Insulin receptor: its subunit structure as determined by photoaffinity labeling

This communication reviews briefly the preparation of two photoreactive arylazido insulins and the use of these insulin derivatives to photolabel the insulin receptor of rat adipocytes. These experiments showed that the receptor contains disulfide linked subunits of 130,000, 90,000, and 40,000 daltons. When not reduced by dithiothreitol, three receptor species of 380,000, 300,000, and 230,000 daltons were detected. Based on the results of photolabeling we propose that the 300,000-dalton species is composed of one 130,000-, one 90,000-, and two 40,000-dalton subunits in disulfide linkages.     

Demonstration of two subtypes of insulin-like growth factor receptors by affinity cross-linking

The structure of receptors for insulin-like growth factors in rat liver plasma membranes and the BRL 3A2 rat liver cell line has been examined by chemical cross-linking with disuccinimidyl suberate and sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing and nonreducing conditions. Two receptor subtypes have been identified: (i) 125I multiplication-stimulating activity cross-linked to liver membranes or intact cells appeared in a complex of Mr = 260,000 (reduced) and 220,000 (nonreduced) and (ii) 125I-insulin-like growth factor I cross-linked to BRL 3A2 cells appeared predominantly in two bands of Mr greater than 300,000 without disulfide reduction and in a Mr = 130,000 complex following reduction. The two subtypes of insulin-like growth factor receptors identified by structural analysis correspond to previously observed differences in their specificity for insulin and insulin-like growth factors.     

The subunit structure of the follitropin receptor. Chemical cross-linking of the solubilized follitropin-receptor complex

Homobifunctional cross-linkers were utilized to characterize high affinity (Ka = 2.2 X 10(-10) M-1) follitropin (FSH) receptors in immature bovine testis. Following the formation of radioiodinated human FSH (125I-hFSH)-receptor complexes, the membranes were solubilized with Triton X-100 or beta-octyl glucoside and the supernatants from ultracentrifugation (220,000 X g) subjected to gel filtration (Sephadex G-200) to separate the labeled hormone-receptor complexes from the unbound 125I-hFSH. The appearance of a high molecular weight (greater than or equal to 200,000) radioactive component in the elution profile was abolished when an excess of unlabeled hFSH was included in the initial incubation. After concentration by ultrafiltration, the 125I-hFSH-receptor complex, as well as the free hormone, was treated with a variety of chemical cross-linkers and subjected to analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Bands of Mr = 65,000 and 83,000 observed in the autoradiograph of the hormone-receptor complex was not present in autoradiographs of free 125I-hFSH, nor were they present when an excess of unlabeled hFSH was included in the initial binding incubation mixtures. The 65,000 and 83,000 Mr bands were, therefore, considered to represent cross-linked complexes of labeled hFSH (Mr = 38,000) or its subunits (hFSH alpha, Mr = 16,000; hFSH beta, Mr = 21,000) and components of the FSH receptor. The bands were observed on autoradiographs when the extraction of the membranes was performed with either Triton X-100 or beta-octyl glucoside and when cross-linking was accomplished with disuccinimidyl suberate, ethylene glycol bis(succinimidyl succinate), or bis[2-(succinimido oxycarbonyl)oxyethyl]sulfone. The Mr of the native FSH receptor in the calf testis has been estimated at 146,000. Our studies demonstrate the multimeric nature of the FSH receptor. However, FSH is also composed of subunits, so that due to the complexity of the system, it was not possible to arrive at a precise assessment of the Mr or quaternary structure of the receptor subunits.     

Conformation and mode of organization of amphiphilic membrane components: a conformational analysis.

The insulin receptor is an integral membrane glycoprotein (Mr approximately 300,000) composed of two alpha-subunits (Mr approximately 130,000) and two beta-subunits (Mr approximately 95,000) linked by disulphide bonds. This oligomeric structure divides the receptor into two functional domains such that alpha-subunits bind insulin and beta-subunits possess tyrosine kinase activity. The amino acid sequence deduced from cDNA of the single polypeptide chain precursor of human placental insulin receptor revealed that alpha- and beta-subunits consist of 735 and 620 residues, respectively. The alpha-subunit is hydrophilic, disulphide-bonded, glycosylated and probably extracellular. The beta-subunit consists of a short extracellular region which links the alpha-subunit through disulphide bridges, a hydrophobic transmembrane region and a longer cytoplasmic region which is structurally homologous with other tyrosine kinases like the src oncogene product and EGF receptor kinases. The cellular function of insulin receptors is dual: transmembrane signalling and endocytosis of hormone. The binding of insulin to its receptor on the cell membrane induces transfer of signal from extracellular to cytoplasmic receptor domains leading to activation of cell metabolism and growth. In addition, hormone-receptor complexes are internalized leading to intracellular proteolysis of insulin, whereas receptors are recycled to the membrane. These phenomena are kinetically well-characterized, but their molecular mechanisms remain obscure. Insulin receptor in different tissues and animal species are homologous in their structure and function, but show also significant differences regarding size of alpha-subunits, binding kinetics, insulin specificity and receptor-mediated degradation. We suggest that this heterogeneity of receptors may be linked to the diversity in insulin effects on metabolism and growth in various cell types. The purified insulin receptor phosphorylates its own beta-subunit and exogenous protein and peptide substrates on tyrosine residues, a reaction which is insulin-sensitive, Mn2+-dependent and specific for ATP. Tyrosine phosphorylation of the beta-subunit activates receptor kinase activity, and dephosphorylation with alkaline phosphatase deactivates the kinase. In intact cells or impure receptor preparations, a serine kinase is also activated by insulin. The cellular role of two kinase activities associated with the insulin receptor is not known, but we propose that the tyrosine- and serine-specific kinases mediate insulin actions on metabolism and growth either through dual-signalling or sequential pathways.(ABSTRACT TRUNCATED AT 400 WORDS)     

Relationship between the affinity and proteolysis of the insulin receptor. Evidence that higher affinity receptors are preferentially degraded

125I-Insulin binding to rat liver plasma membranes initiated two processes that occurred with similar time courses: an increase of receptor affinity for hormone and degradation of the Mr 135,000 alpha subunit of the insulin receptor to a fragment of Mr 120,000. Inhibitors of serine proteinases prevented alpha subunit degradation without affecting the affinity change. This shows that the change of affinity is not produced by receptor proteolysis and that the intact alpha subunit of the insulin receptor can exist as a higher or lower affinity species. Hormone binding was much more rapid than receptor proteolysis and the initial rate of alpha subunit degradation was independent of the concentration of occupied lower affinity receptors. Only persistent hormone binding and the accumulation of higher affinity insulin-receptor complexes led to significant receptor proteolysis. As the incubation time between 125I-insulin and membranes increased, the rate at which hormone dissociated from Mr 135,000 complexes diminished, whereas hormone dissociated from Mr 120,000 complexes slowly after brief or extended incubations. These observations suggest that 125I-insulin binds to membranes to form low affinity complexes that are not substrates for proteolysis. A slow conformational change produces higher affinity hormone-receptor complexes that are selectively degraded. Thus, the conversion between states of affinity may play a role in the regulation of receptor proteolysis and, consequently, insulin action in cells.     

Substructural analysis of the insulin receptor by microsequence analyses of limited tryptic fragments isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the absence or presence of dithiothreitol

Human placental insulin receptor contains 47 Cys per an alpha beta dimer. Most of the 94 Cys in an intact alpha 2 beta 2 receptor are expected to form interchain or intrachain disulfide bonds, since there appears to be only one free cysteine residue in each beta subunit. In order to gain more insight into the three-dimensional organization of the insulin receptor, we have used limited trypsin digestion, SDS-PAGE, and protein microsequencing. The present study revealed the following; major tryptic cleavages occurred at alpha 164, alpha 270, alpha 582, and beta 1115, generating Mr 175,000, 130,000, 100,000, 70,000, and 55,000 disulfide-linked complexes. Under reducing conditions, tryptic fragments of Mr values = 30,000, 70,000, 20,000, 55,000, and 20,000 were identified to be alpha(1-164), alpha(165-582), alpha(165-270), alpha(271-582), and alpha(583-C-terminal), respectively. The major beta subunit tryptic fragment of Mr = 55,000 was assumed to have beta(724-1115) or beta(N-terminal-392). The Mr 175,000 complex appeared to contain two alpha(1-164) and two alpha(165-582), whereas the Mr 70,000 complex contained alpha(583-C-terminal) and beta(724-1115). Tryptic cleavage at alpha 582 apparently produced one Mr 175,000 and two Mr 70,000 complexes, suggesting that the alpha(583-C-terminal) domain interacts with the extracellular domain of the beta subunit by disulfide bonds. Tryptic cleavage at alpha 270 resulting in a formation of one Mr 100,000 complex consisting of two alpha(1-270) and two Mr 130,000 complexes consisting of alpha(271-C-terminal) and beta(724-1115) suggests that Cys residues involved with disulfide bonds between the two alpha subunits are located in the alpha(1-270) domain. The identification of the Mr 55,000 complex consisting of small tryptic fragments between alpha(122-270) indicates that 40 Cys residues in the two alpha(122-270) domains are inter- and intramolecularly associated by disulfide bonds. The alpha(1-121) domain does not appear to be linked to any other domains by disulfide bonds. These results are consistent with the structural model that the N-terminal domains of alpha subunits (122-270) are disulfide-linked together while the C-terminal domain (583-C-terminal) of the alpha subunit is linked to the N-terminal domain of the beta subunit by disulfide bonds.     

Characterization of porcine plasma fibronectin and its fragmentation by porcine liver cathepsin B

Fibronectin was isolated from porcine plasma by affinity chromatography with gelatin-linked Sepharose 4B. Porcine fibronectin had a chemical composition similar to those of human and other fibronectins and reacted with antiserum raised against human fibronectin. It showed hemagglutination activity with trypsin-treated rabbit erythrocytes, though the activity was far less than that of human fibronectin. Porcine plasma fibronectin consisted of two subunit chains of about 230,000-daltons linked by disulfide bonds(s). Limited proteolysis of this protein with porcine liver cathepsin B yielded five major fragments which were investigated by affinity chromatography with gelatin- and heparin-linked Sepharose 4B. One fragment (Mr = 50,000) was bound to gelatin but not to heparin, while the remaining four were bound to heparin but not to gelatin, suggesting that plasma fibronectin takes a discrete domain structure with respect to interaction with these two macromolecules. The three larger heparin-binding fragments, Mr = 175,000, 150,000, and 130,000 were eluted with different concentrations of a mixture of NaCl and urea from the heparin-column, suggesting that they have different interactions with heparin, the 130,000-dalton fragment being the one with the strongest interaction. After reduction with 2-mercaptoethanol, the 175,000-dalton fragment was converted to the 150,000-dalton region fragment, which, together with the unchanged 150,000-dalton fragment, appeared to be equivalent in amount to the 130,000-dalton fragment. This finding suggests that the 150,000- and 130,000-dalton fragments may have originated from different subunit chains. Since the 175,000-dalton fragment was not produced by cathepsin B digestion of fibronectin which had been treated with plasmin, it was concluded that the 175,000-dalton fragment contained interchain disulfide bond(s) which had linked the native subunit chains. These results suggest that porcine plasma fibronectin has non-identical subunit chains composed of domains which differ in interaction with heparin and in susceptibility to cathepsin B.     

Domain structure of fibronectin and its relation to function. Disulfides and sulfhydryl groups.

Hamster cell fibronectin is a glycoprotein consisting of two 230,000-dalton subunits in a disulfide-bonded dimer. The molecule is composed of domains which can be separated by partial proteolytic cleavage. The carbohydrates, disulfide bonds, and a single free sulfhydryl group per chain are distributed nonuniformly among these regions. All the interchain disulfides are within 10,000 daltons of the end of the molecule and are removed by mild proteolysis which also generates 200,000- and 25,000-dalton fragments which do not contain interchain disulfides. The 200,000-dalton fragment contains all or most of the carbohydrate side chains, and the free sulfhydryl group, but is relatively poor in cystine. The 25,000-dalton fragment is carbohydrate-free and cystine-rich but has no free sulfhydryl groups. There is heterogeneity in carbohydrate content among the monomeric chains of intact fibronectin and the 200,000-dalton fragments. The gelatin binding site of fibronectin is in the 200,000 fragment. Intact disulfide bonds are required for binding of fibronectin to cells and to gelatin and blockage of the free sulfhydryl groups prevents binding of fibronectin to cells, suggesting that intermolecular disulfide bonding may be important.     

Detection of vinculin-binding proteins with an 125I-vinculin gel overlay technique

Vinculin is an adhesion plaque component localized on the cytoplasmic side of the cell membrane where stress fibers end. To detect vinculin- binding proteins, we have developed an 125I-vinculin gel overlay method. SDS PAGE was used to separate different protein preparations. After fixing the proteins in the gel with methanol-acetic acid, the SDS was removed with ethanol and the proteins renatured in buffer. The gel was then incubated with 125I-vinculin. After extensive washing to remove nonspecifically associated label, the gel was dried and autoradiographed. Chick embryo fibroblasts, their Rous sarcoma virus transformants, and HeLa cells were found to contain two proteins (Mr 220,000 and 130,000) that bound 125I-vinculin strongly and another (Mr 42,000) that bound it moderately. The 130,000-mol-wt protein was identified as vinculin itself, which suggests that it may self- associate. The 42,000-mol-wt protein was identified as actin with which vinculin is known to interact. The identity of the 220,000-mol-wt protein is not known. It is not cellular fibronectin, myosin, or filamin. When fibroblast proteins were separated into Triton X-100 soluble and insoluble fractions, most of the vinculin and the 220,000- mol-wt protein was found to be in the soluble fraction. Chicken gizzard also contained these vinculin-binding proteins along with three others of Mr 190,000, 170,000, and 100,000.     

Nerve growth factor receptor from rabbit sympathetic ganglia membranes. Relationship between subforms

The receptor for nerve growth factor (NGF) was purified from Triton X-100 extracts of sympathetic ganglia membranes by affinity chromatography on NGF-Sepharose. Elution of purified receptor was accomplished at pH 5 in the presence of 1 M NaCl. Sodium dodecyl sulfate gel electrophoresis of the purified iodinated receptor showed three major bands at Mr = 126,000, Mr = 105,000, and Mr = 81,000. Affinity labeling of the purified receptor using 125I-NGF and the photoreactive agent N-hydroxysuccinimidyl-p-azidobenzoate resulted in two major cross-linked complexes corresponding to Mr = 135,000 and Mr = 110,000. This labeling pattern is similar to that observed with sympathetic ganglia membranes (Massague, J., Guillette, B. J., Czech, M. P., Morgan, C. J., and Bradshaw, R. A. (1981) J. Biol. Chem. 256, 9419-9424) and indicates that these two forms do not arise from the cross-linking procedure. Reaction of the photoaffinity labeled NGF receptors with increasing amounts of trypsin resulted in a progressive decrease in the high molecular weight complex with a concomitant increase in the low molecular weight form. When the larger complex was isolated by electroelution from a sodium dodecyl sulfate gel and treated with trypsin, a species corresponding to Mr = 100,000 was generated. These observations are best explained by a precursor-product relationship for the two NGF receptor species of sympathetic neurons.     

Distinct biologically active receptors for insulin, insulin-like growth factor I, and insulin-like growth factor II in cultured skeletal muscle cells

The expression of insulin-like growth factor (IGF) receptors at the cell surface and the changes in IGF responsiveness during differentiation were studied in the L6 skeletal muscle cell line. Throughout the entire developmental sequence, distinct receptors for IGF I and IGF II that differed in structure and peptide specificity could be demonstrated. During differentiation, both 125I-IGF I and 125I-IGF II binding to the L6 cells decreased as a result of a 3-4-fold reduction in receptor number, whereas 125I-insulin binding increased. Under nonreducing conditions, disuccinimidyl suberate cross-linked 125I-IGF I and 125I-IGF II to two receptor complexes with apparent Mr greater than 300,000 (type I) and 220,000 (type II). Under reducing conditions, the apparent molecular weight of the type I receptor changed to Mr 130,000 (distinct from the 120,000 insulin receptor) and the type II receptor changed to 250,000. IGF I and IGF II both stimulated 2-deoxy-D-glucose and alpha-aminoisobutyric acid uptake in the L6 cells with a potency close to that of insulin, apparently through interaction with their own receptors. The stimulatory effects of IGF II correlated with its affinity for the type II but not the type I IGF receptor, as measured by inhibition of affinity labeling, whereas the effects of IGF I correlated with its ability to inhibit labeling of the type I receptor. In spite of the decrease in type I and type II receptor number, stimulation of 2-deoxy-glucose and alpha-aminoisobutyric acid uptake by the two IGFs increased during differentiation.     

The isolation of human beta-interferon receptor by wheat germ lectin affinity and immunosorbent column chromatographies

Radioiodinated human beta-interferon-Ser 17 (Betaseron) was reversibly cross-linked to Daudi cells by dithiobis(succinimidylpropionate). The radioactive ligand was cross-linked to three macromolecules forming labeled complexes of apparent Mr values of 130,000, 220,000, and 320,000. Betaseron, human alpha-interferon, human interleukin 2 but not recombinant human gamma-interferon competed with the labeled ligand for binding to these putative receptor(s). Human leukocyte-produced gamma-interferon competed weakly with 125I-Betaseron for binding to Daudi cells. The Betaseron-receptor complex(es) was purified by passage through a wheat germ lectin column followed by chromatography on an anti-interferon immunosorbent column and semipreparative gel electrophoresis. The cross-linked ligand-receptor complex was shown to be highly purified by sodium dodecyl sulfate and acetic acid:urea:Triton X-100 polyacrylamide gel electrophoresis. It can be dissociated into the labeled Betaseron (Mr = 17,000) ligand and a receptor moiety which has an apparent molecular weight of 110,000. The chromatographic behavior of the ligand-receptor complex on wheat germ lectin column suggests that the receptor is a glycoprotein. The described procedure yielded about 1 microgram of Betaseron receptor from 10(10) Daudi cells, estimated to contain a maximum of about 15 micrograms of the receptor.     

Inhibition of HeLa cell protein synthesis following poliovirus infection correlates with the proteolysis of a 220,000-dalton polypeptide associated with eucaryotic initiation factor 3 and a cap binding protein complex

Following poliovirus infection of HeLa cells, the synthesis of cellular proteins is inhibited but translation of poliovirus mRNA proceeds. The defect in the recognition of host cell mRNA may be due to a change in a cap recognition complex which, when added to an infected cell lysate, restores the ability to translate capped mRNAs. We employed immunoblotting techniques to examine initiation factors in crude lysates from uninfected and poliovirus-infected HeLa cells. Using an antiserum against eucaryotic initiation factor 3, we detected an antigen of approximate molecular weight 220,000 in uninfected cell lysates but not in infected cell lysates. Antigenically related polypeptides of 100,000 to 130,000 daltons, presumably degradation products, were detected in the infected cell lysate. The time course for degradation of the 220,000-dalton polypeptide correlates with that for inhibition of cellular protein synthesis in vivo. A portion of the population of 220,000-dalton polypeptides apparently associates with initiation factor eIF3 but is readily dissociated in buffers containing high salt. Affinity-purified antibodies against the polypeptide recognize a protein of the same size in a purified preparation of a cap binding protein complex obtained by cap-affinity chromatography. We postulate that the 220,000-dalton polypeptide is an essential component of the cap recognition complex and that its degradation in poliovirus-infected cells results in the inhibition of host cell translation. These results are in the first demonstration of a specific structural defect in an initiation factor resulting from poliovirus infection.     

Analysis of the major large polypeptides of rat seminal vesicle secretion: SVS I, II, and III

Rat seminal vesicle secretion (SVS) contains a variety of protein complexes that seem to be linked by interchain disulfide bonds. Upon reduction and analysis by sodium dodecyl sulfate (SDS) gel electrophoresis, this pattern resolves to 3 major high molecular weight (SVS I-100,000, SVS II-50,000, SVS III-37,000) and 3 major low molecular weight protein bands (SVS IV, V, and VI). A two-dimensional SDS gel (1st dimension unreduced, 2nd dimension reduced) permitted identification of the components of the cross-linked species. In the native secretion, SVS I forms a series of oligomers that include both SVS II and III. Essentially all of SVS III is involved in these complexes, while the bulk of SVS II occurs instead as an apparent homodimer. The smaller proteins (SVS IV-VI) are not involved in covalently crosslinked complexes. The reduced forms of the larger polypeptides were isolated by a variety of procedures involving agarose gel filtration in 6M guanidine hydrochloride, reversed-phase high pressure liquid chromatography, ammonium sulfate fractionation, and preparative polyacrylamide gel electrophoresis. Based on its size, solubility, and amino acid composition, SVS II was identified as the major clottable protein of the secretion.     

Characterization of the somatogenic receptor in rat liver. Hydrodynamic properties and affinity cross-linking

Rat liver somatogenic receptors have been characterized by gel permeation chromatography, sucrose density gradients in H2O and D2O, and affinity cross-linking using 125I-bovine growth hormone (bGH) as a specific somatogenic receptor ligand. Cross-linking of 125I-bovine growth hormone to a Triton X-100-treated low density fraction isolated from livers of late pregnant rats followed by sodium dodecylsulfate-polyacrylamide gel electrophoresis under reducing conditions showed three major binders with Mr 95,000, 86,000, and 43,000 and a minor binder of Mr 55,000, after correction for bound ligand assuming a 1:1 binding ratio of ligand-receptor. The Mr 86,000, 55,000, and 43,000 species were recovered in the detergent-soluble supernatant after high-speed centrifugation, whereas the Mr 95,000 species remained Triton X-100 insoluble. Detergent-soluble 125I-bGH-receptor complexes were further analyzed by sedimentation into sucrose density gradients. The sedimentation coefficient was S20,w = 5.2 S and the partial specific volume v = 0.72 ml/g. Gel permeation chromatography on a Sepharose S-400 column indicated a Stokes radius of 61 A for the 125I-bGH-receptor-Triton X-100 complex. Based on these figures, the molecular weight of the complex was calculated as 131,100. The molecular weight of the ligand-free receptor-Triton X-100 complex was calculated as Mr 109,100. Affinity cross-linking and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the 61 A peak from Sephacryl S-400 chromatography (cf. above) showed two binding entities, one major and one minor with Mr values 86,000 and 43,000, respectively, in the absence of reductant. When electrophoresis was run in the presence of reductant the Mr 43,000 species was the major binding entity. Furthermore, two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis (first dimension, nonreducing and second dimension, reducing) showed that a disulfide-linked binder at Mr 43,000 is contained within the Mr 86,000 species. As with pregnant rats, female and male rats both showed 125I-bovine growth hormone binders of Mr 95,000, 84,000, 55,000, 43,000, and additionally an Mr 35,000 binder.     

Molecular characteristics of nerve growth factor receptors on PC12 cells

Cross-linking of 125I-nerve growth factor (NGF) to PC12 cells with the photoreactive heterobifunctional agent N-hydroxysuccinimidyl-4-azidobenzoate results in the labeling of two major bands with Mr 158,000 and 100,000 and a minor band with Mr 225,000 as determined by polyacrylamide gel electrophoresis under denaturing and reducing conditions. Binding of 125I-NGF to and cross-linking into all these species is abolished in the presence of excess unlabeled NGF but not in the presence of unlabeled epidermal growth factor, insulin, or bovine pancreatic trypsin inhibitor. When PC12 cells with bound 125I-NGF are incubated in excess unlabeled NGF at 0 degree C prior to cross-linking, only the Mr 158,000 species remains. In addition, binding of 125I-NGF to the Mr 158,000 complex is trypsin-resistant, whereas binding to the Mr 100,000 complex is not. These experiments identify the Mr 158,000 species as the slow NGF-receptor complex (chase stable at 0 degree C) and the smaller Mr 100,000 species as the fast NGF-receptor complex (trypsin sensitive). Furthermore, 125I-NGF bound to the former but not to the latter species is displaced by very-low concentrations of NGF, showing that at least a significant fraction of the high-molecular-weight slow receptor is also a high-affinity receptor. This identification is supported by the finding that chick sensory neurons which possess both high- and low-affinity receptors exhibit two major labeled bands with Mr 145,000 and 105,000 as a result of cross-linking with 125I-NGF, whereas a cell population enriched in non-neuronal cells, which possess only low-affinity receptors, exhibits only the Mr 105,000 component. A shift in molecular weight of both species after pretreatment with neuraminidase indicates that both complexes contain sialoglycoproteins and rules out the possibility that differences in sialic acid content are responsible for the difference in molecular weight of the two complexes. The relative amount of the labeling of these two complexes is not affected by the presence of protease inhibitors nor by a variation of 5000-fold in cross-linker concentration. These results place some limits on possible models for the NGF receptors and their interconversion.     

Binding of the glucocorticoid receptor to the rat liver nuclear matrix. The role of disulfide bond formation

The nuclear matrix is a putative skeletal structure which has been implicated in many nuclear functions. To assess a possible role of the nuclear matrix in glucocorticoid action, purified rat liver nuclei containing glucocorticoid-receptor complexes were treated with DNase I +/- RNase A followed by 1.6 M NaCl, thus yielding salt-extractable and salt-resistant (nuclear matrix) fractions. The subnuclear distribution of hormone-receptor complexes was determined by following the fate of unmetabolized radiolabel after injection of labeled triamcinolone acetonide into adrenalectomized animals and subjecting various subfractions to immunoblotting using a monoclonal antibody which recognizes the glucocorticoid receptor. Both techniques indicated that 50-70% of the total nuclear hormone-receptor complexes were recovered in the nuclear matrix fraction. Previous results (Kaufmann, S. H., and Shaper, J. H. (1984) Exp. Cell Res. 155, 477-495) suggest that a variety of nuclear polypeptides become nuclease- and salt-resistant as a result of the formation of intermolecular disulfide bonds. The following evidence suggests that disulfide bonds mediate the association between the glucocorticoid receptor and the nuclear matrix. When nuclei were isolated in the absence of sulfhydryl-blocking and -cross-linking reagents, sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions revealed that the receptor was present as a high molecular weight disulfide-cross-linked complex. When nuclei were isolated in the presence of the irreversible sulfhydryl-blocking reagent iodoacetamide, the disulfide bonds which cross-linked the receptor into high molecular weight complexes were absent; and 85-100% of the hormone-receptor complexes were salt-extractable. When nuclei (isolated in the absence of iodoacetamide) were treated with the sulfhydryl-cross-linking reagent sodium tetrathionate, greater than 95% of the nuclear hormone-receptor complexes became resistant to extraction with nucleases and 1.6 M NaCl. The implications of these results for other matrix-associated nuclear functions are discussed.     

Identification of lipoprotein-binding proteins in rat liver Golgi apparatus membranes

The low density lipoprotein (LDL) receptor has been shown to be a plasma membrane glycoprotein responsible for the cellular binding and endocytosis of plasma lipoproteins. Inasmuch as the Golgi apparatus has been shown to participate in glycoprotein processing and in the assembly of plasma lipoproteins by hepatic and intestinal epithelial cells, the present studies were designed to test the hypothesis that lipoprotein receptors are present within Golgi membranes. Utilizing ligand blotting with a variety of iodinated lipoproteins, several lipoprotein-binding proteins were identified in rat liver Golgi membranes at apparent molecular weights (Mr) 200,000, 160,000, 130,000, 120,000, 100,000, 80,000, and 70,000. The 130,000 protein was the most prominent and was identified as the mature LDL receptor by its binding characteristics and an Mr characteristic of the plasma membrane receptor. Enzymatic deglycosylation studies suggested that the 120,000 and 100,000 proteins were LDL receptor precursors lacking sialic acid. Antibody to the LDL receptor recognized all the bands on immunoblots except the 70,000 protein, with the 130,000 protein being the most prominent. Isolation of the Golgi fractions in the presence of protease inhibitors did not eliminate any of the proteins recognized by the antibody but did result in sharper bands on the blots. Additionally, we investigated the hypothesis that conditions that regulate plasma membrane LDL receptors also cause detectable changes in receptors in Golgi membranes. All the binding proteins were increased in Golgi membranes from rats treated with 17-alpha-ethynylestradiol. Colchicine caused an accumulation of 120,000 Mr protein, suggesting blockage of final sialylation in the trans Golgi. When protein synthesis was inhibited by cycloheximide, there was no reduction of mature LDL receptors in Golgi membranes, consistent with recycling of receptors through this organelle.     

Sulfhydryl/disulfide forms of rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase

Using radiation inactivation and immunoblotting techniques, evidence for functionally active forms of rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase with molecular weights of about 100,000 and 200,000 was obtained. In liver microsomes isolated from rats fed both mevinolin and colestipol, the Mr 100,000 form was the predominant species, whereas in microsomes from animals fed only colestipol, the Mr 200,000 species was the major form. This Mr 200,000 form could be converted to the Mr 100,000 form by addition of dithiothreitol or beta-mercaptoethanol. Although both forms appear to possess catalytic activity, the Mr 200,000 species displays sigmoidal kinetics with respect to the concentration of NADPH, whereas the Mr 100,000 form exhibits typical hyperbolic kinetics.     

Identification of an isozymic form of acetyl-CoA carboxylase

Acetyl-CoA carboxylase (ACC) is a major rate-limiting enzyme of fatty acid biosynthesis; its product, malonyl-CoA, also contributes to the regulation of fatty acid oxidation and elongation. Using monospecific antibodies directed against rat liver ACC and N- and C-terminal antipeptide antibodies raised against predicted sequences of the cloned ACC of Mr 265,000, we have identified a unique biotin-containing cytosolic protein of molecular mass 280,000 daltons that is distinct from this 265,000-dalton protein. This protein is uniquely expressed in rat cardiac and skeletal muscle but is co-expressed with the 265,000-dalton protein in rat liver, mammary gland, and brown adipose tissue. In the fed rat, white adipose tissue contains only the 265,000-dalton protein. Like the 265,000-dalton protein, the 280,000-dalton protein is present predominantly in the cytosolic fraction of liver. In the liver, the content of both proteins is diminished on fasting and increases on fasting/refeeding with a high carbohydrate diet. In contrast, the cardiac and skeletal muscle 280,000-dalton protein content is unaltered by nutritional manipulation. Avidin-Sepharose isolates of citrate-dependent ACC from the heart reveal only the 280,000-dalton protein, while white adipose tissue isolates show only the 265,000 form. These species differ in the sensitivity to citrate activation and in the Km for acetyl-CoA. Antibodies reactive with the 280,000-dalton protein on immunoblotting precipitate ACC activity in heart isolates, while white adipose ACC is precipitated only by antibodies specific for the 265,000-dalton species. However, in ACC isolates where both proteins are present, a heteroisozyme complex can be detected both by immunoprecipitation and by a sandwich enzyme-linked immunosorbent assay. We conclude that the 280,000-dalton protein is an isozyme of ACC, distinct from the previously cloned 265,000-dalton species. Its presence in cardiac and skeletal muscle, where fatty acid synthesis rates are low, suggest that it might play alternative roles in these tissues such as regulation of fatty acid oxidation or microsomal fatty acid elongation.