Plant basic 7 S globulin-like proteins have insulin and insulin-like growth factor binding activity.

Basic 7 S globulin (Bg) is a cysteine-rich glycoprotein present in soybean seeds. Mature Bg is composed of high- and low-kDa subunits linked by disulfide bonding. A ligand blotting experiment using [125I]insulin and [125I]insulin-like growth factor-I and -II showed that Bg subunits are able to bind not only to insulin but to insulin-like growth factors-I and -II. Bg-like proteins from other legume species cross-reacted with anti-Bg antibody also bind to insulin and insulin-like growth factors. Bg-like protein in carrot cells was found to have insulin binding activity. Bg-like proteins may be involved in an insulin-like regulatory mechanism in many plant species.     

Insulin binds to and promotes the phosphorylation of a Mr 210 000 component of its receptor in detergent extracts of rat liver microsomes

Insulin in the presence of Mn2+ and [gamma 32P]ATP promoted the phosphorylation of two proteins of Mr 95 000 and Mr 210 000 in detergent extracts of rat liver microsomes. The Mr 210 000 protein was identified as a component od the insulin receptor by immunoprecipitation. It also bound [125I]insulin specifically, was phosphorylated largely on a tyrosine residue and could not be cleaved to smaller subunits under extreme reducing conditions. The Mr 210 000 protein appears to be a component of a sub-population of liver membrane insulin receptors in which insulin-binding and insulin-stimulated tyrosine kinase phosphorylation site(s) reside in a single polypeptide chain.     

Structure and ligand specificity of the Drosophila melanogaster insulin receptor.

The insulin-binding and protein tyrosine kinase subunits of the Drosophila melanogaster insulin receptor homolog have been identified and characterized by using antipeptide antibodies elicited to the deduced amino acid sequence of the alpha and beta subunits of the human insulin receptor. In D. melanogaster embryos and cell lines, the insulin receptor contains insulin-binding alpha subunits of 110 or 120 kilodaltons (kDa), a 95-kDa beta subunit that is phosphorylated on tyrosine in response to insulin in intact cells and in vitro, and a 170-kDa protein that may be an incompletely processed receptor. All of the components are synthesized from a proreceptor, joined by disulfide bonds, and exposed on the cell surface. The beta subunit is recognized by an antipeptide antibody elicited to amino acids 1142 to 1162 of the human insulin proreceptor, and the alpha subunit is recognized by an antipeptide antibody elicited to amino acids 702 to 723 of the human proreceptor. Of the polypeptide ligands tested, only insulin reacts with the D. melanogaster receptor. Insulinlike growth factors type I and II, epidermal growth factor, and the silkworm insulinlike prothoracicotropic hormone are unable to stimulate autophosphorylation. Thus despite the evolutionary divergence of vertebrates and invertebrates, the essential features of the structure and intrinsic functions of the insulin receptor have been remarkably conserved.     

Screening microorganisms for insulin binding reveals binding by Burkholderia multivorans and Burkholderia cenocepacia and novel attachment of insulin to Aeromonas salmonicida via the A-layer

Exposure to microorganisms is considered an environmental factor that can contribute to Type 1 diabetes. Insulin-binding proteins (IBPs) on microorganisms may induce production of antibodies that can react with the human insulin receptor (HIR) with possible consequences in developing a diabetic autoimmune response against HIR and insulin. The interaction of insulin with microorganisms was studied by screening 45 microbial species for their ability to bind insulin. Binding assays were performed using labelled insulin to identify insulin-binding components on the microorganisms. Burkholderia multivorans and Burkholderia cenocepacia isolated from patients with cystic fibrosis (CF) and the fish pathogen Aeromonas salmonicida were the only strains of those tested, which showed insulin-binding components on their cell surfaces. Further work with A.?salmonicida suggested that the insulin-binding activity of A.?salmonicida is due to the A-layer. A mutant of A.?salmonicida lacking the A-layer showed binding, but at a much reduced rate suggesting another insulin-binding component in addition to the high affinity of the A-protein. Soluble protein lysates were subjected to Western ligand blotting using peroxidase-labelled insulin to detect IBPs. Two positive IBPs were apparent at approximately 30 and 20?kDa in lysates from Burkholderia strains, but no IBP was detected in A.?salmonicida lysates.     

Insulin-Induced Outgrowth of Pseudopodial Cables from Cultured Micromere-Derived Cells Isolated from Sea Urchin Embryos at the 16 Cell Stage, with Special Reference to the Insulin-Receptor.

Cultured cells derived from micromeres isolated from sea urchin embryos at the 16 cell stage are known to show outgrowth of pseudopodial cables followed by spicule rod formation when cultured in the presence of horse serum. Micromere-derived cells cultured with bovine insulin showed pseudopodial cable growth but did not produce spicule rods. Micromere-derived cells reversibly bound to insulin through out the period between 3 and 20 hr of culture. The dissociation constant of insulin with these cells was about 5.1 × 10⁻¹⁰M during the whole culture period examined. Horse serum, as well as blastocoelic fluid obtained from early gastrulae, concentration-dependently reduced the amount of insulin bound to these cells, but the bound insulin was scarcely replaced by any proteins tested, such as bovine serum albumin. The micromere-derived cells were bound to have an insulin-binding protein, that may be the receptor for insulin or insulin-like proteins. The insulin-binding protein had a smaller molecular weight than the insulin receptor of mammalian cells. The binding of insulin with this protein in micromere-derived cells probably results in pseudopodial cable growth.     

The insulin-binding domain of insulin receptor is encoded by exon 2 and exon 3.

Insulin receptors are disulfide-linked oligotetramers composed of two heterodimers each containing a 130-kDa alpha subunit and a 90-kDa beta subunit. Insulin binds to the extracellular alpha subunit, and in the process stimulates the autophosphorylation of the beta subunit and the expression of tyrosine kinase activity. Studies combining the use of photoaffinity labeling and immunoprecipitation with anti-peptide antibody have directly demonstrated that the cysteine-rich domain, encoded by exon 3, in the alpha subunit is part of the insulin-binding site of the receptor. Experiments with chimeric insulin receptors and chimeric insulin-like growth factor I receptors have confirmed that the cysteine-rich domain constitutes a part of the insulin-binding site. In addition, results from these experiments suggest that the N-terminal sequence, encoded by exon 2, in the alpha subunit also participates in insulin binding. In this review it is proposed that, assuming two insulin-binding sites per each holoreceptor oligotetramer, each insulin-binding domain may contain respectively two sub-domains for hydrophobic and charge contact with insulin, and that high-affinity binding would require the interaction of both subunits with the possibility of each subunit reciprocally contributing one of the sub-domains.     

A thiol-sensitive degradative process of liver uncouples autophosphorylation of the insulin receptor from insulin binding.

Insulin receptors derived from highly purified rat liver plasma membranes and Golgi membranes showed differences in insulin-mediated receptor autophosphorylation, even though their insulin-binding characteristics were similar. This difference was related to the generation of a Mr-84,000 fragment of the Mr-90,000 beta subunit of the plasma-membrane receptor, a fragment that was not present in the receptor from Golgi membranes, in the absence of a change in the insulin-binding alpha subunit. When autophosphorylation activity was based on insulin binding, the activity of the plasma-membrane-derived insulin receptor was decreased to 25-30% that of the Golgi-derived receptor. Endoglycosidase F digestion produced changes in the Mr values for both species, but they were not converted into a single subunit, thereby suggesting differences in the protein component of the two subunits. Although the proteinase inhibitors phenylmethanesulphonyl fluoride, ovomucoid and aprotinin failed to block the formation of the Mr-84,000 fragment, the presence of iodoacetamide or EDTA during liver homogenization markedly inhibited fragment generation and allowed the plasma-membrane insulin receptor to retain an autophosphorylation activity comparable with that present in insulin receptors from Golgi membranes. Thus a thiol-sensitive, cation-dependent, degrading activity has been identified that can uncouple the insulin-binding activity of the plasma-membrane insulin receptor from its tyrosine kinase activity.     

Adipocyte insulin-binding species: The size and subunit composition of the larger binding species

Several investigators have reported that there are both large and small insulinbinding proteins in plasma membranes; the larger protein demonstrates nonlinear Scatchard binding, and the smaller protein has linear binding. We now present evidence that the larger insulin-binding species consists of four proteins of different sizes. Rat epididymal adipocyte plasma membranes were prebound with ¹²⁵I-insulin and then exposed to 1 mM disuccinimidyl suberate for 15 min at 2°C. The membranes were solubilized in 0.1% Triton X-100 and applied to a Sepharose 6B column. Peaks of radioactivity from the column were dialyzed, lyophilized, and analyzed by dodecyl-sulphate gel electrophoresis (5%, 100/1; mono/bisacrylamide). Autoradiograms of the gels were scanned with a densitometer. The Sepharose chromatogram revealed four radioactive peaks: peak 1 at column void volume; peak 2, K_av = 0.27; peak 3, K_av = 0.77; and peak 4, K_av = 1.09. Dodecyl sulphate electrophoresis of fractions in peak 2 demonstrated four bands on autoradiography; peak 1 did not enter the gel and peaks 3 and 4 ran with the dye front. Molecular weight estimates of the four insulin-binding species in peak 2 were 600, 500, 420, and 350 K. On dithiothreitol reduction each insulin-binding species yielded subunits of M_r ? 135 and 18 K. The three largest binding species demonstrated an additional 45-K dalton protein on dithiothreitol reduction, and the 500-K and 420-K binding species also yielded a 49-K dalton protein. These results suggest that the large insulin-binding protein in rat epididymal adipocytes contains several insulin-binding species, and that these insulin-binding species differ in the number of and the type of subunits they contain. In addition, it may be postulated that the nonlinear Scatchard binding associated with the larger binding protein is a consequence of the heterogeneity of the insulin-binding species in this Sepharose peak.     

Insulin binds to type V collagen with retention of mitogenic activity

The abilities of eight extracellular matrix proteins, fibronectin, vitronectin, laminin, and collagen types I, II, III, IV, and V to bind insulin were examined by binding studies with insulin conjugated with peroxidase. At a physiological pH and ionic strength, type V collagen bound to insulin most strongly. The other types of collagen, laminin, and vitronectin also bound insulin with affinity lower than that of type V collagen. The insulin-binding site of type V collagen was in a 30-kDa CNBr fragment of the alpha 1 (V) chain. Analysis of the amino acid sequence showed that this 30-kDa fragment was identical to the heparin-binding fragment of type V collagen. The insulin-binding sites of laminin and vitronectin were located in the A chain and in the heparin-binding domain, respectively. Insulin bound to type V collagen stimulated the synthesis of DNA by mouse mammary tumor MTD cells, indicating that bound insulin retained mitogenic activity.     

Selection of DNA aptamers against insulin and construction of an aptameric enzyme subunit for insulin sensing

We selected DNA aptamers against insulin and developed an aptameric enzyme subunit (AES) for insulin sensing. The insulin-binding aptamers were identified from a single-strand DNA library which was expected to form various kinds of G-quartet structures. In vitro selection was carried out by means of aptamer blotting, which visualizes the oligonucleotides binding to the target protein at each round. After the 6th round of selection, insulin-binding aptamers were identified. These identified insulin-binding aptamers had a higher binding ability than the insulin-linked polymorphic region (ILPR) oligonucleotide, which can be called a "natural" insulin-binding DNA aptamer. The circular-dichroism (CD) spectrum measurement of the identified insulin-binding DNA aptamers indicated that the aptamers would fold into a G-quartet structure. We also developed an AES by connecting the best identified insulin-binding aptamer with the thrombin-inhibiting aptamer. Using this AES, we were able to detect insulin by measuring the thrombin enzymatic activity without bound/free separation.     

Tumor necrosis factor alpha in various tissues of insulin-resistant obese Koletsky rats: relations to insulin receptor characteristics.

Tumor necrosis factor alpha (TNFalpha) was found to be significantly increased in skeletal muscles and retroperitoneal fat of obese insulin-resistant Koletsky rats as compared to control Wistar rats. This increase was accompanied by a depression of insulin receptor protein tyrosine kinase (PTK) activity. Neither the insulin-binding capacity nor insulin receptor affinity were related to this TNFalpha increase in these tissues. In the liver, no significant changes of TNFalpha content and only a lowering of insulin-binding capacity were found. It is concluded that an increased TNFalpha content in muscles and fat (but not in the liver) contributes to insulin resistance by lowering insulin receptor protein tyrosine kinase activity, while other insulin receptor characteristics (insulin-binding capacity and affinity of insulin receptors to the hormone) do not seem to be influenced by this factor.     

Characterization of Blue-Green Fluorescence in the Mesophyll of Sugar Beet (Beta vulgaris L.) Leaves Affected by Iron Deficiency

Protease C1, an enzyme from soybean (Glycine max [L.] Merrill cv Amsoy 71) seedling cotyledons, was previously determined to be the enzyme responsible for the initial degradation of the alpha' and alpha subunits, but not the beta subunit, of beta-conglycinin storage protein. The sizes of the proteolytic products generated by the action of protease C1 suggest that the cleavage sites on the alpha' and alpha subunits of beta-conglycinin may be located in their N-terminal domain, which is not found in the beta subunit of beta-conglycinin. To check this hypothesis, storage proteins from other plant species that are homologous to either the alpha'/alpha or the beta subunit of beta-conglycinin were tested as substrates. As expected, the convicilin from pea (Pisum sativum), a protein homologous to the alpha' and alpha subunits of beta-conglycinin, was digested by protease C1. The vicilins from pea as well as vicilins from adzuki bean (Vigna angularis), garden bean (Phaseolus vulgaris), black-eyed pea (Vigna unguiculata), and mung bean (Vigna radiata), storage proteins that are homologous to the beta subunit of soybean beta-conglycinin, were not degraded by protease C1. Degradation of soybean beta-conglycinin involves a sequential attack of the alpha subunit at multiple sites, culminating in the formation of a stable intermediate of 53.5 kD and a final product of 48.0 kD. The cleavage sites resulting in this formation of the intermediates and final product were determined by N-terminal analysis. These were compared to the known amino acid sequences of the three beta-conglycinin subunits. Results showed these two polypeptides to be generated by proteolysis of the alpha subunit at regions bearing long strings of acidic amino acid residues.     

Photoaffinity labeling of the insulin receptor in H4 hepatoma cells: Lack of cellular receptor processing

Photoaffinity labeling techniques were used to identify insulin-binding components of the plasma membrane in insulin-responsive, monolayer-cultured hepatoma cells. The activated, photosensitive reagent, an n-hydroxysuccinimide ester of 4-azidobenzoic acid, was coupled with highly purifed insulin, and the hormone derivative was subsequently iodinated, bound to cell surface receptors of intact H4 cells, and photoactivatcd. After dissolution of the cells, labeled proteins were analyzed by SDS/polyacrylamide gel electrophoresis under reducing conditions. The main labeled band exhibited an apparent molecular weight of 130,000. Two minor components of apparent mol wt 95,000 and 40,000 were also identified. Specific labeling of all 3 bands was inhibited by simultaneous incubation of the cells with native insulin, but not by the heterologous hormone, glucagon, prior to photoactivation. Binding of azidobenzoyl-insulin to H4 cells was time-dependent, as was the correlated labeling of receptor components. Band-labeling by the photosensitive insulin derivative was totally light-dependent; spontaneous covalent linking of insulin and receptor was not observed. The labeled receptor-related proteins were not degraded by the cells under our experimental conditions.     

Elucidation of the quaternary structure of the insulin receptor.

Photoreactive insulin analogues specifically label predominantly one polypeptide in the insulin receptor of rat liver plasma membranes. We have used the bifunctional reagent disuccinimidyl suberate to cross-link this polypeptide to its neighbouring, but not necessarily labelled, subunits. The results of these studies show that (1) there are at least three types of subunit in the receptor, with apparent Mr (Mapp.) values of 65 000, 95 000 and 120 000; (2) the receptor appears to consist of two Mapp. 120 000, one Mapp. 95 000 and one Mapp. 65 000 subunits; (3) the Mapp. 65 000 subunit, which has not been previously reported, may be only loosely attached to the receptor, and does not interact directly with the insulin-binding subunit (M app. 120 000).     

Purification of the catalytically active phosphorylated form of insulin receptor kinase by affinity chromatography with O-phosphotyrosyl-binding antibodies

The catalytically active, tyrosyl-phosphorylated form of insulin receptor kinase was isolated from human placenta by a procedure which exploits the propensity for the intact alpha 2 beta 2 form of insulin receptor to undergo insulin-promoted autophosphorylation at tyrosyl residues and concomitant activation as a tyrosyl kinase. Purification of tyrosyl-phosphorylated insulin receptor was effected by adsorption on and elution (with a hapten) from a column of O-phosphotyrosyl-binding antibody immobilized on protein A-Sepharose (Ab-protein A). The starting material for the purification process was protein which had been solubilized from placental membranes and purified by chromatography on immobilized wheat germ agglutinin. After chromatography on Ab-protein A to remove preexisting O-phosphotyrosyl-containing proteins, the fraction which did not adsorb to the Ab-protein A column was incubated with insulin and briefly treated with ATP so as to maximize selective autophosphorylation of insulin receptor. This material was then subjected to chromatography on Ab-protein A. Although the amount of the intact alpha 2 beta 2 form of insulin receptor present in the starting material was only a small fraction of the protein (approximately 0.2%) and only approximately 20% of the insulin-binding forms of the receptor present, it was eluted (with 10 mM p-nitrophenyl phosphate) from the column in greater than or equal to 80% purity. Chromatography on Ab-protein A appears to have an advantage over the alternative affinity chromatographic procedures which utilize immobilized insulin or antiinsulin receptor antibody to adsorb insulin receptor, since these procedures do not resolve the intact alpha 2 beta 2 form of insulin receptor from the nicked insulin-binding forms of the receptor which do not undergo insulin promoted autophosphorylation.     

Cytosolic insulin-binding proteins of mouse liver cells

It has been recently shown that insulin retains its biological activity after receptor-directed internalization and it may affect the cell metabolism by interaction with cytosolic insulin-binding proteins (CIBPs). Using affinity chromatography combined with SDS-PAGE and MALDI-TOF mass-spectrometry we have identified 7 proteins from mouse liver cells that specifically bind to the insulin, including adenylate kinase 2 (25.6 kD), kinesin superfamily protein 20B (26.0 kD), hepatic arginase 1 (34.8 kD), fructose-bisphosphate aldolase B (39.5 kD), 4-hydroxyphenylpyruvate dioxygenase (45.1 kD), betaine-homocysteine methyl-transferase (45.0 kD) and KRIT1 (83.4 kD).     

The expression and processing of two recombinant 2S albumins from soybean (Glycine max) in the yeast Pichia pastoris

Soybean seeds contain two 2S albumin storage proteins (AL1 and AL3) which may contribute to their industrial processing quality and allergenicity. We show that these proteins (AL1 and AL3) are well expressed by the methylotrophic yeast Pichia pastoris and that one of the secreted proteins (AL3) has a similar conformation and stability to that purified from soybean seeds. Further, we show that the subunits are post-translationally processed within the same loop region as the native protein but with some differences in the precise sites. This internal processing provides useful information on the endoproteolytic activity in P. pastoris. We also show that, similar to many plant allergens, the 2S albumins from soybean are stable to heat and chemical treatments.     

Mobilization of storage proteins in soybean seed (Glycine max L.) during germination and seedling growth

During germination and early growth of the seedling, storage proteins are degraded by proteases. Currently, limited information is available on the degradation of storage proteins in the soybean during germination. In this study, a combined two-dimensional gel electrophoresis and mass spectrometry approach was utilized to determine the proteome profile of soybean seeds (Glycine max L.; Eunhakong). Comparative analysis showed that the temporal profiles of protein expression are dramatically changed during the seed germination and seedling growth. More than 80% of the proteins identified were subunits of glycinin and β-conglycinin, two major storage proteins. Most subunits of these proteins were degraded almost completely at a different rate by 120h, and the degradation products were accumulated or degraded further. Interestingly, the acidic subunits of glycinin were rapidly degraded, but no obvious change in the basic chains. Of the five acidic subunits, the degradation of G2 subunit was not apparently affected by at least 96h but the levels decreased rapidly after that, while no newly appearing intermediate was detected upon the degradation of G4 subunit. On the other hand, the degradation of β-conglycinin during storage protein mobilization appeared to be similar to that of glycinin but at a faster rate. Both α and α' subunits of β-conglycinin largely disappeared by 96h, while the β subunits degraded at the slowest rate. These results suggest that mobilization of subunits of the storage proteins is differentially regulated for seed germination and seedling growth. The present proteomic analysis will facilitate future studies addressing the complex biochemical events taking place during soybean seed germination.     

Identification of marker proteins for the adulteration of meat products with soybean proteins by multidimensional liquid chromatography-tandem mass spectrometry

Soybean proteins are frequently added to processed meat products for economic reasons and to improve their functional properties. Monitoring of the addition of soybean protein to meat products is of high interest due to the existence of regulations forbidding or limiting the amount of soybean proteins that can be added during the processing of meat products. We have used chromatographic prefractionation on the protein level by perfusion liquid chromatography to isolate peaks of interest from extracts of soybean protein isolate (SPI) and of meat products containing SPI. After enzymatic digestion using trypsin, the collected fractions were analyzed by nanoflow liquid chromatography-tandem mass spectrometry. Several variants and subunits of the major seed proteins, glycinin and beta-conglycinin, were identified in SPI, along with two other proteins. In soybean-protein-containing meat samples, different glycinin A subunits could be identified from the peak discriminating between samples with and without soybean proteins added. Among those, glycinin G4 subunit A4 was consistently found in all samples. Consequently, this protein (subunit) can be used as a target for new analytical techniques in the course of identifying the addition of soybean protein to meat products.     

Regulation of the insulin receptor kinase by hyperinsulinism

A murine fibroblast cell line transfected with human insulin receptor cDNA, NIH 3T3 HIR3.5, was observed to display insulin-induced down-regulation of insulin-binding activity in a time- and concentration-dependent manner. Maximal inhibition of insulin-binding activity (54%) occurred within 16 h of exposure to 100 nM insulin in vivo, where in vivo refers to intact cells in tissue culture. The decrease in cellular insulin-binding activity was the consequence of a decrease in the number of cell-associated insulin receptors as determined by Scatchard analysis of insulin binding, 125I-insulin affinity cross-linking, and Western blotting of the insulin receptor beta subunit. Acute insulin treatment in vivo (1-60 min) resulted in the activation of the insulin receptor protein tyrosine kinase as determined by in vitro phosphorylation of glutamic acid:tyrosine (4:1), where in vitro refers to broken cell preparations. This acute in vivo insulin activation of the insulin receptor tyrosine kinase resulted in a greater stimulation (1.4-1.9-fold) of tyrosine kinase activity in the glutamic acid:tyrosine (4:1) assay than the maximal stimulation produced by insulin treatment in vitro. In contrast, long term (24 h) insulin treatment in vivo resulted in a 50-70% decrease in intrinsic protein tyrosine kinase activity of the insulin receptors compared with that of acutely activated (1 min) insulin receptors. Under these conditions, the insulin receptor protein kinase activity remained insulin independent in the in vitro substrate kinase assay. Surprisingly, the insulin-independent activated (1 min in vivo insulin-treated) and uncoupled (24 h in vivo insulin-treated) insulin receptors displayed similar stoichiometries of 32P incorporation into the beta subunit by in vitro autophosphorylation when compared with the control insulin receptors, ranging from 1.5 to 1.8 mol of phosphate incorporated/mol of insulin receptor. Phosphoamino acid analysis demonstrated that the phosphoserine/phosphothreonine content of in vivo 32P-labeled insulin receptors increased markedly within a 1-h exposure to insulin in vivo, whereas insulin-induced receptor desensitization was not apparent until 10-24 h after exposure to insulin. These data suggest that insulin treatment in vivo results initially in the activation of the insulin receptor kinase followed by a subsequent uncoupling of protein kinase activity. This insulin-induced desensitization of the insulin receptor kinase does not correlate with the extent of beta subunit serine/threonine phosphorylation.