Surface-exposed proteins of 3T3-L1 adipocytes: identification of phosphorylated, insulin-translocated, and recycling proteins.

Twenty-seven adipocyte-specific, cell surface-exposed proteins were detected by derivatizing undifferentiated and differentiated 3T3-L1 cells with membrane impermeant sulfosuccinimidyl 2-(biotinamido)ethyl-1,3-dithiopropionate at 0 degrees C. Biotinylated proteins were adsorbed onto streptavidin-agarose, resolved on two-dimensional polyacrylamide gels, and detected by autoradiography or silver staining. Of the surface-exposed proteins specific to adipocytes, three were phosphorylated and seven were glycoproteins that bound to wheat germ agglutinin and eluted with N-acetylglucosamine. Eleven of the adipocyte-specific proteins were bound to streptavidin-agarose after the cells were biotinylated at 20 degrees C and then stripped with glutathione at 0 degrees C to isolate plasma membrane proteins that localize to recycling endosomes as well as the cell surface. When insulin-deprived cells were acutely treated with insulin, only a few proteins, including one protein tentatively identified as the GLUT4 glucose transporter, were found to increase in concentration at the cell surface. These latter results imply that up-regulation of glucose transport by the translocation of GLUT4 to the cell surface in response to insulin occurs by exocytic fusion of an intracellular compartment having a limited number of proteins.     

Compartmentation and turnover of the low density lipoprotein receptor in skin fibroblasts

The low density lipoprotein receptor (LDLR) was immunoprecipitated from [35S]methionine-labeled skin fibroblasts derivatized at 4 or 18 degrees C with an impermeant biotinylating reagent. Separation of derivatized and underivatized receptor from immunoprecipitates by selective binding to streptavidin-agarose allowed assessment of receptor protein cellular compartmentation and rates of intercompartmental transfer. At both 4 and 18 degrees C the amount of LDLR that is derivatized in cells labeled to near steady state saturates after 1-2 h of reaction at, respectively, 47 and 70% of total immunoprecipitable receptor protein. On the basis of temperature titration experiments, protein exposed only to the cell surface reacts at 4 degrees C; raising the temperature of biotinylation to 18 degrees C provides access to an additional pool of receptor protein. Remaining LDLR is derivatized at 37 degrees C. LDLR unreactive at 18 degrees C largely resides in membrane compartment(s) devoid of plasma membrane on the basis of its fractionation on Percoll gradients. While total cellular LDLR and 4 degrees C-derivatized LDLR labeled to steady state turn over in a first order manner (t1/2 = 12-13 h), the specific activity of pulse-labeled, 4 degrees C-accessible protein peaks after 1-2 h of chase and reaches a reduced level by 3 h of chase. These latter results show that the newly synthesized LDLR is transiently enriched at the cell surface prior to achieving equilibrium distribution between the cell surface and intracellular pools.     

Transcytosis in MDCK cells: identification of glycoproteins transported bidirectionally between both plasma membrane domains

MDCK cells display fluid-phase transcytosis in both directions across the cell. Transcytosis of cell surface molecules was estimated by electron microscopic analysis of streptavidin-gold-labeled frozen sections of biotinylated cells. Within 3 h, approximately 10% of the surface molecules, biotinylated on the starting membrane domain, were detected on the opposite surface domain irrespective of the direction of transcytosis. This suggests that the transcytosis rates for surface molecules are equal in both directions across the cell as shown previously for fluid-phase markers. A biochemical assay was established to identify transcytosing glycoproteins in MDCKII-RCAr cells, a ricin- resistant mutant of MDCK. Due to a galactosylation defect, surface glycoproteins of these cells can be labeled efficiently with [3H]galactose. Transcytosis of [3H]galactose-labeled glycoproteins to the opposite membrane domain was detected by surface biotinylation. Detergent-solubilized glycoproteins derivatized with biotin were adsorbed onto streptavidin-agarose and separated by SDS-PAGE. A subset of the cell surface glycoproteins was shown to undergo transcytosis. Transport of these glycoproteins across the cell was time and temperature dependent. By comparative two-dimensional gel analysis, three classes of glycoproteins were defined. Two groups of glycoproteins were found to be transported unidirectionally by transcytosis, one from the apical to the basolateral surface and another from the basolateral to the apical surface. A third group of glycoproteins which has not been described previously, was found to be transported bidirectionally across the cell.     

Dynamic nature of the association of large tumor antigen and p53 cellular protein with the surfaces of simian virus 40-transformed cells

A molecular complex of simian virus 40 large tumor antigen (T-Ag) and p53 cellular protein is present on the surface of simian virus 40-transformed mouse cells. The stability of the association of the two proteins with the cell surface was characterized. Cells were either surface iodinated by the lactoperoxidase technique or metabolically labeled with [35S]methionine, and surface antigens were detected by differential immunoprecipitation with specific antibodies immediately after labeling or after incubation at 37 degrees C. A rapid, concomitant disappearance of T-Ag and p53 from the cell surface was observed. The half-life of iodinated surface T-Ag was less than 30 min, whereas that of [35S]methionine-labeled surface T-Ag was 1 to 2 h. Although T-Ag and p53 were rapidly lost, both were also rapidly replaced on the cell surface, since newly exposed molecules could be detected when cells were reiodinated after a 2-h chase period. Control experiments established that the loss of the surface molecules was not induced by the iodination reaction. The appearance of surface T-Ag was prevented when cellular protein synthesis was inhibited with cycloheximide. The disappearance and replacement of T-Ag and p53 appeared to be energy-independent processes, as neither was inhibited by sodium azide or 2,4-dinitrophenol. Incubation of iodinated cells at 4 degrees C did block the loss of T-Ag and p53. These observations suggest that T-Ag and p53 are coordinately turned over in the plasma membrane. The nature of the association of the T-Ag-p53 complex with the cell surface can best be described as highly dynamic.     

Patterns of proteins synthesized in the R15 neuron of Aplysia. Temporal studies and evidence for processing

The time-course of changes in the pattern of newly synthesized proteins in the R15 neuron of the parietovisceral ganglion of Aplysia californica has been studied at 14 degrees C. 5% polyacrylamide gels containing sodium dodecyl sulfate (SDS) have been used to separate newly synthesized (leucine-labeled) proteins from the neuron. We have demonstrated that the pattern of newly synthesized proteins from the R15 neuron does not change significantly if 5-h pulses of labeled leucine are given during the first 72 h of in vitro incubation of the excised ganglion. However, the level of leucine incorporation begins to decline somewhere between 17 and 43 h after the ganglion is isolated; at 43 and 69 h the levels of incorporation fell to 29 and 10% of the initial level, respectively. A number of conclusions have been drawn from the use of a sequential, double-label type of experiment in the same cell. There is processing of SDS-soluble, 12,000-dalton (12k) material to 6,000-9,000-dalton (6-9k) material. These materials are the two major peaks on gels after long labeling periods and together account for about 35% of all newly synthesized proteins. After synthesis of 12k material, there is a gradual disappearance of 12k (half-life about 8 h) and simultaneous appearance of 6-9k material on the gels, as the postsynthesis "chase" period of ganglia incubation is increased. The processing of 12k to 6-9k material occurs even in the presence of anisomycin, a protein syntehsis inhibitor, during the chase period. While the rate of 12k to 6-9k conversion can vary from cell to cell, it appears to remain consistent within, and is characteristic of, any individual R15. We detect no circadian rhythm in either the rate of 12k synthesis or the rate of 12k to 6-9k processing with 5-h label periods. These results are discussed in relation to the roles of 12k and 6-9k material in the R15 neuron.     

Interleukin-3 stimulates the tyrosine phosphorylation of the 140-kilodalton interleukin-3 receptor

Murine interleukin-3 (mIL-3) stimulates the rapid and transient tyrosine phosphorylation of a number of proteins in mIL-3-dependent B6SUtA1 cells. Two of these proteins, p68 and p140, are maximally phosphorylated at tyrosine residues within 2 min of addition of mIL-3. Because 125I-mIL-3 can be cross-linked to both 70- and 140-kDa proteins on intact B6SUtA1 cells, we investigated whether the tyrosine phosphorylated p68 and p140 were these two mIL-3 receptor proteins. Addition of antiphosphotyrosine antibodies (alpha PTyr Abs) to cell lysates from B6SUtA1 cells, to which 125I-mIL-3 had been disuccinimidyl suberate-cross-linked, resulted in the immunoprecipitation of 125I-mIL-3 complexed to both 70- and 140-kDa proteins. To determine if the observed immunoprecipitation pattern was due to the direct interaction of alpha-PTyr Abs with these two mIL-3 receptor proteins or with tyrosine-phosphorylated proteins that were associated with the receptor proteins, cell lysates were treated with 2% sodium dodecyl sulfate, 5% 2-mercaptoethanol, and boiled for 1 min. After removal of sodium dodecyl sulfate and 2-mercaptoethanol, alpha PTyr Abs immunoprecipitated 125I-mIL-3 cross-linked to only the 140-kDa protein. To confirm this finding, 32P-labeled B6SUtA1 cells were treated with biotinylated or fluoresceinated mIL-3. Addition of immobilized streptavidin or antifluorescein antibodies, respectively, to cell lysates from these cells resulted in the enrichment of only a 140-kDa tyrosine phosphorylated protein. Taken together, these results strongly suggest that only the 140-kDa receptor protein is tyrosine phosphorylated upon mIL-3 binding.     

Immobilization of lipase on hydrophobic nano-sized magnetite particles

As a tool for the stable enzyme reuse, enzyme immobilization has been studied for several decades. Surface-modified nano-sized magnetite (S-NSM) particles have been suggested as a support for the immobilization of enzyme in this study. Based on the finding that a lipase is strongly adsorbed onto a hydrophobic surface, NSM particles (8–12 nm) were made hydrophobic by binding of sodium dodecyl sulfate via a sulfate ester bond. Various types of measurements, such as transmission electron microscopy, X-ray diffraction, infrared spectroscopy, vibration sample magnetometer, and thermo gravimetric analysis, were conducted in characterizing S-NSM nanoparticles. S-NSM particles were used for the adsorption of porcine pancreas lipase (PPL). A dodecyl carbon chain is expected to form a spacer between the surface of the NSM and the lipase adsorbed. The immobilized PPL showed the higher specific activity of oil hydrolysis than that of free one. Immobilized PPL could be recovered by magnetic separation, and showed the constant activity during the recycles.     

A study on the separation of reconstituted proteoliposomes and unincorporated membrane proteins by use of hydrophobic affinity gels, with special reference to band 3 from bovine erythrocyte membranes

Alkyl-Sepharose 4B with octyl, decyl, or dodecyl groups as an alkyl chain was a good adsorbent for any type of detergents and a variety of proteins, but not for phospholipids in a vesicle form. When these gels were added to the mixtures of reconstituted proteoliposomes prepared by using bovine band 3 and the protein unincorporated into liposomes, free band 3 in solution was adsorbed onto the gels and the proteoliposomes could be recovered by filtration, suggesting that this procedure, when applicable, permits a rapid isolation of proteoliposomes without loss and dilution of the sample. In addition, the results indicated that Bio-Beads SM-2 resin, which is virtually nonadsorbing for most proteins, can be used in removing any kind of detergents from those protein-detergent mixtures.     

Affinity purification of human granulocyte macrophage colony-stimulating factor receptor alpha-chain. Demonstration of binding by photoaffinity labeling

The human granulocyte macrophage colony-stimulating factor (GM-CSF) receptor alpha-chain, a low affinity component of the receptor, was solubilized and affinity-purified from human placenta using biotinylated GM-CSF. Scatchard analysis of 125I-GM-CSF binding to the placental membrane extract disclosed that the GM-CSF receptor had a dissociation constant (Kd) of 0.5-0.8 nM, corresponding to the Kd value of the GM-CSF receptor alpha-chain on the intact placental membrane. Affinity labeling of the solubilized protein using a photoreactive cross-linking agent, N-hydroxysuccinimidyl-4-azidobenzoate (HSAB), demonstrated a single specific band of 70-95 kDa representing a ligand-receptor complex. Approximately 2 g of the placental membrane extract was subjected to a biotinylated GM-CSF-fixed streptavidin-agarose column, resulting in a single major band at 70 kDa on a silver-stained sodium dodecyl sulfate gel. The radioiodination for the purified material disclosed that the purified protein had an approximate molecular mass of 70 kDa and a pI of 6.6. Binding activity of the purified material was demonstrated by photoaffinity labeling using HSAB-125I-GM-CSF, producing a similar specific band at 70-95 kDa as was demonstrated for the crude protein.     

Biotinylated proteins as molecular weight standards on Western blots

Protein molecular weight standards were biotinylated by reaction with biotinyl-N-hydroxysuccinimide ester. The biotinylated proteins were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoretically transferred to nitrocellulose paper. The resolved protein bands were detected by formation of a streptavidin-biotin/horseradish peroxidase complex and reaction with 4-chloro-1-naphthol and hydrogen peroxide. The biotinylated proteins are easy to prepare and are useful as molecular weight standards with most procedures employing immunodetection of proteins following transfer to nitrocellulose paper.     

Internalization and stepwise degradation of heparan sulfate proteoglycans in rat hepatocytes.

Intracellular transport and degradation of membrane anchored heparan sulfate proteoglycans (HSPGs) were studied in cultured rat hepatocytes labeled with [35S]sulfate and [3H]glucosamine. Pulse chase experiments showed that membrane anchored HSPGs were constitutively transported to the cell surface after completion of polymerization and modification of the glycosaminoglycan chains in the Golgi apparatus. The intact HSPGs had a relatively short residence time at the cell surface and in non-degrading compartments (T(1/2) approximately 2-3 h), while [35S]sulfate labeled degradation products were found in lysosomes, and to a lesser extent in late endosomes. These degradation products which were free heparan sulfate chains with little or no protein covalently attached, were approximately half the size of the original glycosaminoglycan chains and were the only degradation intermediate found in the course of HSPG catabolism in these cells. In cells incubated in the presence of the microtubule perturbant vinblastine, or in the presence of the vacuolar ATPase inhibitor bafilomycin A1, and in cells incubated at 19 degrees C, the endocytosed HSPGs were retained in endosomes and no degradation products were detected. Disruption of lysosomes with glycyl-phenylalanine 2-naphthylamide (GPN) revealed a GPN resistant degradative compartment with both intact and partially degraded HSPGs. This compartment probably corresponds to late endosomes. Treatment of hepatocytes with the thiol protease inhibitor leupeptin inhibited the final degradation of the protein moiety of the HSPGs. The protein portion seems to be degraded completely before the glycosaminoglycan chains are cleaved. The degradation of the glycosaminoglycan chains is rapid and complete with one observable intermediate.     

Insulin modulation of hepatic synthesis and secretion of apolipoprotein B by rat hepatocytes

Insulin inhibition of apolipoprotein B (apoB) secretion by primary cultures of rat hepatocytes was investigated in pulse-chase experiments using [35S]methionine as label. Radioactivity incorporation into apoBH and apoBL, the higher and lower molecular weight forms, was assessed after immunoprecipitation of detergent-solubilized cells and media and separation of the apoB forms using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Hepatocyte monolayers were incubated for 12-14 h in medium with and without an inhibitory concentration of insulin. Cells were then incubated for 10 min with label, and, after differing periods of chase with unlabeled methionine, cellular medium and media labeled apoB were analyzed; greater than 90% of labeled apoB was present in cells at 10 and 20 min after pulse, and labeled apoB did not appear in the medium until 40 min of chase. Insulin treatment inhibited the incorporation of label into total apoB by 48%, into apoBH by 62%, and into apoBL by 40% relative to other cellular proteins. Insulin treatment favored the more rapid disappearance of labeled cellular apoBH with an intra-cellular retention half-time of 50 min (initial half-life of decay, t1/2 = 25 min) compared with 85 min in control (t1/2 = 60 min). Intracellular retention half-times of labeled apoBL were similar in control and insulin-treated hepatocytes and ranged from 80 to 100 min. After 180 min of chase, 44% of labeled apoBL in control and 32% in insulin-treated hepatocytes remained cell associated. Recovery studies indicated that insulin stimulated the degradation of 45 and 27% of newly synthesized apoBH and apoBL, respectively. When hepatocyte monolayers were continuously labeled with [35S]methionine and then incubated in chase medium with and without insulin, labeled apoBH was secreted rapidly, reaching a plateau by 1 h of chase, whereas labeled apoBL was secreted linearly over 3-5 h of chase. Insulin inhibited the secretion of immunoassayable apoB but not labeled apoB. Results demonstrate that 1) insulin inhibits synthesis of apoB from [35S]methionine, 2) insulin stimulates degradation of freshly translated apoB favoring apoBH over apoBL, and 3) an intracellular pool of apoB, primarily apoBL, exists that is largely unaffected by insulin. Overall, insulin action in primary hepatocyte cultures reduces the secretion of freshly synthesized apoB and favors secretion of preformed apoB enriched in apoBL.     

Surface proteins on Cloudman melanoma cells

Lactoperoxidase-catalyzed cell surface radioiodination was used to study the size distribution and physiological characteristics of cell surface proteins of Cloudman melanoma cells maintained in vivo (An cell) or in vitro (TC cells). Slight qualitative and quantitative differences were observed between detergent-extracted radiolabeled membrane proteins obtained from An and TC cells when the proteins were resolved by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Striking differences were observed in the membrane residence times of radioiodinated proteins of the An and TC cells when labeled cells were cultured 24 hr. Many labeled surface proteins are lost more rapidly from An cells than from TC cells. The possibility that the release of surface proteins from tumor cell membranes may be an important factor which determines the nature of the host immune response against incipient cancers is considered.     

Use of biotin-labeled nucleic acids for protein purification and agarose-based chemiluminescent electromobility shift assays

We have employed biotin-labeled RNA to serve two functions. In one, the biotin tethers the RNA to streptavidin-agarose beads, creating an affinity resin for protein purification. In the other, the biotin functions as a label for use in a modified chemiluminescent electromobility shift assay (EMSA), a technique used to detect the formation of protein-RNA complexes. The EMSA that we describe avoids the use not only of radioactivity but also of neurotoxic acrylamide by using agarose as the gel matrix in which the free nucleic acid is separated from protein-nucleic acid complexes. After separation of free from complexed RNA in agarose, the RNA is electroblotted to positively charged nylon. The biotin-labeled RNA is readily bound by a streptavidin-alkaline phosphatase conjugate, allowing for very sensitive chemiluminescent detection ( approximately 0.1-1.0 fmol limit). Using our system, we were able to purify both known iron-responsive proteins (IRPs) from rat liver and assess their binding affinity to RNA containing the iron-responsive element (IRE) using the same batch of biotinylated RNA. We show data indicating that agarose is especially useful for cases when large complexes are formed, although smaller complexes are even better resolved.     

Visualization of intracellular trafficking: use of biotinylated ligands in conjunction with avidin-gold colloids

A simple, sensitive method to visualize the binding and internalization of protein ligands by cells in culture is described. A biotinylated toxin was used as ligand, and succinoylated avidin adsorbed onto 5.2 nm gold sols was the electron-dense marker. This method affords direct localization of proteins that are on the cell surface or intracellular without need for techniques that alter membrane integrity.     

Proteins of rod outer segments of toad retina: binding with calmodulin and with GTP

Proteins of purified rod outer segments from toad retina were analysed by electrophoresis on polyacrylamide gel containing sodium dodecyl sulfate. The binding of proteins with calmodulin and with guanosine triphosphate was studied by electroblotting the proteins resolved by electrophoresis onto nitrocellulose sheets and by incubating the blots with labelled ligands. The results indicate that rod outer segments from toad retina contain nine proteins which bind to calmodulin and one protein, different from transducin, that binds to guanosine triphosphate.     

Adenosine dialdehyde blocks the disappearance of two nerve growth factor-induced insoluble proteins

Two nonionic-detergent-insoluble proteins are induced early in the nerve growth factor (NGF)-induced neuronal differentiation of PC12 cells. The pools of these two proteins then disappear from the insoluble fraction after a few days of continued exposure of the cells to NGF. The methylation-inhibiting drug adenosine dialdehyde blocks the disappearance of these insoluble proteins, implicating a methylation-dependent step in the pathway that regulates the fate of these proteins.Abbreviations NGF nerve growth factor - PBS phosphatebuffered saline - SDS sodium dodecyl sulfate     

Identification of heparin-binding proteins in bovine seminal plasma

A group of four similar proteins, BSP-A1, BSP-A2, BSP-A3, and BSP-30-kDa, represent the major acidic proteins found in bovine seminal plasma (BSP). These proteins are secretory products of the seminal vesicles; they bind to spermatozoa upon ejaculation and could represent decapacitation factors. It has been shown that the glycosaminoglycans present in the female reproductive tract are involved in the capacitation of spermatozoa. Therefore, it was of interest to investigate whether BSP-A1, -A2, -A3, and -30-kDa proteins of bovine seminal fluid interact with heparin. Chromatography of alcohol precipitates of bovine seminal fluid on a heparin-Sepharose column resolved these proteins into three peaks. Peaks 1 and 2 (retarded proteins) were eluted upon extensive washing of the column with 0.05 M phosphate buffer, pH 7.4 (equilibrating buffer), and accounted for approximately 25% of the applied proteins. Proteins in peak 3 represented adsorbed proteins and were eluted with phosphate buffer containing 1 M NaCl. Proteins in each peak were characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. Peak 1 contained proteins with molecular weights ranging from 8 to 350 kDa, peak 2 contained a single protein with a molecular weight of 14 kDa, and peak 3 contained proteins with molecular weights of 15.5, 16, 25, and 30 kDa. The proteins in peak 3 were further resolved into unadsorbed (peak 4) and adsorbed (peak 5) proteins on a gelatin-Agarose column. Separation of the proteins of peak 3 and peak 5 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and reducing agents followed by transfer to nitrocellulose and probing with antibodies against the previously well-characterized BSP proteins indicated the presence of BSP-A1, BSP-A2, BSP-A3, and BSP-30-kDa proteins.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plant Plasma Membrane Proteins : II. Biotinylation of Daucus Carota Protoplasts and Detection of Plasma Membrane Polypeptides after Sds-Page

The ability of two biotinylating reagents, sulfosuccinimidobiotin and sulfosuccinimidyl 2-(biotinamido)ethyl-1,3′-dithiopropionate, to label plasma membrane proteins was examined. These compounds form covalent bonds with the free amino groups of proteins and label the proteins with biotin. Biotinylated proteins can be detected with avidin-peroxidase staining. Protoplasts isolated from embryogenic Daucus carota suspension cells were labeled with biotin and the membranes were separated on linear sucrose gradients. The conditions used for labeling the protoplasts did not cause protoplast rupture or loss of viability. The distribution of the biotin label in these linear sucrose gradients was analyzed and compared to the distribution of vanadate-sensitive ATPase activity, a marker for the plasma membrane. Both the biotin label and the vanadate-sensitive ATPase activity were strongly localized in the gradient at peak density of 1.16 gram per cubic centimeter. When the protoplast surface was labeled, biotinylated polypeptides were detected after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and polypeptides of 153, 94, 51, 30, 20, 17, and 14 kilodaltons were shown to be plasma membrane in origin. When a crude membrane pellet was labeled, numerous biotinylated polypeptides were distributed throughout the gradient. Because the position of the biotin label in the gradient is strongly correlated with the distribution of vanadate-sensitive ATPase, it is concluded that these biotinylating reagents are effective and reliable labels for proteins of the plant plasma membrane. Furthermore, these labels permit the positive identification of plasma membrane proteins after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and can serve as convenient markers for solubilization and purification of these proteins.