+ in the extracellular medium than in its presence. This Li+ gradient-induced acidification was dependent on the extracellular pH, was highest at an extracellular pH of 7.05, and decreased with increasing pH to 7.5. The results allow the conclusion that a coupled Na+-H+ exchange is present in pancreatic acinar cells and that the intracellular pH rather than the extracellular Na+ concentration regulates this transport mechanism.

Influence of fibrinogen on fibrin polymerization. Ultracentrifugation studies

During the transformation of fibrinogen to fibrin, excess fibrinogen suppresses further polymerization of fibrin, thereby enabling the nascent fibrin to be transported in a soluble form in blood. The question of possible complex formation between fibrin and fibrinogen was addressed by analyzing fibrin/fibrinogen (1:20, mol/mol) mixtures in the presence of calcium ions in stable linear sucrose density gradients by ultracentrifugation at 37 degrees C. During the period of ultracentrifugation in independent experiments, 40% of desAA-fibrin and 30% of desAABB-fibrin, respectively, precipitated without the participation of fibrinogen. The desAABB-fibrin, recovered in the gradient fractions, appeared as high-molecular-weight polymers (22 S), whereas the recovered desAA-fibrin exhibited only a slight increase in molecular weight (9 S) compared to fibrinogen (8 S). In contrast to this finding, both types of fibrin were totally recovered in gradient fractions provided that fibrinogen was present in the gradient at a uniform concentration of 2 mg/ml. In addition, the presence of fibrinogen but not human serum albumin reduced the size of desAABB-fibrin polymers (17 S). However, stable fibrin-fibrinogen complexes could not be demonstrated, since cosedimentation of differently labelled desAABB-fibrin and fibrinogen was not detectable. These studies suggest a specific but weak interaction of the solubilizing fibrinogen with the soluble fibrin polymers as demonstrated by a rapid exchange of both macromolecules.

Identification of the D-glucose binding polypeptide of the renal Na+-D-glucose cotransporter with a covalently binding D-glucose analog

The covalently binding D-glucose analog 10-N-(bromoacetyl)amino-1-decyl-beta-D-glucopyranoside (BADG) was synthesised and shown to be a high-affinity inhibitor of the renal Na+-D-glucose contransporter. From renal brush-border membranes a protein fraction was isolated, in which the concentration of Na+-dependent phlorizin binding sites per mg protein was enriched 7-fold. In labeling experiments with this protein fraction a polypeptide of Mr approximately 79000 was identified as containing the D-glucose binding site of the renal Na+-D-glucose cotransporter.

3'-Isothiocyanatobenzamido[3H]cholate, a new affinity label for hepatocellular membrane proteins responsible for the uptake of both bile acids and phalloidin

Substitution of the hydroxyl group on C7 of cholic acid by a benzamido group leads to a derivative with inhibiting quality for the inward transport of both bile acids and phallotoxins by isolated liver cells. The tritiated isothiocyanate derivative was prepared (3'- isothiocyanatobenzamido [3H]cholate, [3H] IBCA ) with a specific activity of 70-80 mCi/mmol. The latter compound was used for affinity labeling of liver plasma membranes in order to detect chemically modified proteins involved in the transport of bile acids. [3H] IBCA and the noncovalently binding analogs were recognized by the transport system; they inhibited the uptake of both [14C]cholate and of demethyl[3H] phalloin in vitro. Isothiocyanatobenzamidocholate ( IBCA ) was able to protect isolated hepatocytes against phalloidin. In isolated and purified plasma membranes prepared from liver cells [3H] IBCA binds to saturable sites in an irreversible manner. Micromolar concentrations of unlabeled IBCA or millimolar concentrations of natural substrates prevented [3H] IBCA binding in a concentration dependent manner; some other substrates of the transport system also protected liver membranes against chemical modification. Membranes from AS- 3OD hepatoma cells, well known to transport neither bile acids nor phallotoxins, could not be labeled by [3H] IBCA . The major targets of labeling in hepatocellular plasma membranes were polypeptides with molecular mass of 67, 60, 54, 50, and 37 kDa as shown by SDS-polyacrylamide gel electrophoresis (10% acrylamide). The 67 kDa protein could be found in the aqueous phase after phase separation in Triton X-114. The 54 kDa and 50 kDa proteins remained in the detergent phase and can therefore be regarded as integral membrane proteins.

A stopped flow capillary perfusion method to evaluate contraluminal transport parameters of methylsuccinate from interstitium into renal proximal tubular cells

In order to study the transport of dicarboxylic acids through the contraluminal cell membrane of proximal tubular cells, 3H- methylsuccinate has been synthetized by catalytic hydration of methylfumarate . As the chromatography of radioactive material excreted in the urine after i.v. injection of 3H- methylsuccinate shows, no metabolite is detectable during the first 3 min. After 10 min, less than 10% of the excreted radiolabel is metabolized. To measure the contraluminal influx of 3H- methylsuccinate from the interstitium into cortical tubular cells, the renal vessels were clamped so that the proximal tubular lumina collapsed. Then Ringer solution was injected into the blood capillaries. It contained different concentrations of 3H- methylsuccinate and 14C-inulin as extracellular space marker. After contact times between 1 and 10 s, this fluid was withdrawn from the capillaries and the disappearance of 3H- methylsuccinate relative to 14C-inulin was measured. The morphological compartments in the outer cortex of the clamped glutaraldehyde-fixed kidney were evaluated by a stereological method. For proximal tubular cells a ratio of extracellular water space to intracellular space of 1:3.1 and a ratio extracellular water space to free cell water space of 1:2 was found. It was tested whether the experimental disappearance curves with 4 different starting concentrations of 3H- methylsuccinate fit with the data from four model calculations. It was found that the data and the conditions of transport are consistent with the predictions of a facilitated diffusion model.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretion and contraluminal uptake of dicarboxylic acids in the proximal convolution of rat kidney

The transport of dicarboxylic acids in the proximal convolution was investigated by measuring: a) the zero net flux transtubular concentration difference of DL-methyl-succinate, b) its 2-s influx from the interstitium into tubular cells, and c) its 3.5-s efflux from the tubular lumen. With the first method a luminal concentration exceeding the peritubular concentration was observed, thus indicating a net active transtubular secretion of this slowly metabolized substance. All transport steps, luminal and contraluminal , as well as the overall transport, were Na+-dependent and inhibited by lithium (apparent Ki approximately equal to 1.8 mmol/l). The overall transport of methylsuccinate , as well as the contraluminal influx into proximal tubular cells, could be inhibited by paraaminohippurate and H2-DIDS with an apparent Ki of approximately equal to 1.8 mmol/l, by taurocholate with an apparent Ki approximately equal to 3.1 mmol/l and by pyruvate with an apparent Ki approximately equal to 5 mmol/l, but not by sulfate, thiosulfate, L-lactate, oxalate and urate. As judged from the inhibition of contraluminal methylsuccinate influx by 48 dicarboxylic acids (aliphatic and aromatic), a specificity pattern was observed similar to that of inhibition of luminal efflux of 2-oxoglutarate [22]: a preference of dicarboxylates in the transconfiguration with a chain length of 4-5 carbons; little change in the inhibitory potency with CH3-, OH-, SH- and O=, but strong reduction with a NH3+ in the 2 position; only a small reduction of inhibitory potency with 2,3 disubstituted SH and OH analogs; preference of the dicarboxylic benzene in the 1,4 position and of the diacetyl benzene in the 1,2 position. The data indicate a Na+-dependent dicarboxylic transport system at the contraluminal cell side of the proximal tubule which is very similar to the luminal transport system for dicarboxylic acids.

Further characterization of membrane proteins involved in the transport of organic anions in hepatocytes. Comparison of two different affinity labels: 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid and brominated taurodehydrocholic acid

4,4'-Diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid (H2DIDS) known as an irreversible inhibitor of the anion transport in red blood cells (Cabantchik, Z.I. and Rothstein, A. (1972) J. Membrane Biol. 10, 311-330) blocks also the uptake of bile acids and of some foreign substrates in isolated hepatocytes (Petzinger, E. and Frimmer, M. (1980) Arch. Toxicol. 44, 127-135). [3H]H2DIDS was used for labeling of membrane proteins probably involved in anion transport of rat liver cells. The membrane proteins modified in vitro by [3H]H2DIDS were compared with those labeled by brominated taurodehydrocholic acid. The latter is one of a series of suitable taurocholate derivatives, all able to bind to defined membrane proteins of hepatocytes and also known to block the uptake of bile acids as well as of phallotoxins and of cholecystographic agents (Ziegler, K., Frimmer, M., Möller, W. and Fasold, H. (1982) Naunyn-Schmiedeberg's Arch. Pharmacol. 319, 254-261). The radiolabeled proteins were compared after SDS-electrophoresis with and without reducing agent present, solubilization by detergents, two-dimensional electrophoresis and after separation of integral and peripheral proteins. Our results suggest that the anion transport system of liver cells cannot distinguish between bile acids and the anionic stilbene derivative (DIDS). The labeling pattern for both kinds of affinity labels was very similar. Various combinations of separation techniques gave evidence that the radiolabeled membrane proteins are not subunits of a single native channel protein.

The kinetics of intramolecular cross-linking of the band 3 protein in the red blood cell membrane by 4,4'-diisothiocyano dihydrostilbene-2,2'-disulfonic acid (H2DIDS)

The two isothiocyanate groups of the anion transport inhibitor 4,4'-diisothiocyano dihydrostilbene-2-2'-disulfonate (H2DIDS) may react covalently with two lysine residues called a and b that reside on the chymotryptic 60,000 Dalton and 35,000 Dalton segments, respectively, of the band 3 protein of the human erythrocyte membrane. Under suitable conditions, the reaction leads to the establishment of intramolecular cross-links between a and b (M.L. Jennings & H. Passow, 1979, Biochim. Biophys. Acta 554:498-519). In the present work, the time course of the reactions with a and b, and of the establishment of the cross-link were investigated experimentally and compared with simple mathematical models of the reaction sequence. The rates of reaction with a and b were found to increase with increasing pH. Regardless of pH, the rate of reaction with a exceeds that with b several-fold. Once the H2DIDS molecule has reacted with a, the rate of the subsequent reaction of the other isothiocyanate group with b is reduced by about 1/30. The reactions that follow the unilateral attachment to site b are not yet clear. A more detailed analysis of the time course of the cross-linking reaction suggests that a satisfactory description of the kinetics requires the assumption that the H2DIDS binding site may exist in two different states, and that the transition from one state to the other is associated with changes of the reactivities of either lys a alone or of both lys a and b. This led to the formulation of the two-states model of the H2DIDS binding site, which is supported by other pieces of independent evidence. The analysis of the pH dependence of the rate of thiocyanylation of b shows that the apparent pK value of that lysine residue is about 9.9 to 10.0 and hence slightly lower than the intrinsic pK of a lysine residue in an aqueous environment.

Characterization of an ATPase on the inside of rat-liver nuclear envelopes by affinity labeling

Nuclear envelope membranes from rat liver cells contain ATPases, one of which can be inhibited and irreversibly labeled by (S-dinitrophenyl)-6-mercaptopurine riboside triphosphate. Inhibition and covalent substitution of the ATPase are achieved only after disruption of the nuclei, the ATP analogue is inactive on the ATPase activity of whole nuclei or on vesicles of the membrane prepared after a modified heparin method of Bornens and Courvalin. Electron micrographs and scanning micrographs helped to establish the characterization of closed vesicles and intact nuclei. With the aid of (alpha-32P)-labeled, and of the (beta, gamma-32P)-labeled analogue, it was possible to demonstrate the incorporation of the nucleotide into a few protein regions of the nuclear membrane disc electrophoresis pattern.

Affinity labels for membrane components involved in the uptake of bile acids and of phallotoxins by hepatocytes. Development of covalently binding derivatives of bile acids and of compounds related to cholecystographic agents

A series of covalently binding derivatives of bile acids, fusidic acid and of compounds similar to cholecystographic agents were synthesized. Nearly all of them inhibited the development of protrusions on the surface of isolated hepatocytes regularly seen after treatment with phalloidin. The same compounds inhibited the uptake of demethylphalloin and of cholate in a concentration dependent manner. Two kinds of effects could be distinguished: The irreversible part of the inhibition depended on the incubation period and could not be removed by washing procedures. The reversible one was independent on the duration of the preincubation. Final results indicated that the tested derivatives inhibited either both transports, and the phalloidin response of liver cells to the same degree and in the same manner, or were found to be ineffective in all tests. The above parallelism supports the hypothesis that phallotoxins may be translocated by a carrier system normally responsible for the uptake of bile acids from the portal blood.

Chemical modification of membrane proteins by brominated taurodehydrocholate in isolated hepatocytes; relationship to the uptake of cholate and of phalloidin and to the sensitivity of hepatocytes to phalloidin

In vitro treatment of isolated rat hepatocytes with brominated taurodehydrocholic acid (BTC) reduced their sensitivity against phalloidin and inhibited the uptake of phalloidin as well as of cholate in an irreversible and concentration dependent manner. BTC was taken up itself by liver cells; this process was inhibited by 4,4'-diisothiocyano 2,2'-stilbene disulfonate (DIDS). When hepatocytes were incubated with 35S-BTC their plasma membranes contained five labeled protein species with molecular weights of 67,000, 49,000, 38,000, 32,000 and 24,000 as shown by SDS-electrophoresis. No marked difference was observed when isolated plasma membranes from livers were directly treated with the affinity label. DIDS suppressed covalent binding of 35S-BTC to membrane components drastically. Incubation of phalloidin insensitive AS-30D ascites hepatoma cells with 35S-BTC did not result in a chemical modification of the above five proteins. This agrees with an earlier observation that hepatoma cells are unable to take up phalloidin and bile acids (Petzinger et al. 1979; Rufeger and Grundmann 1977; Kroker et al. 1978).