Photoaffinity labeling of the follitropin receptor.
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Affinity Labels↗
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Photoaffinity labeling of melanocyte-stimulating hormone receptors.
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Adrenergic alpha-Agonists↗
Design, implementation and pitfalls of photoaffinity labelling experiments in in vitro preparations. General principles.
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Photoaffinity labelling of Plasmodium falciparum proteins involved in phospholipid transport.
Erythrocytes infected with mature-stage malaria parasites accumulate phospholipids from exogenous sources. We show that the transport of N-(7-nitrobenzy-2-oxa-1,3-diazol-4-yl)-1,2- dipalmitoyl-sn-glycero-3-phosphatidylethanolamine (N-NBD-DPPE), from the erythrocyte membrane to the intracellular malaria parasite, is dependent upon metabolic energy. A photoreactive phospholipid analogue, N-[125I]iodo-4-azidosalicylamidyl-1, 2-dilauryl-sn-glycero-3-phosphatidylethanolamine (N-125I-ASA-DLPE), has been synthesised and used in an attempt to identify proteins involved in phospholipid trafficking in malaria-infected erythrocytes. This photoreactive probe was found to preferentially label a protein with an apparent molecular weight of 22 kDa. Photolabelling of the 22-kDa protein was enhanced upon ATP depletion of malaria-infected erythrocytes.
Adenosine Triphosphate↗
E coli tRNAPhe modified at the 3-(3-amino-3-carboxypropyl) uridine with a photoaffinity label is fully functional for aminoacylation and for ribosomal interaction.
E. coli tRNAPhe was modified at its 3-(3-amino-3-carboxypropyl)uridine residue with the N-hydroxysuccinimide ester of N-(4-azido-2-nitrophenyl) glycine. Exclusive modification of this base was shown by two-dimensional TLC analysis of the T1 oligonucleotide and nucleoside products of nuclease digestion. The fully modified tRNA could be aminoacylated to the same level as control tRNA. The aminoacylated tRNA was as active as control tRNA in non-enzymatic binding to the P site of ribosomes, and in EFTu-dependent binding to the ribosomal A site. The functional activity of this photolabile modified tRNA allows it to be used to probe the A and P binding sites on ribosomes and on the other proteins that interact with tRNA. Crosslinking to the ribosomal P site has been shown.
Affinity Labels↗
Synthesis of gamma-[4-(benzoylphenyl)methylamido]-7-methylguanosine 5'-triphosphate, a photoaffinity-label for cap-binding proteins.
An easy and rapid method for the synthesis of the photoreactive cap analogue gamma-[4-(benzoylphenyl)methylamido]-7-methylguanosine-5'-triphosp hate (BP-m7GTP) has been developed. Synthesis of [gamma-32P]BP-m7GTP was carried out as a two-step procedure. The first step included the phosphorylation of GDP with [32P]orthophosphate to [gamma-32P]GTP and its methylation to [gamma-32P]m7GTP. After purification, [gamma-32P]m7GTP was condensed with 4-aminomethyl-benzophenone-HCl to [gamma-32P]BP-m7GTP. Using this probe, specific labeling of nuclear cap binding proteins is demonstrated.
Affinity Labels↗
Photoaffinity labelling of the human platelet thromboxane A2/prostaglandin H2 receptor.
The synthesis, binding and photoincorporation of a thromboxane A2/prostaglandin H2 (TXA2/PGH2) analog (9,11-dimethylmethano-11,12-methano-16-(3-[125I]iodo-4-azidophenyl )-13,14- dihydro-13-aza-15 alpha beta-omega-tetranor-TXA2) [( 125I]PTA-Azido) to washed human platelets was characterized. Kinetic analysis of the binding of [125I]PTA-Azido at 30 degrees C yielded a k1 of 1.83.10(7) M-1.min-1 and k -1 of 0.195 min-1, Kd = k -1/k1 = 11 nM. Incubation of washed human platelets with [125I]PTA-Azido followed by photolysis resulted in the radiolabelling of a number of platelet proteins as assessed by SDS-PAGE autoradiography. The radiolabelling of three of these protein bands could be either uniformly blocked or reduced with a series of structurally dissimilar TXA2/PGH2 receptor antagonists or agonists and corresponded to proteins with a molecular mass of 43, 39 and 27 kDa. In addition, the incorporation of [125I]PTA-Azido into the three proteins was stereoselectively blocked by a pair of optically active stereoisomers that are TXA2/PGH2 receptor antagonists. Two-dimensional gel electrophoresis indicated that the 43 kDa protein possessed a pI value of 5.6 and that the 27 kDa protein exists in at least three isoforms with pI values of 4.9, 5.1 and 5.3. The labelling pattern was not altered by a mixture of proteinase inhibitors. The data suggest that one or more of these specifically radiolabelled proteins may represent the human platelet TXA2/PGH2 receptor.
Affinity Labels↗
Preparation of N epsilon B28-monoazidobenzoyl insulin-like growth factor I and photoaffinity labeling of insulin-like growth factor I receptor.
Recombinant human insulin-like growth factor I (hIGF-I) was reacted with azidobenzoyl hydroxysuccinimide to produce a mixture of photoactive hIGF-I derivatives. The mixture was purified by reversed-phase HPLC to yield three mono-substituted azidobenzoyl hIGF-Is. One of the derivatives was identified by amino acid sequencing as N epsilon B28-monoazidobenzoyl hIGF-I. This derivative was indistinguishable from native hIGF-I when bioassayed in Rat-1 fibroblasts. A 120-kDa band, the alpha subunit of the IGF-I receptor, was specifically labeled in Rat-1 plasma membranes by this photoprobe. The labeling of this band was reduced by hIGF-I at 1 nM and completely abolished by hIGF-I, but not insulin, at 100 nM, indicating the specificity of the photolabeling of the IGF-I receptor by this fully active IGF-I photoprobe.
Affinity Labels↗
Photoaffinity labeling of plasma membrane receptors for cytochalasins in Ehrlich tumor cells.
Treatment of purified Ehrlich ascites cell plasma membranes either with [3H]cytochalasin B or [3H]19-O-acetylchaetoglobosin A under photolytic conditions produced several radioactive polypeptides which were characterized by SDS-PAGE analyses. The major proteins so photolabeled were in the 60,000-80,000 Da range, with less labeling found in polypeptides smaller than 43,000 and greater than 90,000 Da. Immunofluorescent staining failed to identify the major photolabeled component as actin. It is concluded, in keeping with prior investigations using other cell types, that the predominant proteins photolabeled by cytochalasins are affiliated with the glucose-transport system.
Affinity Labels↗
Photoaffinity labeling of the androgen receptor from human skin fibroblasts.
The reproducible photolabeling of the androgen receptor from human skin fibroblasts, using [3H]methyltrienolone (R-1881) as ligand is described. Crude nuclei were irradiated for 2 min using a UV lamp with an emission line at 352 nm and a CuSO4 filter. After KCl extraction, proteins were precipitated with trichloroacetic acid, washed with ether and assayed for radioactivity. Specific binding was determined as the difference in bound radioactivity between cells incubated with [3H]R-1881 +/- a 200-fold excess of unlabeled dihydrotestosterone (DHT). The photolabeled proteins were analyzed on SDS-polyacrylamide gel electrophoresis yielding one peak of 90 kDa and in several cases, one of 43 kDa. These peaks comprised 60 +/- 20% of the saturable binding recovered on the gels. The overall efficiency of photolabeling was between 1 and 5%. The amount of covalently bound radioactivity was proportional to the number of cells used. The labeling was inhibited by R-1881, DHT, the anti-androgens hydroxyflutamide and cyproterone acetate and to a lesser extent by estradiol and progesterone. No covalent attachment of R-1881 to any protein was observed when nuclei from patients with androgen insensitivity were irradiated, whether or not the cells were receptor positive or negative. In conclusion the androgen receptor from human skin fibroblast can be efficiently photolabeled and could be used as a marker to follow receptor purification. The absence of photolabeling of nuclear extracts from receptor-positive androgen-insensitive patients may reflect some abnormality of the receptor.
Affinity Labels↗
Investigation of opioid receptor binding protein in adult and fetal mouse brain membranes by photoaffinity labeling.
125I-[D-Ala2,p-N3-Phe4-Met5]Enkephalin was used to investigate the opioid receptor from brain membranes of adult (6 months old) and fetal (16 days gestation) mice. The photolabeled membranes were analyzed by sodium dodecyl sulfate gel electrophoresis. A 46,000-Da protein was specifically photo-labeled in the adult mouse brain membranes. The photolabeling of this protein was inhibited in the presence of [D-Ala2,Met5]enkephalin and a peptide ligand specific for the mu-opioid receptor. No specific labeling of any protein was detected in the brain membranes of the fetal mice. This apparent lack of the opioid receptor in the fetal mouse brain suggests that the appearance of the opioid receptor in the adult mouse brain is subject to developmental regulation.
Animals↗
Photoaffinity labeling and binding studies reveal the existence of two types of phencyclidine receptors in the NCB-20 cell line.
The mouse neuroblastoma-Chinese hamster brain hybrid cell line NCB-20 is the only clonal cell line in which binding studies indicate the presence of phencyclidine (PCP) receptor-like sites. We report here that polypeptide components of NCB-20 cell PCP sites were identified with the photolabile PCP derivative [3H]N-[1-(3-azidophenyl)cyclohexyl]piperidine ([3H]AZ-PCP). The pharmacological selectivity of [3H]AZ-PCP binding (under reversible conditions) was similar to that observed for [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to NCB-20 cell membranes. Inhibition of [3H]TCP binding by AZ-PCP, dexoxadrol or MK-801 was biphasic, suggesting the presence of two types of PCP sites on NCB-20 cells. Photolysis of NCB-20 cell membranes pre-equilibrated with [3H]AZ-PCP, followed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), revealed the presence of 5 major labeled bands (Mr 90,000, 68,000, 49,000, 40,000 and 33,000), a pattern similar to that observed for rat brain membranes. MK-801 and D-2-amino-5-phosphonovaleric acid (D-(-)-AP5) selectively inhibited the labeling of Mr 68,000 and 90,000 polypeptides. These results indicate that the labeled bands represent constituents of at least two different PCP binding proteins. The Mr 68,000 and 90,000 components appear to correspond to a high-affinity site, which comprises approximately 20% of total [3H]TCP sites in these cells, and exhibits the pharmacology expected for the PCP receptor of the N-methyl-D-aspartate (NMDA)-gated channel.
Animals↗
Intracellular binding of ethidium studied by photoaffinity labeling in vivo.
The azide analog of 14C-labeled ethidium bromide was mixed with yeast cells and when photolyzed by visible light, formed covalent complexes with all yeast cell organelles. The 14C counts were found in DNA, RNA and protein of yeast subcellular fractions, illustrating the complexity of binding of a drug which appears highly specific in its actions.
Affinity Labels↗
Photoaffinity labeling of platelet thrombin-binding proteins.
The interaction of thrombin and platelets was studied with a heterobifunctional photoactivable crosslinking agent. Radiolabeled thrombin that was modified with ethyl-N-5-azido-2-nitrobenzoylaminoacetimidate formed two types of complex with platelet proteins: platelet-associated complexes and supernatant complexes. The platelet-associated complexes formed within 20 s. Autoradiography after electrophoresis with sodium dodecyl sulfate indicated that these complexes had apparent masses of 210, 185, 155 and 125 kDa. Formation of the complexes was blocked by hirudin; this is consistent with crosslinking that was a direct consequence of the binding of thrombin to a specific receptor, since hirudin blocks thrombin-induced platelet activation and the saturable binding of thrombin to platelets. The labeled supernatant complex had an apparent mass of about 490 kDa. It also formed in the supernatant solution of platelets after activation with a divalent cation ionophore, suggesting a complex of thrombin with a secreted protein. The supernatant complex did not involve fibrinogen or alpha 2-macroglobulin, but a similar complex was formed with partially purified secreted glycoprotein G (thrombin-sensitive protein, thrombospondin). Formation of the complex was blocked by hirudin. A similar complex was formed after prolonged (1 h) incubation without photoactivation. It is concluded that thrombin forms high-affinity, hirudin-sensitive complexes with secreted glycoprotein G, as well as with platelet surface proteins.
Blood Platelets↗
Photoaffinity labeling of thaumatin-binding protein in monkey circumvallate papillae.
Thaumatin I is an intensely sweet-tasting protein. It was photo-crosslinked with taste papillae of crab-eating monkey by using a conjugated photo-affinity reagent [3H]azidobenzoylthaumatin I. Serial sections of SDS-polyacrylamide gel electrophoresis of the 0.1 M sodium phosphate buffer-soluble fraction from taste papillae had a large peak of radioactivity at the Mr region of approx. 70,000; fractions from non-taste papillae did not. Excess unlabeled thaumatin I reduced the photo-crosslinking at the 70 kDa region; acetylated thaumatin I (which is not sweet) did not. The results show that taste papillae of the monkey contain a protein of Mr approx. 50,000, which binds to thaumatin I (Mr 22,209) but not to completely acetylated thaumatin I. The possibility that the thaumatin-binding protein is a sweet receptor protein is discussed.
Acetylation↗
Photoaffinity labelling of the mu-opioid receptor is dependent on the nature of the photosensitive group of carfentanil derivatives.
Arylazido and aryldiazonium derivatives of carfentanil, bearing their photoactivatable function at the same position on the molecule, were synthesized. In the dark both of them exhibited similar binding affinity profiles and behaved as mu-selective and reversible ligands of the opioid receptor sites. Following irradiation, only the azido analog displayed the properties of an efficient, irreversible and selective label of the mu-opioid receptor, allowing physicochemical requirements for alkylation of this receptor subtype to be examined. Evidence is presented to consider this azido compound a promising tool for characterizing the mu-opioid receptor protein.
Affinity Labels↗
Photoaffinity labeling of plasma membrane proteins involved in the transport of loop diuretics into hepatocytes.
To identify proteins involved in the hepatocellular uptake of loop diuretics, [3H]bumetanide was photoactivated by light flash in the presence of either intact isolated rat hepatocytes, rat liver basolateral plasma membranes or integral membrane proteins extracted from the basolateral plasma membranes. Proteins of 52-54, 48, 33, 27, 25 and 23 kDa in sodium dodecyl sulfate (SDS) gel electrophoresis were radiolabeled on intact hepatocytes. On liver basolateral plasma membranes a 50-52 kDa protein was the most intensely labeled protein. After separation into integral and associated membrane proteins by extraction with Triton X-114, radioactive labeling was only found in integral membrane proteins with a molecular weight of 50-52 kDa. Photoactivated bumetanide irreversibly inhibited the hepatocellular uptake of cholate, taurocholate but not of serine. Binding proteins for photoactivated bumetanide were absent on AS 30-D ascites hepatoma cells. Labeling of all proteins was sodium dependent in intact hepatocytes but was sodium independent in plasma membranes. Labeling was prevented by non-labeled bumetanide and by the loop diuretics piretanide and furosemide. Labeling protection was further achieved with organic anions such as bromosulfophthalein, rifampicin, probenecid and by the bile acids taurocholate, deoxycholate and dehydrocholate. The radiolabeled proteins did not belong to the bumetanide-sensitive NaCl/KCl co-transport system which apparently does not occur in intact isolated rat hepatocytes.
Affinity Labels↗
Photoaffinity labelling and radiation inactivation of the leukotriene B4 receptor in human myeloid cells.
The leukotriene (LT) B4 receptor has been characterized in the human monocyte leukemia THP-1 cell line. Scatchard analysis of [3H]LTB4 specific binding to THP-1 cell membranes revealed a single population of high affinity (KD = 56 pM) and saturable (2000 receptors/cell) binding sites. [3H]LTB4 specific binding was enhanced by divalent cations, but inhibited by both monovalent cations and a non-hydrolysable GTP analogue. Treatment with GTP analogue resulted in a concentration-dependent reduction in the number of high affinity binding sites, accompanied by the appearance of an equal number of binding sites of lower affinity (KD = 1250 pM). In contrast, Scatchard analysis with human polymorphonuclear leukocyte (PMN) membranes consistently revealed two populations of LTB4 receptors (KD = 48 pM and 270 pM). Treatment with GTP analogue, however, converted all these detectable binding sites to the lower affinity state. These data suggest that the LTB4 receptor in both THP-1 cell and PMN membranes exists in interconverting affinity states modulated by G-protein coupling. The similarity between the LTB4 receptors present in these two cell types was also substantiated by target-size analysis by radiation inactivation, which estimated a comparable molecular mass of 56.5 kDa and 52.8 kDa for the THP-1 cell and PMN LTB4 receptors, respectively. Finally, the presence of a single LTB4 receptor in PMN was demonstrated by direct photolabelling. Irradiation of frozen [3H]LTB4 equilibrium binding assay incubations resulted in complete photolysis of [3H]LTB4. Subsequent resolution of the tritiated PMN proteins by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis (PAGE) revealed one major radioactive peak migrating with an apparent molecular weight of 61,000. This peak was identified as the LTB4 receptor since radiolabelling could be completely inhibited by the presence of excess unlabelled LTB4 or the LTB4-receptor antagonist, L-662,328. Photolabelling was also partially inhibited by pretreatment with GTP analogue, consistent with G-protein uncoupling reagents reducing receptor affinity without complete inhibition. In summary, the LTB4 receptor identified in human myeloid cells is a G-protein coupled receptor with interconvertible high and low affinity states, having a molecular mass of 53-61 kDa.
Affinity Labels↗