Binding of apoA-IV-phospholipid complexes to plasma membranes of rat liver

Rat apoA-IV complexes with dimyristoyl phosphatidylcholine (apoA-IV-DMPC) have been prepared and their ability to bind to purified rat liver plasma membranes investigated. Binding equilibrium at 37 degrees C was reached in 30 minutes. Saturation binding experiments and subsequent analysis of the results with Scatchard plots gave results consistent with the presence of a single saturable binding site. DMPC or POPC unilamellar vesicles could not compete with apoA-IV-DMPC for binding; apoA-I-DMPC competed only partially. ApoE-poor HDL effectively competed with apoA-IV-DMPC. The fact that binding could be greatly reduced (greater than 70%) by preincubating the membrane with pronase (18 micrograms/ml), supports the conclusion that a membrane protein is involved in binding. Based on these results, we speculate that the rapid catabolism of apoA-IV in plasma may be mediated by a specific uptake mechanism in the liver. The implications of these results support the hypothesis that apoA-IV is involved in reverse cholesterol transport.     

Binding of fluorescent lanthanides to rat liver mitochondrial membranes and calcium ion-binding proteins.

(1) Tb3+ binding to mitochondrial membranes can be monitored by enhanced ion fluorescence at 545 nm with excitation at 285 nm. At low protein concentrations (less than 30 mug/ml) no inner filter effects are observed. (2) This binding is localized at the external surface of the inner membrane and is unaffected by inhibitors of respiration or oxidative phosphorylation. (3) A soluble Ca2+ binding protein isolated according to Lehninger, A.L. ((1971) Biochem. Biophys. Res. Commun. 42, 312-317) also binds Tb3+ with enhanced ion fluorescence upon excitation at 285 nm. The excitation spectrum of the isolated protein and of the intact mitochondria are indicative of an aromatic amino acid at the cation binding site. (4) Further characterization of the Tb3+-protein interaction revealed that there is more than one binding site per protein molecule and that these sites are clustered (less than 20 A). Neuraminidase treatment or organic solvent extraction of the protein did not affect fluorescent Tb3+ binding. (5) pH dependency studies of Tb3+ binding to the isolated protein or intact mitochondria demonstrated the importance of an ionizable group of pK greater than 6. At pH less than 7.5 the amount of Tb3+ bound to the isolated protein decreased with increase in pH as monitored by Tb3+ fluorescence. With intact mitochondria the opposite occurred with a large increase in Tb3+ fluorescence at higher pH. This increase was not observed when the mitochondria were preincubated with antimycin A and rotenone.     

Three actin-binding proteins are developmentally regulated in rat liver plasma membranes

Actin-binding membrane proteins (linking microfilaments to the cell membrane) are involved in cytoskeleton-membrane interactions which are supposed to undergo profound changes during cell proliferation and development. In this study 8 polypeptides were shown to bind F-actin directly in the liver cell membranes of mature rats. From these, the abundance of three polypeptides, of 130, 50 and 36 kDa, was observed to increase considerably during postnatal development, which indicates a developmental change in the cytoskeleton-membrane interactions.     

Identification of a fodrin-like protein in rat liver basolateral membranes

A 240 KDa calmodulin- and actin-binding protein has been identified in the plasma membrane of rat liver. This protein is mainly associated with subplasmamembrane fractions enriched in the basolateral domain and very little of it is found in the canalicular membrane fraction. An 80 KDa actin-binding protein is found only in the canalicular fraction.     

Characterization of rat liver cell plasma membranes.

A method is described by which bile canalicular membranes (BCM) can be prepared, together with canaliculus-free plasma membrane (PM), both essentially free of contamination. The recovery of both fractions together was estimated to be 46%. The concentrations of total lipid, total phospholipid and cholesterol were substantially greater in the BCM, and polyacrylamide gel electrophoresis revealed differences in protein composition. The differences in lipid and protein composition of these two plasma membrane fractions are presumably related to their very different physiological functions.     

Photoaffinity labelling of nucleoside-transport proteins in plasma membranes isolated from rat and guinea-pig liver.

Nitrobenzylthioinosine (NBMPR) was employed as a probe of the nucleoside transporters from rat and guinea-pig liver. Purified liver plasma membranes prepared on self-generating Percoll density gradients exhibited 16-fold (rat) and 10-fold (guinea pig) higher [3H]NBMPR-binding activities than in crude liver homogenates (3.69 and 14.7 pmol/mg of protein for rat and guinea-pig liver membranes respectively, and 0.23 and 1.47 pmol/mg of protein for crude liver homogenates respectively). Binding to membranes from both species was saturable (apparent Kd 0.14 and 0.63 nM for rat and guinea-pig membranes respectively) and inhibited by uridine, adenosine, nitrobenzylthioguanosine (NBTGR) and dilazep. Uridine was an apparent competitive inhibitor of high-affinity NBMPR binding to rat membranes (apparent Ki 1.5 mM). There was a marked species difference with respect to dipyridamole inhibition of NBMPR binding (50% inhibition at 0.2 and greater than 100 microM for guinea-pig and rat respectively). These results are consistent with a role of NBMPR-binding proteins in liver nucleoside transport. Exposure of rat and guinea pig membranes to high-intensity u.v. light in the presence of [3H]NBMPR resulted in the selective radio-labelling of membrane proteins which migrated on sodium dodecyl sulphate/polyacrylamide gels with apparent Mr values in the same range as that of the human erythrocyte nucleoside transporter (45 000-66 000). Covalent labelling of these proteins was abolished when photolysis was performed in the presence of non-radio-active NBTGR as competing ligand.     

Binding and degradation of 125I-glucagon by highly purified rat liver plasma membranes

¹²⁵I-glucagon binding and degradation were studied in highly purified plasma membranes from rat livers. Specific ¹²⁵I-glucagon binding increased rapidly with time at 30°C and reached a maximum between 30 and 120 min. At 120 min the labelled material present in the supernatants from incubation mixtures had extensively lost its ability to rebind to fresh membranes whatever the glucagon concentration. This impairment was not due to the release of a degradative activity into the incubation mixture, suggesting a membrane-mediated process. The presence of proteinase inhibitors (bacitracin/aprotinin) resulted both in an increase in specific ¹²⁵I-glucagon binding to membranes and an improvement in the ability of the labelled material from the supernatant to rebind to fresh membranes. When analysed by Bio-Gel P-10 chromatography the loss in the ability of the labelled material in the supernatants to rebind to fresh membranes correlated with a decrease in the labelled material which eluted as ¹²⁵I-glucagon from the column. Chromatographic analysis overestimated ¹²⁵I-glucagon when compared to the radioreceptor assay. The labelled material extracted from membranes by Triton X-100 solubilization or dissociated from membranes after exposure to an excess of unlabelled glucagon mainly eluted as ¹²⁵I-glucagon. However, a significant amount (20–30%) of the labelled material eluted in the low molecular weight region.     

Uridine transport in basolateral plasma membrane vesicles from rat liver

Summary The characteristics of uridine transport were studied in basolateral plasma membrane vesicles isolated from rat liver. Uridine was not metabolized under transport measurement conditions and was taken up into an osmotically active space with no significant binding of uridine to the membrane vesicles. Uridine uptake was sodium dependent, showing no significant stimulation by other monovalent cations. Kinetic analysis of the sodium-dependent component showed a single system with Michaelis-Menten kinetics. Parameter values were K _M 8.9 m and V _max 0.57 pmol/mg prot/sec. Uridine transport proved to be electrogenic, since, firstly, the Hill plot of the kinetic data suggested a 1 uridine: 1 Na⁺ stoichiometry, secondly, valinomycin enhanced basal uridine uptake rates and, thirdly, the permeant nature of the Na⁺ counterions determined uridine transport rates (SCN^– > NO ₃ ^– > Cl^– > SO ₄ ^2– ). Other purines and pyrimidines cis-inhibited and trans-stimulated uridine uptake.This work has been partially supported by grant PM90-0162 from D.G.I.C.Y.T. (Ministerio de Educación y Ciencia, Spain). B.R.-M. is a research fellow supported by the Nestlé Nutrition Research Grant Programme.     

Small GTP-binding proteins on rat liver lysosomal membranes.

GTP-binding proteins have been identified on the membranes of highly purified dextran-filled lysosomes (dextranosomes) and Triton-filled lysosomes (tritosomes) obtained from rat liver. Autoradiography of blots of lysosomal membrane proteins incubated with [alpha-32P]GTP revealed the presence of several specific GTP-binding proteins with a relative molecular mass (M(r)) predominantly in the range of 26-30 kDa. These GTP-binding proteins migrated slower in polyacrylamide gels than purified c-Ha-ras protein expressed in E. coli, whose apparent M(r) was 23 kDa in the same blot. The relative contents of GTP-binding proteins in lysosomal membranes were comparable or greater than that of plasma membranes and of microsomes. Chemical extraction showed that lysosomal GTP-binding proteins were more tightly associated with the membranes than with microsomal GTP-binding proteins. The possible involvement of lysosomal GTP-binding proteins in cellular functions including vacuolar (lysosomal) acidification and organellar dynamics are discussed.     

Tocopherol-binding proteins in membranes of rat liver mitochondria

The use of the adsorption chromatography on the hydroxyl apatite makes it possible to yield and partly purify the triton X-100-solubilized mitochondrial protein fraction of the rat liver able to bind specifically [3H]-alpha-tocopherol. The method permits removing simultaneously both free detergent and [3H]-alpha-tocopherol from the protein mixture without disturbance of the established equilibrium. When compared with methods used for the removal of free hydrophobic ligands in the in vitro binding experiments, the applied method is the most effective.     

Binding of aminoacyl-tRNA to rat liver ribosomal proteins

Rat liver ribosome treatment with ethanol and 1 M NH₄Cl releases some 31–33 ribosomal proteins. This split protein fraction binds Phe-tRNA, Ac-Phe-tRNA, Met-tRNA_M and f-Met-tRNA_F in the absence of K⁺ and Mg⁺⁺ ions. When the split protein fraction is passed through Sephadex G-100 only six proteins are retained in the column: S10, S14, S15, S19, L35, and L36. The aminoacyl-tRNA binding activity of this protein fraction retained in the Sephadex G-100 column is similar to that of the total split protein fraction, suggesting that the above six proteins, or only some of them, are involved in the binding reaction.     

High affinity binding proteins for pancreatic polypeptide on rat liver membranes.

We report here the identification on rat liver plasma membranes and microsomes of proteins that bind pancreatic polypeptide (PP) with high affinity and specificity (plasma membranes: KD = 4.6 nM, Bmax = 3.28 pmol/mg protein; microsomes: KD = 3.45 nM, Bmax = 18.7 pmol/mg protein). These binding proteins appeared coupled to a G-protein, since 0.1 mM guanosine 5'-O-(3-thiotriphosphate) decreased the affinity by half. When 125I-labeled PP-binding protein complexes covalently cross-linked with disuccinimido suberate were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, two radioactive bands with M(r) values of 52,000 and 38,000 were demonstrated. Both bands were inhibited by unlabeled PP with an IC50 of approximately 5 nM (but not by neuropeptide Y or peptide YY). After the cross-linked complexes were solubilized from liver microsomes with 0.2% Triton X-100 and gel-filtered, they did not interact with the lectins wheat germ agglutinin, Ulex europaeus agglutinin, Ricinus communis agglutinin, and soy bean agglutinin. That these binding proteins may not be glycosylated was further supported by the failure of either peptide N-glycosidase F and endo-beta-N-acetylglucosaminidase F to alter the size of the PP-binding protein complexes on gel electrophoresis. These PP-binding proteins may serve as receptors and mediate a hepatic effect of PP.     

Isolation of Raja erinacea basolateral liver plasma membranes: characterization of lipid composition and fluidity.

Developing a method for isolating skate (Raja erinacea) basolateral liver plasma membranes, as well as characterizing the lipid composition and fluidity of these membranes, was the primary purpose of this study. Membranes were isolated using self-generating Percoll gradients. Marker enzyme studies indicate that this preparation is highly enriched in the basolateral domain of the liver plasma membrane and largely free of contamination by intracellular organelles or canalicular membranes. Further, these membranes contain the agency responsible for Na(+)-dependent alanine transport. This finding indicates that this membrane preparation will be useful for the study of skate liver plasma membrane transport processes. The lipid composition and fluidity (as assessed by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene) of the skate basolateral liver plasma membrane shows little variation among preparations. Further, DPH anisotropy plotted as a function of temperature yields a straight line (r = 0.99) which indicates that there is no lipid phase change in these membranes from 4 degrees to 37 degrees C. The membrane preparation does contain substantial phospholipase A2 activity. The function of this enzyme is, in part, to modify membrane lipid composition and fluidity in response to temperature variations; therefore, this finding suggests that in situ lipid metabolizing enzymes may play a central role in the adaptation of skate basolateral liver plasma membranes to changes in the ambient temperature.