Direct cross-linking of 125I-labeled glucagon to its membrane receptor by UV irradiation

Regulation of two GTPases Toc159 and Toc34 in the translocon of the outer envelope of chloroplasts

The GTPases Toc159 and Toc34 of the translocon of the outer envelope of chloroplasts (TOC) are involved in recognition and transfer of precursor proteins at the cytosolic face of the organelle. Both proteins engage multiple interactions within the translocon during the translocation process, including dimeric states of their G-domains. The units of the Toc34 homodimer are involved in the recognition of the transit peptide representing the translocation signal of precursor proteins. This substrate recognition is part of the regulation of the GTPase cycle of Toc34. The Toc159 monomer and the Toc34 homodimer recognize the transit peptide of the small subunit of Rubisco at the N- and at the C-terminal region, respectively. Analysis of the transit peptide interaction by crosslinking shows that the heterodimer between both G-domains binds pSSU most efficiently. While substrate recognition by Toc34 homodimer was shown to regulate nucleotide exchange, we provide evidence that the high activation energy of the GTPase Toc159 is lowered by substrate recognition. The nucleotide affinity of Toc34G homodimer and Toc159G monomer are distinct, Toc34G homodimer recognizes GDP and Toc159G GTP with highest affinity. Moreover, the analysis of the nucleotide association rates of the monomeric and dimeric receptor units suggests that the heterodimer has an arrangement distinct from the homodimer of Toc34. Based on the biochemical parameters determined we propose a model for the order of events at the cytosolic side of TOC. The molecular processes described by this hypothesis range from transit peptide recognition to perception of the substrate by the translocation channel.

Factors affecting insulin-regulated hepatic gene expression

Obesity has become a major concern of public health. A common feature of obesity and related metabolic disorders such as noninsulin-dependent diabetes mellitus is insulin resistance, wherein a given amount of insulin produces less than normal physiological responses. Insulin controls hepatic glucose and fatty acid metabolism, at least in part, via the regulation of gene expression. When the liver is insulin-sensitive, insulin can stimulate the expression of genes for fatty acid synthesis and suppress those for gluconeogenesis. When the liver becomes insulin-resistant, the insulin-mediated suppression of gluconeogenic gene expression is lost, whereas the induction of fatty acid synthetic gene expression remains intact. In the past two decades, the mechanisms of insulin-regulated hepatic gene expression have been studied extensively and many components of insulin signal transduction pathways have been identified. Factors that alter these pathways, and the insulin-regulated hepatic gene expression, have been revealed and the underlying mechanisms have been proposed. This chapter summarizes the recent progresses in our understanding of the effects of dietary factors, drugs, bioactive compounds, hormones, and cytokines on insulin-regulated hepatic gene expression. Given the large amount of information and progresses regarding the roles of insulin, this chapter focuses on findings in the liver and hepatocytes and not those described for other tissues and cells. Typical insulin-regulated hepatic genes, such as insulin-induced glucokinase and sterol regulatory element-binding protein-1c and insulin-suppressed cytosolic phosphoenolpyruvate carboxyl kinase and insulin-like growth factor-binding protein 1, are used as examples to discuss the mechanisms such as insulin regulatory element-mediated transcriptional regulation. We also propose the potential mechanisms by which these factors affect insulin-regulated hepatic gene expression and discuss potential future directions of the area of research.

Biochemical and functional characterization of UDP-galactose 4-epimerase from Aeromonas hydrophila

Bacteria of genus Aeromonas, responsible for a variety of pathological conditions in humans and fish, are ubiquitous waterborne bacteria. Aeromonas produces several virulent factors including a complex of lipopolysaccharide and surface array protein, involved in colonization. UDP-galactose 4-epimerase (GalE) catalyzes the production of UDP-galactose, a precursor for lipopolysaccharide biosynthesis, and thus is an important drug target. GalE exhibits interspecies variation and heterogeneity at its structural and functional level and therefore, the differences between the GalE of the host and the pathogen can be exploited for drug designing. In the present study, we report biochemical and functional characterization of the recombinant GalE of Aeromonas hydrophila. Unlike GalE reported from all other species, the purified recombinant GalE of A. hydrophila was found to exist as a monomer. This is the first report of UDP-galactose 4-epimerase from any species being a monomer. The molecular mass of the 6xHis-rGalE was determined to be 38271.477 (m/z). The 6xHis-rGalE with a K(m) of 0.5 mM for UDP-galactose exhibited optimum activity at 37 degrees C and pH 8-9. Spectrofluorimetric and CD analysis confirmed that the thermal inactivation was due to structural changes and not due to the NAD-dissociation. A relatively more ordered structure of the enzyme at pH 8 and 9 as compared to that at pH 6 or 7 suggests a key role of the electrostatic interactions in maintaining its native tertiary structure.

Interaction of [fluorescein-Trp25]glucagon with the human glucagon receptor expressed in Drosophila Schneider 2 cells

The human glucagon receptor was expressed at high density in Drosophila Schneider 2 (S2) cells. Following selection with G418 and induction with CuSO4, the cells expressed the receptor at a level of 250 pmol/mg of membrane protein. The glucagon receptor was functionally coupled to increases in cyclic AMP in S2 cells. Protein immunoblotting with anti-peptide antibodies revealed the expressed receptor to have an apparent molecular mass of 48 kDa, consistent with low levels of glycosylation in this insect cell system. Binding of [fluorescein-Trp25]glucagon to S2 cells expressing the glucagon receptor was monitored as an increase in fluorescence anisotropy along with an increase in fluorescence intensity. Anisotropy data suggest that the mobility of the fluorescein is restricted when the ligand is bound to the receptor. Kinetic analysis indicates that the binding of glucagon to its receptor proceeds via a bimolecular interaction, with a forward rate constant that is several orders of magnitude slower than diffusion-controlled. These data would be consistent with a conformational change upon the binding of agonist to the receptor. The combination of [fluorescein-Trp25]glucagon with the S2 cell expression system should be useful for analyzing glucagon receptor structure and function.

Characterization and purification of the solubilized pituitary adenylate-cyclase-activating polypeptide-1 receptor from porcine brain using a biotinylated ligand

A specific receptor for the brain-gut neuropeptide pituitary adenylate-cyclase-activating polypeptide (PACAP-1 receptor) was solubilized with Chapso from porcine brain plasma membranes and purified. Binding of 125I-PACAP(1-27) to the solubilized material was reversed equipotently by unlabeled PACAP(1-27) and PACAP(1-38). Soluble receptors retained the binding affinities and specificities of the plasma membrane fraction. Scatchard analysis of equilibrium-binding data indicated the existence of a single high-affinity binding site (Kd = 0.23 nM, Bmax = 1.2 pmol/mg protein). Binding of 125I-PACAP(1-27) to solubilized receptors was not affected by guanosine nucleotides, suggesting that solubilization dissociates the PACAP-1 receptor/guanosine-nucleotide-binding protein complex. Affinity cross-linking of 125I-PACAP(1-27) to soluble PACAP-1 receptors identified a specifically labeled 60-kDa protein. Enzymic deglycosylation of soluble affinity-labeled receptors reduced the apparent molecular mass by 10 kDa. The solubilized receptor glycoprotein was purified 4-5-fold by lectin-adsorption chromatography on wheatgerm agglutinin immobilized on agarose. S-Biotinyl[Ala28-34, Cys35]PACAP(1-35) was synthesized, immobilized on streptavidin-coated magnetic Sepharose beads and used to further affinity-purify wheatgerm-agglutinin-eluted receptor material. This more than 6000-fold enriched PACAP-1-receptor-preparation retained single-class high-affinity binding and consisted of an almost homogenous 55-60-kDa protein identified by silver staining. In conclusion, we established a rapid method for purification of PACAP-1 receptors, allowing further studies to be performed by protein chemistry.

Isolation and partial characterization of a specific alpha-fetoprotein receptor on human monocytes

Since a large body of data has suggested a significant role for alpha-fetoprotein (AFP) in the regulation of the immune response at a number of levels, we examined the possibility of a specific receptor for AFP on the immune recognition cell, the monocyte/macrophage. Microscopic autoradiography exhibited an obvious binding of AFP almost exclusively on human peripheral monocytes but not on lymphocytes. In a human monocyte cell line (U937) Scatchard plot analysis indicated the presence of two distinct AFP-specific binding sites with a Kd of 5 x 10(-11) M, 49 binding sites per cell, and 2.5 x 10(-7) M, 7,800 binding sites per cell. 125I-ASD-AFP, AFP-radiolabeled bifunctional photoactivatable thio-cleavable cross-linker, was used to isolate the AFP binding protein from U937 cells. After ultraviolet photoactivation, 125I-sulfosuccinimidyl 2-(p-azido-salicylamido)ethyl-1,3'-dithiopropionate was covalently linked to the putative receptor. Autoradiography of SDS gradient PAGE under reducing conditions showed a major radiolabeled band at between 62 and 65 kD. To confirm the specificity of the finding, recombination of AFP with the isolated receptor was examined in artificially reconstituted membrane vesicles, which also resulted in a single band at approximately 62-65 kD by SDS-PAGE autoradiography. From the data above, we concluded that human monocytes possess a specific AFP binding protein on the membrane, a putative receptor, which may be involved with the physiological regulation of the immune response.

The GTP-insensitive component of high-affinity [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding in the rat hippocampus corresponds to an oxidized state of the 5-hydroxytryptamine1A receptor

Previous studies on central 5-hydroxytryptamine1A (5-HT1A) receptors have consistently shown the existence of a GTP-insensitive component of agonist binding, i.e., binding of [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) that persists in the presence of 0.1 mM GTP or guanylylimidodiphosphate (GppNHp). The molecular basis for this apparent heterogeneity was investigated pharmacologically and biochemically in the present study. The GppNHp-insensitive component of [3H]8-OH-DPAT binding increased spontaneously by exposure of rat hippocampal membranes or their 3-[3-(cholamidopropyl)dimethylammonio]-1-propane sulfonate-soluble extracts to air; it was reduced by preincubation of solubilized 5-HT1A binding sites in the presence of dithiothreitol and, in contrast, reversibly increased by preincubation in the presence of various oxidizing reagents like sodium tetrathionate or hydrogen peroxide. In addition, exposure of hippocampal soluble extracts to short-cross-linking reagents specific for thiols produced an irreversible increase in the proportion of GppNHp-insensitive over total [3H]8-OH-DPAT binding. The pharmacological properties of this GppNHp-insensitive component of [3H]8-OH-DPAT binding were similar to those of 5-HT1A sites in the absence of nucleotide. Sucrose gradient sedimentation of solubilized 5-HT1A binding sites treated by dithiothreitol or sodium tetrathionate showed that oxidation prevented the dissociation by GTP of the complex formed by the 5-HT1A receptor binding subunit (R[5-HT1A]) and a guanine nucleotide-binding protein (G protein). Moreover, the oxidation of -SH groups by sodium tetrathionate did not prevent the inactivation of [3H]8-OH-DPAT specific binding by N-ethylmaleimide, in contrast to that expected from an interaction of both reagents with the same -SH groups on the R[5-HT1A]-G protein complex. These data suggest that the appearance of GTP-insensitive [3H]8-OH-DPAT specific binding occurs as a result of the (spontaneous) oxidation of essential -SH groups (different from those preferentially inactivated by N-ethylmaleimide) on the R[5-HT1A]-G protein complex.

Purification of an angiotensin II binding protein by using antibodies to a peptide encoded by angiotensin II complementary RNA

We have generated a monospecific antibody to a synthetic peptide encoded by an RNA complementary to the mRNA for angiotensin II (AII) and determined whether this antibody recognizes the AII receptor. We demonstrate that the antibody competes specifically with 125I-labeled AII for the same binding site on rat adrenal membranes. Furthermore, we show that this antibody inhibits the secretion of aldosterone from cultured rat adrenal cells, suggesting that the antibody recognizes the biologically relevant AII receptor. Finally, we demonstrate that antibody to the complementary peptide can be used to immunoaffinity-purify a protein of Mr 66,000 that specifically binds radiolabeled AII.

Gastric inhibitory polypeptide receptor in hamster pancreatic beta cells. Direct cross-linking, solubilization and characterization as a glycoprotein

125I-labelled gastric inhibitory polypeptide (125I-GIP) is directly cross-linked to its specific receptor in hamster pancreatic beta cell membranes by using an ultraviolet irradiation procedure. This approach results in the identification of a GIP-protein complex of apparent Mr 64,000. The labelling of this protein species is specific since it is inhibited when incubating the membranes with increasing doses of native GIP (0.1 nM-1 microM) together with 125I-GIP, half-maximal inhibition being elicited by 5 nM peptide. Reduction of the GIP-protein complex by 100 mM dithiothreitol induces a decrease of the electrophoretic mobility of the complex. Alternatively pretreatment of membranes with dithiothreitol (up to 1 M) does not prevent the binding of 125I-GIP to its receptor. When prelabelled membranes are extracted by 0.5% Triton X-100 (v/v) and the extract is layered on a Sephadex G-50 column, a high peak of radioactivity is eluted with the void volume of the column. Treatment of this peak by 10 min ultraviolet irradiation followed by SDS-PAGE leads to identification of a major band of Mr 64,000. When the peak is further layered on Sephacryl S-200 it yields a single peak of radioactivity corresponding to a protein species with a Stokes radius of 3.2 nm and an apparent Mr of 65,000. The solubilized GIP-receptor complex is specifically adsorbed by Sepharose coupled to wheat germ agglutinin and concanavalin A and eluted from these lectins by their respective sugars. In conclusion the GIP receptor in pancreatic beta cells is a protein monomer of apparent Mr 59 000; its structure is maintained by intrachain disulfide bridges, these bonds being, however, not involved in the interaction of GIP with its receptor; the GIP receptor is a glycoprotein containing N-acetylglucosamine, mannose and probably sialic acid in its carbohydrate moiety.