Photogenerated reagents for membranes: selective labeling of intrinsic membrane proteins in the human erythrocyte membrane

The nucleophilicity of a dialkylcarbene: unusual activation parameters for additions of adamantanylidene to simple alkenes

Computational and experimental results demonstrate that adamantanylidene (1) behaves as a highly reactive nucleophile toward common alkenes. It is the only known saturated nucleophilic carbene that lacks direct or vinylogous heteroatomic substitution. The activation energy and enthalpy for addition of 1 to methyl acrylate are the most negative values yet encountered in any carbene-alkene addition.

Characterization of the effects of aryl-azido compounds and UVA irradiation on the viral proteins and infectivity of human immunodeficiency virus type 1

Hydrophobic UV-activatable compounds have been shown to partition into the hydrophobic region of biological membranes to selectively label transmembrane proteins, and to inactivate enveloped viruses. Here, we analyze various UV-activatable azido- and iodo-based hydrophobic compounds for their ability to inactivate a model-enveloped virus, human immunodeficiency virus (HIV-1 MN). Treatment of HIV-1 with 1,5-diazidonapthalene (DAN), 1-iodo, 5-azidonaphthalene (INA), 1-azidonaphthalene (AzNAP) or 4,4'-diazidobiphenyl (DABIPH) followed by UVA irradiation for 2 min resulted in complete viral inactivation, whereas treatment using analogous non-azido-containing controls had no effect. Incorporation of an azido moiety within these hydrophobic compounds to promote photoinduced covalent reactions with proteins was found to be the primary mechanism of viral inactivation for this class of compounds. Prolonged UVA irradiation of the virus in the presence of these azido compounds resulted in further modifications of viral proteins, due to the generation of reactive oxygen species, leading to aggregation as visualized via Western blot analysis, providing additional viral modifications that may inhibit viral infectivity. Furthermore, inactivation using these compounds resulted in the preservation of surface antigenic structures (recognized by neutralizing antibodies b12, 2g12 and 4e10), which is favorable for the creation of vaccines from these inactivated virus preparations.

Photo-activation of the hydrophobic probe iodonaphthylazide in cells alters membrane protein function leading to cell death

Background

Photo-activation of the hydrophobic membrane probe 1, 5 iodonaphthylazide (INA) by irradiation with UV light (310–380 nm) results in the covalent modification of transmembrane anchors of membrane proteins. This unique selectivity of INA towards the transmembrane anchor has been exploited to specifically label proteins inserted in membranes. Previously, we have demonstrated that photo-activation of INA in enveloped viruses resulted in the inhibition of viral membrane protein-induced membrane fusion and viral entry into cells. In this study we show that photo-activation of INA in various cell lines, including those over-expressing the multi-drug resistance transporters MRP1 or Pgp, leads to cell death. We analyzed mechanisms of cell killing by INA-UV treatment. The effects of INA-UV treatment on signaling via various cell surface receptors, on the activity of the multi-drug resistance transporter MRP1 and on membrane protein lateral mobility were also investigated.

Results

INA treatment of various cell lines followed by irradiation with UV light (310–380 nm) resulted in loss of cell viability in a dose dependent manner. The mechanism of cell death appeared to be apoptosis as indicated by phosphatidylserine exposure, mitochondrial depolarization and DNA fragmentation. Inhibition by pan-caspase inhibitors and cleavage of caspase specific substrates indicated that at low concentrations of INA apoptosis was caspase dependent. The INA-UV treatment showed similar cell killing efficacy in cells over-expressing MRP1 function as control cells. Efflux of an MRP1 substrate was blocked by INA-UV treatment of the MRP1-overexpressing cells. Although INA-UV treatment resulted in inhibition of calcium mobilization triggered by chemokine receptor signaling, Akt phosphorylation triggered by IGF1 receptor signaling was enhanced. Furthermore, fluorescence recovery after photobleaching experiments indicated that INA-UV treatment resulted in reduced lateral mobility of a seven transmembrane G protein-coupled receptor.

Conclusion

INA is a photo-activable agent that induces apoptosis in various cancer cell lines. It reacts with membrane proteins to alter the normal physiological function resulting in apoptosis. This activity of INA maybe exploited for use as an anti-cancer agent.

Ebola virus inactivation with preservation of antigenic and structural integrity by a photoinducible alkylating agent

Current methods for inactivating filoviruses are limited to high doses of irradiation or formalin treatment, which may cause structural perturbations that are reflected by poor immunogenicity. In this report, we describe a novel inactivation technique for Zaire Ebola virus (ZEBOV) that uses the photoinduced alkylating probe 1,5-iodonaphthylazide (INA). INA is incorporated into lipid bilayers and, when activated by ultraviolet irradiation, alkylates the proteins therein. INA treatment of ZEBOV resulted in the complete loss of infectivity in cells. Results of electron microscopy and virus-capture assays suggested the preservation of conformational surface epitopes. Challenge with 50,000 pfu of INA-inactivated, mouse-adapted ZEBOV did not cause disease or death in mice. A single vaccination with INA-inactivated ZEBOV (equivalent to 5 x 10(4) pfu) protected mice against lethal challenge with 1000 pfu of ZEBOV. INA-inactivated virus induced a protective response in 100% of mice when administered 3 days before challenge. Thus, INA may have significant potential for the development of vaccines and immunotherapeutics for filoviruses and other enveloped viruses.

Photoinduced reactivity of the HIV-1 envelope glycoprotein with a membrane-embedded probe reveals insertion of portions of the HIV-1 Gp41 cytoplasmic tail into the viral membrane

The interactions of HIV-1 Env (gp120-gp41) with CD4 and coreceptors trigger a barrage of conformational changes in Env that drive the membrane fusion process. Various regions of gp41 have profound effects on HIV entry and budding. However, the precise interactions between gp41 and the membrane have not been elucidated. To examine portions of membrane proteins that are embedded in membrane lipids, we have studied photoinduced chemical reactions in membranes using the lipid bilayer specific probe iodonaphthyl azide (INA). Here we show that in addition to the transmembrane anchor, amphipatic sequences in the cytoplasmic tail (CT) of HIV-1 gp41 are labeled by INA. INA labeling of the HIV-1 gp41 CT was similar whether wild-type or a mutant HIV-1 was used with uncleaved p55 Gag, which does not allow entry. These results shed light on the disposition of the HIV-1 gp41 CT with respect to the membrane. Moreover, our data have general implications for topology of membrane proteins and their in situ interactions with the lipid bilayer.

A photochemical approach to the lipid accessibility of engineered cysteinyl residues

Ordinary electrophilic reagents react too slowly in a nonpolar environment to be useful for the determination of the accessibility to lipid of continuous stretches of residues mutated to cysteine. By contrast, photoactivated 5-iodonaphthyl-1-azide (INA) reacted readily with 2-mercaptoethanol and dodecanethiol in nonpolar solvents and in liposomes. Continuous stretches of residues in the amphipathic N-terminal helix and first transmembrane helix of the bacterial potassium channel KcsA were replaced with cysteine, and the mutants were expressed in Escherichia coli and isolated in inner membranes. These membranes were dissolved in detergent and reconstituted into asolectin liposomes incorporating INA. The extent of light-induced reaction of INA with each cysteine was assayed by subsequent reaction with the gel-shifting, SH-specific methoxy-polyethylene glycol-2-pyridine disulfide. The pattern of apparent second-order rate constants for the photoreactions of eight substituted cysteines in the N-terminal helix conformed to other measures of lipid exposure. The pattern of the rate constants for the photoreactions of 15 cysteines in the first transmembrane helix had peaks every third residue, which partly conformed to other measures of lipid exposure.

Reversible penetration of alpha-glutathione S-transferase into biological membranes revealed by photosensitized labelling in situ

Fluorescent lipid analogue 3,3'-dioctadecyloxacarbocyanine incorporated into biological membranes was used to induce photoactivation of a hydrophobic probe 5-[125I]iodonaphthyl-1-azide (125INA) by energy transfer and to thereby confine subsequent radiolabelling of proteins to the lipid bilayer. This approach was applied in bovine chromaffin cells to discover cytosolic proteins that reversibly penetrate into membrane domains. alpha-Glutathione S-transferase (alpha-GST) was identified as the only labelled protein in bovine chromaffin-cell cytosol, indicating that it inserts reversibly into the membrane lipid bilayer. The selectivity of the labelling towards the lipid bilayer is demonstrated by showing that influenza virus haemagglutinin becomes labelled by 125INA only after the insertion of this protein into the target membrane. The molar 125INA:protein ratio was used as a quantitative criterion for evaluation of the penetration of proteins into the membrane lipid bilayer. This ratio was calculated for four integral membrane proteins and four soluble proteins that interact with biological membranes. The values for four integral membrane proteins (erythrocyte anion transporter, multidrug transporter gp-170, dopamine transporter and fusion-competent influenza virus haemagglutinin) were 1, 8, 2 and 2, respectively, whereas for soluble proteins (annexin VII, protein kinase C, BSA and influenza virus haemagglutinin) the values were 0.002, 0, 0.002 and 0.02, respectively. The molar ratio for alpha-GST was found to be 1, compatible with the values obtained for integral membrane proteins.

Conformational changes and fusion activity of vesicular stomatitis virus glycoprotein: [125I]iodonaphthyl azide photolabeling studies in biological membranes

The interaction of VSV glycoprotein (VSV G) with biological membranes was studied by photosensitized labeling. The method is based on photosensitized activation by the fluorescent lipid analog 3,3'-dioctadecyloxacarbocyanine (DiO) of a hydrophobic probe, [125I]iodonaphthyl azide (125INA), that rapidly partitions into the membrane bilayer of virus and cells. 125INA labeling of proteins and lipids can be confined to the site of chromophore localization by photosensitized labeling. Photoactivation using visible light of target membrane labeled with DiO and 125INA, to which unlabeled virions are bound, results in exclusive labeling of envelope glycoproteins inserted into the target membrane [Pak et al. (1994) J. Biol. Chem. 269, 14614]. In this study, we labeled lipid symmetric erythrocyte ghosts with 125INA and DiO. Photosensitized activation of VSV prebound to labeled ghosts with visible light resulted in VSV G labeling under fusogenic conditions. Photoactivation of 125INA by UV light, which is nonspecific, produced labeled VSV G at both acidic and neutral pH. Photosensitized labeling of VSV G by DiO-125INA-ghosts was also observed at pH 5.5, 4 degrees C, in the absence of mixing between viral and cellular lipids, suggesting insertion of the ectodomain of VSV G. Soluble VSV G lacking the transmembrane domain inserted into DiO-125INA-ghosts under the same conditions as intact VSV G. DiO inserted into intact VSV appeared to be a suitable fluorophore for continuous kinetic measurements of membrane fusion by fluorescence dequenching. Our photosensitized labeling results establish biochemical correlates for the three states of VSV G, which we had proposed based on kinetic data [Clague et al., Biochemistry 29, 1303]. In addition, we found that VSV G insertion into the target membrane is reversible, suggesting a "velcro"-like attachment of the fusogenic domain with the target membrane.

Topological disposition of lysine 943 in native Na+/K(+)-transporting ATPase

Because of the conflicting conclusions that have been reached regarding the location of the two putative membrane-spanning segments from cysteine 911 through isoleucine 929 and from isoleucine 946 through cysteine 964 in the alpha subunit of native ovine Na+/K(+)-transporting ATPase, the disposition of lysine 943 with respect to the plane of the lipid bilayer was investigated. Sealed, right-side-out vesicles were modified with pyridoxal phosphate and Na[3H]BH4 in the presence and absence of saponin, a reagent that creates holes in the membranes. Modified alpha polypeptide was isolated, and digested with trypsin and chymotrypsin to release the desired peptides, QQGMK and QQGMK([3H]pyr)NK (where [3H]pyr designates the modification on lysine 943). These peptides, after cyclization of their amino-terminal glutamines, were isolated with an immunoadsorbent specific for the amino-terminal sequence pyroglutamyl-QGM-followed by high-pressure liquid chromatography on a C-18 reverse phase column. Comparisons were made of the extent of incorporation of radioactivity into lysine 943 between sealed vesicles and sealed vesicles pretreated with saponin. An increase in incorporation into lysine 943 of 5-fold to 18-fold was seen in vesicles pretreated with saponin prior to the modification with pyridoxal phosphate. This increase in incorporation is consistent with a cytoplasmic location for lysine 943. This conclusion places the residues on the carboxy-terminal side of the putative membrane-spanning segment from cysteine 911 through isoleucine 929 and the amino-terminal side of the putative membrane-spanning segment from isoleucine 946 through cysteine 964 in the ovine alpha subunit on the cytoplasmic side of the membrane.

Photoaffinity labeling of cytochrome P450 2B4: capture of active site heme ligands by a photocarbene

Spiro[adamantane-2,2'-diazirine], which produces adamantyl carbene upon photolysis, binds tightly to P450 2B4 (KS = 3.2 microM), giving a normal substrate binding difference spectrum. Irradiation of 2-[3H]adamantane diazirine at 365 nm in the presence of native, ferric P450 2B4 resulted in first-order photolysis (t1/2 = 1.8 min). The main product was 2-[3H]adamantanol, with about 6% of the radioactivity covalently bound to P450 2B4. With the ferrous carbonyl form of P450 2B4, 2-adamantanol production decreased and protein labeling increased to 12%. When ferric cyanide 2B4 was used, 2-adamantanecarbonitrile was formed in addition to 2-adamantanol. The nitrile appears to have resulted from capture of the iron-bound cyanide ligand by the carbene. The use of multiple cycles of photolysis increased the percentage of protein labeling to 76%. Photolabeling was inhibited by known 2B4 substrates and inhibitors. Also, N-demethylation of benzphetamine and generation of a substrate binding difference spectrum by benzphetamine were both inhibited stoichiometrically with the fraction of radiolabeled protein. The labeled protein was permanently converted to the high-spin state, as indicated by the characteristic change in the absorbance spectrum, demonstrating irreversible occupation of the substrate binding site by the adamantyl residue. Mild acid hydrolysis of radiolabeled 2B4 at the five Asp-Pro bonds generated a 2-kDa peptide which carried 78% of the radioactivity. These results are interpreted as the result of the active site carbene reacting by three competing pathways: capture of the heme sixth ligand to yield either 2-adamantanol or 2-adamantanecarbonitrile, capture of an unbound active site water molecule to yield adamantanol, and covalent attachment to a protein residue. Thus, the P450 2B4 active site appears to contain at least one unbound water molecule in addition to the heme aquo sixth ligand, even when substrate is present.

Fragmentation of dimyristoylphosphatidylcholine vesicles by apomyoglobin

Previously we have reported results of a preliminary study on the micellization of phosphatidylcholine vesicles by apomyoglobin at pH 4 (J. W. Lee and H. Kim, 1988, FEBS Lett. 241, 181-184). The micellization study has been extended here to investigate the effect of the lipid to protein ratio, temperature, size of vesicles, and pH. The pH-dependent study indicated that micellization occurs when the protein assumes either a molten globular or random coil structure. Time-dependent hydrophobic labeling by 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)-diazirine showed that there is an initial increase in contact between the protein and hydrophobic acyl chain of lipid followed by a decrease in the interaction. This may be explained as the initial stage of vesicle aggregation which is subsequently superseded by the fragmentation. These reactions are discussed in term of protein unfolding at low pH.

Identification of a voltage-responsive segment of the potential-gated colicin E1 ion channel

The voltage dependence of channel activity of the bactericidal protein colicin E1 was found to be correlated with insertion into the membrane bilayer of a specific segment of the 178-residue COOH-terminal thermolytic colicin channel peptide. The insertion into the bilayer was detected by an increase in labeling by one of two different lipophilic photoaffinity probes or by a decrease in iodination of peptide tyrosines from the external solution. Imposition of a potassium diffusion potential of -100 mV resulted in an increase of 35-60% in the labeling of the peptide by the lipophilic probe in the bilayer and a concomitant decrease in labeling of Tyr residues in the peptide by the iodination reagent in the external solution. The change in labeling decreased upon dissipation of the membrane potential with a half-time of about 1 min. The labeling change was localized to a 36-residue peptide segment bounded by alanine-425 and by tryptophan-460. This segment containing seven positively charged residues at low pH is a voltage-sensitive region that inserts into the membrane bilayer when the channel is turned on by the potential and is extruded from it when the voltage is removed and the channel is turned off.

3-(Trifluoromethyl)-3-(m-isothiocyanophenyl)diazirine: synthesis and chemical characterization of a heterobifunctional carbene-generating crosslinking reagent

A new hydrophobic heterobifunctional photocrosslinking reagent 3-(trifluoromethyl)-3-(m-isothiocyanophenyl)diazirine (TRIMID), a carbene precursor, and its radioiodinated analogue [125I]TRIMID, have been synthesized and chemically characterized. The reagents were applied for membrane protein modification in human erythrocyte membranes and purple membranes from Halobacterium halobium. Covalent labeling of the anion transport protein (band 3) via the isothiocyanate function was confirmed. Radiolabeled TRIMID was detected in at least two thermolysin-generated transmembrane fragments of the anion transport protein, and half-maximal inhibition of the erythrocyte anion transport activity was attained with 2.2 mM reagent. In bacteriorhodopsin (BR), a common binding site for the monofunctional phenylisothiocyanate and the bifunctional crosslinking reagent was identified: preincubation of purple membranes with TRIMID suppressed phenylisothio-[14C]-cyanate binding to BR. [125I]TRIMID was recovered in V-1, the N-terminal segment of BR, which includes the phenylisothiocyanate binding site Lys-41. Light-induced intramolecular crosslinking of band 3-derived thermolytic fragments was not observed, although the carbene was generated in situ and photocrosslinking of the protease V8 fragments of BR was not detected. Chemical and physicochemical characteristics of the new reagent are discussed with regard to limitations imposed for photoinduced site-directed crosslink formation.

Evidence for the presence of water within the hydrophobic core of membranes

The photoreactive ganglioside derivative N-diazirinyl-lyso-GM1 was incorporated into liposomes and calf brain microsomes. After photoactivation at 350 nm it was found to dimerize with phospholipids such as phosphatidylcholine and phosphatidylserine and with cholesterol. The predominant covalent reaction product, however, was the alcohol, resulting from the reaction with water. It amounted to about 45% of the covalent reaction products in calf brain microsomes and to about 58% in pure phosphatidylcholine liposomes. Based on the temperature dependence of the photoreaction of N-diazirinyl-lyso-GM1 in liposomes consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphoryl-choline or 1,2-distearoyl-sn-glycero- 3-phosphorylcholine and on affinity labeling experiments with cholera toxin we propose that the predominant reaction of N-diazirinyl-lyso-GM1 with water is due to the presence of water within the hydrophobic core of artificial and biological membranes.

Topological disposition of the sequences -QRKIVE- and -KETYY in native (Na+ + K+)-ATPase

The dispositions with respect to the plane of the membrane of lysine-905 in the internal sequence -EQRKIVE- and of lysine-1012 in the carboxy-terminal sequence -RRPGGWVEKETYY of the alpha-polypeptide of sodium and potassium ion activated adenosinetriphosphatase have been determined. These lysines are found in peptides released from the intact alpha-polypeptide by the extracellular protease from Staphylococcus aureus strain V8 and by trypsin, respectively. Synthetic peptides containing terminal sequences of these were used to prepare polyclonal antibodies, which were then used to prepare immunoadsorbents directed against the respective peptides. Sealed, right-side-out membrane vesicles containing native (Na+ + K+)-ATPase were labeled with pyridoxal phosphate and sodium [3H]borohydride in the absence or presence of saponin. The labeled alpha-polypeptide was isolated from these vesicles and digested with appropriate proteases. The incorporation of radioactivity into the peptides binding to the immunoadsorbent directed against the sequence pyrERXIVE increased 3-fold in the presence of saponin as a result of the increased accessibility of this portion of the protein to the reagent when the vesicles were breached by saponin; hence, this sequence is located on the cytoplasmic face of the membrane. It was inferred that the carboxy-terminal sequence -KETYY is on the extracytoplasmic face since the incorporation of radioactivity into peptides binding to the immunoadsorbent directed against the sequence -ETYY did not change when the vesicles were breached with saponin.

A photoactivable phospholipid analogue that specifically labels membrane cytoskeletal proteins of intact erythrocytes

A radioactive photoactivable analogue of phosphatidylethanolamine, 2-(2-azido-4-nitro-benzoyl)-1-acyl-sn-glycero-3-phospho[14C]ethanolamine ([14C]AzPE), was synthesized. Upon incubation with erythrocytes in the dark, about 90% of [14C]AzPE spontaneously incorporated into the cells; of this fraction, about 90% associated with the membrane, all of it noncovalently. Upon photoactivation, 3-4% of the membrane-associated probe was incorporated into protein. Analysis of this fraction by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, as well as extraction of labeled membranes with alkali or detergent, showed that the probe preferentially labeled cytoskeletal proteins. [14C]AzPE appears to be a useful tool for the study of lipid-protein interactions at the cytoplasmic face of the plasma membrane of intact cells.

Depth-dependent photolabelling of membrane hydrophobic core with 9-diazofluorene-2-butyric acid

Hydrophobic photoactivable reagents, which readily partition into membranes, have proved very useful for studying membrane hydrophobic core. These reagents have been linked to fatty acids in order to obtain amphipathic photoactivable reagents which label membranes more effectively. By varying the length of these amphipathic reagents, an attempt to label membrane hydrophobic core at different depths can be made. We report here 9-diazofluorene-2-butyric acid as a new photoactivable reagent which labels the single bilayer vesicles prepared from egg phosphatidylcholine. The labelling site on the fatty acyl chains could be traced to be between the carbon atom 4 and 6. The new probe thus labels the membrane at a site which is proximal to what can be predicted from its length and transverse location in membranes.

Hydrophobic labelling of membrane-embedded proteins with lipophilic reagents. Incorporation of [125I]INA and [125I]TID into B lymphocyte membrane immunoglobulins

Hydrophobic labelling is frequently used in the study of membrane-inserted domains of intrinsic proteins. However, the published procedures, fail to incorporate sufficient radioactivity into membrane immunoglobulins of B lymphocytes to permit investigation of their subunit structures and associations with other proteins. In order to increase the specific radioactivity of [125I]iodonaphthylazide ([125I]INA), an improved method for the synthesis of the reagent was developed. In addition, the optimal conditions for labelling B lymphocytes with [125I]INA and the commercially available reagent 3-(trifluoromethyl)-3-(3'-[125I]iodophenyl)diazirine ([125I]TID) were elaborated. Under these optimized conditions, Ig molecules labelled with [125I]INA and [125I]TID were isolated and analysed in detail by SDS-PAGE. The usefulness of the two reagents for the investigation of lipid-embedded domains of membrane proteins is discussed.

Interaction of Clostridium perfringens delta toxin with erythrocyte and liposome membranes and relation with the specific binding to the ganglioside GM2

The specific interaction of the cytolytic Clostridium perfringens delta toxin with membrane GM2 was indicated by: (i) characterization of this glycolipid in the membrane of sheep and goat erythrocytes, which are lysed by the toxin, whereas GM2 was undetectable in insensitive rabbit erythrocytes, (ii) demonstration of 125I-toxin binding to GM2, by autoradiography, following incubation with thin-layer chromatograms containing separated neuroblastoma gangliosides, and (iii) toxin fixation by phospholipid-cholesterol unilamellar vesicles containing either sheep gangliosides or GM2. In order to investigate the intramembrane events leading to membrane disruption following toxin binding, the photoreactive probe 12(4-azido-2-nitrophenoxy)stearoyl 1-14C glucosamine, which inserts into the outer layer and labels integral membrane proteins, was used to establish whether delta toxin penetrates into target cell membrane. No toxin labeling was found, suggesting that toxin action takes place at the membrane surface. This contention is supported by the observation that despite toxin binding, GM2 liposomes did not release entrapped 14C-glucose. Treatment of toxin with carboxypeptidases, but not aminopeptidases, abolished both toxin binding capacity onto erythrocytes and its combination with antitoxin neutralizing antibodies, suggesting that the carboxy terminal end of the toxin is critical for binding to cell membrane.

Photolabeling of myelin basic protein in lipid vesicles with the hydrophobic reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine

The hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine([125I]TID) was used to label myelin basic protein or polylysine in aqueous solution and bound to lipid vesicles of different composition. Although myelin basic protein is a water soluble protein which binds electrostatically only to acidic lipids, unlike polylysine it has several short hydrophobic regions. Myelin basic protein was labeled to a significant extent by TID when in aqueous solution indicating that it has a hydrophobic site which can bind the reagent. However, myelin basic protein was labeled 2-4-times more when bound to the acidic lipids phosphatidylglycerol, phosphatidylserine, phosphatidic acid, and cerebroside sulfate than when bound to phosphatidylethanolamine, or when in solution in the presence of phosphatidylcholine vesicles. It was labeled 5-7-times more than polylysine bound to acidic lipids. These results suggest that when myelin basic protein is bound to acidic lipids, it is labeled from the lipid bilayer rather than from the aqueous phase. However, this conclusion is not unequivocal because of the possibility of changes in the protein conformation or degree of aggregation upon binding to lipid. Within this limitation the results are consistent with, but do not prove, the concept that some of its hydrophobic residues penetrate partway into the lipid bilayer. However, it is likely that most of the protein is on the surface of the bilayer with its basic residues bound electrostatically to the lipid head groups.

Demonstration that lysine-501 of the alpha polypeptide of native sodium and potassium ion activated adenosinetriphosphatase is located on its cytoplasmic surface

Evidence that the peptide HLLVMKGAPER, which can be released from intact sodium and potassium ion activated adenosinetriphosphatase by tryptic digestion, is located on the cytoplasmic surface of the native enzyme has been obtained. An immunoadsorbent directed against the carboxy-terminal sequence of this tryptic peptide has been constructed. The peptide KGAPER was synthesized by solid-phase techniques. Antibodies against the sequence -GAPER were purified by immunoadsorption, using the synthetic peptide attached to agarose beads. These antibodies, in turn, were coupled to agarose beads to produce an immunoadsorbent. Sealed, right-side-out vesicles, prepared from canine kidneys, were labeled with pyridoxal phosphate and sodium [3H]borohydride in the absence or presence of saponin, respectively. A tryptic digest of these labeled vesicles was passed over the immunoadsorbent. Large increases in the incorporation of radioactivity into the peptides bound by the immunoadsorbent were observed in the digests obtained from the vesicles exposed to saponin. From the results of several control experiments examining the labeling reaction as applied to these vesicles, it could be concluded that this increase in incorporation resulted only from the access that the reagents gained to the inside of the vesicles in the presence of saponin and that the increase in the extent of modification was due to the cytoplasmic disposition of this segment in the native enzyme.

Selective labeling of proteins in biological systems by photosensitization of 5-iodonaphthalene-1-azide

The apolar azide of 5-iodonaphthalene-1-azide (Ina) partitions into the lipid bilayer of biological membranes. Upon photolysis at 314 nm, it is rapidly converted into the reactive nitrene, which efficiently attaches covalently to lipid-embedded domains of proteins and, to a lesser extent, to membrane phospholipids. Above 370 nm, Ina absorption is negligible and photolysis at these wavelengths does not occur. However, on addition of the photosensitizing molecule 3-aminopyrene, trifluoperazine, or 8-anilinonaphthalene-1-sulfonate, followed by irradiation at 380 nm, efficient conversion of Ina to reactive species was observed, as measured by [125I]Ina-labeling of membrane proteins and inactivation of the hormonal response of adenylate cyclase. Irradiation at 480 nm in the presence of a fluorescein derivative of n-undecylamine also resulted in a pattern of [125I]Ina-labeled membrane proteins and hormone uncoupling indistinguishable from that obtained following direct photolysis at 314 nm. Photosensitization of the azide molecules is confined to the vicinity of the photosensitizer chromophore. This allowed selective labeling of chromophore-bearing proteins in solution or in membranes. Bovine serum albumin-fluorescein conjugate, in the presence of nonderivatized soluble proteins, was exclusively labeled by [125I]Ina when irradiated at 480 nm, but random labeling occurred on photolysis at 314 nm. Likewise, rhodopsin in rod outer segment membranes from frog retina was exclusively labeled by [125I]Ina upon photosensitization at 380 nm. Random labeling again occurred on direct irradiation at 314 nm. The results suggest that selective labeling in complex biological systems may be achieved by photosensitized activation of azides.

Identification of subunits of acetylcholine receptor that interact with a cholesterol photoaffinity probe

All four subunits of the acetylcholine receptor in membrane vesicles isolated from Torpedo californica have been labeled with [3H]cholesteryl diazoacetate. As this probe incorporates into lipid bilayers analogously to cholesterol, this result indicates that acetylcholine receptor interacts with cholesterol. This investigation also demonstrates that this probe is a useful reagent for studying the interaction of cholesterol with membrane proteins.

Covalent attachment of sulfhydryl-specific, electron spin resonance spin-labels to Fab' fragments of murine monoclonal antibodies that recognize human platelet membrane glycoproteins. Development of membrane protein specific spin probes

A general method for the production of high-affinity, nitroxide-labeled, protein-specific spin probes is described in this paper. Fab' fragments are generated from protein-specific, murine monoclonal antibodies by pepsin digestion and mild reduction with cysteine. The free sulfhydryl group located in the carboxy-terminal region of these molecules and produced de novo by this manipulation is then alkylated by reaction with 4-maleimido-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-maleimide), thereby generating spin-labeled Fab' fragments of these monoclonal antibodies. Two prototypic monoclonal antibodies were tested, each specific for a different integral membrane glycoprotein of human blood platelets. The results indicate that Fab' spin probes generated by this method retain the ability to bind to these glycoproteins within the membrane of intact platelets. These reagents thus represent probes that can be generally used to monitor integral membrane protein mobility on the surface of the intact cell.

A carbene-yielding amino acid for incorporation into peptide photoaffinity reagents

3-[p-[3-(Trifluoromethyl)-3H-diazirin-3-yl]phenyl]alanine has been prepared in 10 steps from p-bromobenzyl alcohol. The alcohol was converted to 3-(alpha-iodo-p-tolyl)-3-(trifluoromethyl)-3H-diazirine, which was used to alkylate N-(diphenylmethylene)glycine ethyl ester. After deprotection the amino acid was obtained with an overall yield of 18%. The D- and L-isomers of 3-[p-[3-(trifluoromethyl)-3H-diazirin-3-yl]phenyl]alanine have also been resolved, and 3-[p-[3-(trifluoromethyl)-3H-diazirin-3-yl]phenyl]alanine labeled with tritium has been prepared. The advantages of using this amino acid as a building block for peptide photoaffinity reagents is discussed.

Photoaffinity labelling of mitochondrial NADH dehydrogenase with arylazidoamorphigenin, an analogue of rotenone

A photoaffinity-labelling analogue of the respiratory inhibitor rotenone was synthesized from the naturally occurring rotenoid amorphigenin. The analogue inhibits NADH-ubiquinone oxidoreductase activity at concentrations comparable with those of rotenone. Photolysis of the radiolabelled analogue bound to isolated NADH-ubiquinone oxidoreductase resulted in preferential incorporation of radioactivity into a polypeptide of Mr 33 000, particularly at low concentrations of the inhibitor. Preparations of the enzyme differ in a parallel fashion in the content of this polypeptide, the degree of photolabelling by the analogue and their sensitivity to rotenone, providing further evidence that the 33 000-Mr protein forms part of the rotenone-binding site.

Topology of beef heart cytochrome c oxidase from studies on reconstituted membranes

The orientation of purified beef heart cytochrome c oxidase, incorporated into vesicles by the cholate dialysis procedure [Carroll, R.C., & Racker, E. (1977) J. Biol. Chem. 252, 6981], has been investigated by functional and structural approaches. The level of heme reduction obtained by using cytochrome c along with the membrane-impermeant electron donor ascorbate was 78 +/- 2% of that obtained with cytochrome c and the membrane-permeant reagent N,N,N',N'-tetramethyl-p-phenylenediamine. Electron transfer from cytochrome c is known to occur exclusively from the outer surface of the mitochondrial inner membrane (C side), implying that at least 78% of the oxidase molecules are oriented in the same way in these vesicles as in the intact mitochondria. Trypsin, which cleaves subunit IV near its N terminus, modifies only 5-7% of this subunit in intact vesicles. This removal of the N-terminal residues has been shown to occur only in mitochondrial membranes with their inner side (M side) exposed. Diazobenzene [35S]sulfonate [( 35S]DABS) likewise modifies subunit IV only in submitochondrial particles. Labeling of intact membranes with [35S]DABS resulted in incorporation of only 4-8% of the total counts that could be incorporated into this subunit in membranes made leaky to the reagent by addition of 2% Triton X-100. Therefore, both the functional and structural data show that at least 80% and probably more of the cytochrome c oxidase molecules are oriented with their C domain outermost and M domains in the lumen of vesicles prepared by the cholate dialysis method.(ABSTRACT TRUNCATED AT 250 WORDS)

2-[8-14C]naphthyl 2-diazo-3,3,3-trifluoropropionate, a new carbene generating reagent for probing hydrophobic membrane domains

A new precursor of a lipophilic photolabel, 2-[8-14C]naphthyl 2-diazo-3,3,3-trifluoropropionate (NADIT) has been synthesized. The suitability of the reagent for labeling the hydrophobic core of membranes is demonstrated by studying its reactivity in chromatophores of Rhodospirillum rubrum G-9+. The label binds preferentially to the phospholipids and intrinsic membrane proteins. In isolated reaction centers treated with NADIT the hydrophobic subunits M and L are more labeled than the H subunit. The high reactivity, dark stability and ease of synthesis favors this very lipophilic reagent to identify the intrinsic hydrophobic sections of membrane proteins.