Thymectomized, irradiated, and bone marrow-reconstituted chimeras have normal cytolytic T lymphocyte precursors but a defect in lymphokine production

A model system has been developed to study extrathymic T cell differentiation; mice have been thymectomized, lethally irradiated, and reconstituted with bone marrow cells depleted of Thy-1+ cells. After 8 wk, the spleen cells of these athymic, bone marrow-reconstituted chimeras contain Thy-1+ precytolytic T lymphocytes (CTL) that are able to respond to antigen only if supernatant from Con A-activated T cells is added to culture. The phenotype of these pre-CTL is similar to that of thymocytes, suggesting that they may be immature T cells. Initial evaluation of the CTL repertoire of these athymic mice demonstrated that the CTL generated to trinitrophenyl-modified syngeneic cells are H-2-restricted, and that the CTL generated to alloantigens have many of the cross-reactivities observed in normal mice but not in nude mice. In this report, we demonstrate a helper T cell defect in these thymectomized chimeras. These chimeras lack an Ly-1+ helper cell required for thymocytes to differentiate to CTL. Further studies revealed that when spleen cells from these thymectomized chimeras were stimulated with Con A, they produced normal levels of interleukin 2. However, these splenocytes were defective in the production of another factor needed for CTL differentiation.     

Induction of allogeneic cytolytic T lymphocytes by partially purified membrane glycoproteins

It has been previously shown that P815 (H-2^d) purified plasma membranes can induce cytolytic activity from primed C57BL/6 (H-2^b) spleen cells. The secondary cytolytic T lymphocyte (CTL) inducing activity is retained when these P815 plasma membranes are solubilized in deoxycholate. Evidence is now presented that the cell surface antigens responsible for CTL induction can be partially purified in active form and these antigens can be incorporated into reconstituted membranes and phospholipid vesicles. The active antigens have the properties expected for H-2 molecules on lentil lectin chromatography and gel filtration.     

Requirement for three distinct lymphokines for the induction of cytotoxic T lymphocytes from thymocytes

The induction of cytotoxic T lymphocytes (CTL) from CTL precursors requires a combination of antigen and lymphokine signals. To investigate lymphokine requirements for CTL generation, we used an assay in which helper T cell and accessory cell-depleted spleen cells or whole thymocytes were cultured with lectin (Con A) and lymphokines. This culture was followed by assessment of lectin-dependent cytolysis. High concentrations of recombinant interleukin 2 (R-IL 2) (100 U/ml) alone were not sufficient for lectin-mediated CTL induction from thymocytes, whereas 20 to 100 U/ml of R-IL 2 alone could induce a significant lectin-mediated CTL response from accessory cell-depleted spleen cells. Using thymocytes as responders, we found purified or recombinant interferon-gamma (IFN-gamma) did not cause cytolytic activity either in the absence of or in the presence of R-IL 2. However, supernatant from Con A-stimulated rat spleen cells (rat Con A SN) in combination with R-IL 2 could induce cytolytic activity, suggesting that several factors are required for CTL induction. Con A SN was fractionated by gel filtration and the fractions were tested for ability to induce CTL. In the presence of a low level of R-IL 2 (5 U/ml), fractions with a Mr of approximately 31,000 could induce CTL, and this activity was referred to as CTL differentiation factor (CDF). The peak fractions containing CDF activity did not have detectable IL 1, IL 2, IFN-gamma, or CSF activity. However, by add-back experiments and the use of blocking antibodies, a monoclonal antibody against the IL 2 receptor or antibodies against murine IFN-gamma, we demonstrated that CTL induction from mature thymocytes (L3T4-, Lyt-2+) requires CDF activity in addition to IL 2 and IFN-gamma.     

Ontogeny of proliferative and cytotoxic responses to interleukin 2 and concanavalin A in murine fetal thymus

The ontogeny of proliferative and cytotoxic responses to concanavalin A (Con A) and interleukin 2 (IL 2) in C57BL/6J (B6) fetal thymus (FT) was investigated. Embryonic thymocytes were either taken from embryos at different times of gestation or from 14 day B6 FT that were maintained as organ cultures for various times. It was found that the B6 FT could proliferate to Con A and EL4 SN (an IL 2 containing culture supernatant) in a synergistic fashion. This synergy between Con A and EL4 SN was first observed at the 16th to 17th day of gestation. A similar differentiation process took place in 14-day FT that had been maintained as organ cultures; the synergy between Con A and EL4 SN was first observed after 3 days in organ culture. This synergy increased with increasing time of organ culture, and was most evident after 10 days. The synergy between Con A and EL4 SN was also observed when the EL4 SN was replaced with IL 2 which had been purified from crude EL4 SN to apparent homogeneity. B6 FT could also form cytotoxic T lymphocytes (CTL) on stimulation with Con A and EL4 SN. Con A-activated CTL (polyspecific) were detected by including phytohemagglutinin in the assay medium. CTL response was first detected in the 17-day fetal thymus by using this assay. In organ cultures, CTL responses were first detected after 4 days in organ culture, and reached peak levels after 12 to 14 days. The CTL precursor (CTL-P) frequencies in the B6 FT after 2, 5, 10, and 14 days in organ culture were less than 1/10,000, 1/2232, 1/297, and 1/70, respectively; the corresponding CTL-P frequency in adult thymus was 1/60. After 6 days in organ culture, B6 FT could also form CTL in response to Con A and pure IL 2. This finding suggests that the ability to synthesize other differentiation factors that are required for CTL responses is acquired at an early time of thymic differentiation.     

Characterization of the mutant-unc D-gene product in a strain of Escherichia coli K12. An altered beta-subunit of the magnesium ion-stimulated adenosine triphosphatase.

Membranes from a mutant strain of Escherichia coli K12 carrying the uncD409 allele were washed in low-ionic-strength buffers in the presence or absence of the proteinase inhibitor p-aminobenzamidine. Unlike membranes from a normal strain, those from strain AN463 (uncD409) did not become proton-permeable, as judged by NADH-induced atebrinfluorescence quenching, when the membranes were washed in the absence of p-aminobenzamide. Furthermore, ATP-dependent atebrin-fluorscence quenching in such washed membranes could not be reconstituted by the addition of solubilized Mg2+-stimulated adenosine triphosphatase preparations. The examination by two-dimensional polyacrylamide-gel electrophoresis of the polypeptide composition of the washed membranes from strain AN463 (uncD409) indicated the presence of a polypeptide of similar molecular weight to the normal beta-subunit of the Mg2+-stimulated adenosine triphosphatase, but with an altered isoelectric point. Both the normal and abnormal beta-subunits were identified in membranes prepared from a partial diploid strain carrying both the unc+ and uncD409 alleles. It is concluded that the uncD gene codes for the beta-subunit of the Mg2+-stimulated adenosine triphosphatase.     

Interaction of concanavalin A with lactogenic receptors in isolated membranes

Specific binding of lactogenic hormones to their receptors in membranes from lactating mouse liver is inhibited by concanavalin A (Con A). Binding to solubilized receptors is not affected by the presence of Con A in the binding reaction. However, these solubilized binding proteins are retained on Con A-Sepharose columns. Prebound hormone-receptor complexes are also retained on Con A-Sepharose. These data indicate that lactogenic receptors have a Con A binding site distinct from the hormone binding site. Moreover, once bound, the hormone is not released by the action of Con A. Somatogenic receptors do not have a Con A binding site and the binding of bovine growth hormone to hepatic membranes is not affected by the presence of Con A in the binding reaction. Inhibition of binding to lactogenic receptors by Con A occurs independently from other membrane perturbing events such as phospholipid methylation.     

Reconstitution of ciliary membranes containing tubulin

Membranes from the gill cilia of the mollusc Aequipecten irradians may be solubilized readily with Nonidet P-40. When the detergent is removed from the solution by adsorption to polystyrene beads, the proteins of the extract remain soluble. However, when the solution is frozen and thawed, nearly all of the proteins reassociate to form membrane vesicles, recruiting lipids from the medium. The membranes equilibrate as a narrow band (d = 1.167 g/cm3) upon sucrose density gradient centrifugation. The lipid composition of reconstituted membranes (1:2 cholesterol:phospholipids) closely resembles that of the original extract, as does the protein content (45%). Ciliary calmodulin is the major extract protein that does not associate with the reconstituted membrane, even in the presence of 1 mM calcium ions, suggesting that it is a soluble matrix component. The major protein of reconstituted vesicles is membrane tubulin, shown previously to differ hydrophobically from axonemal tubulin. The tubulin is tightly associated with the membrane since extraction with 1 mM iodide or thiocyanate leaves a vesicle fraction whose protein composition and bouyant density are unchanged. Subjecting the detergent-free membrane extract to a freeze-thaw cycle in the presence of elasmobranch brain tubulin or forming membranes by warming the extract in the presence of polymerization-competent tubulin yields a membrane fraction with little incorporated brain tubulin. This suggests that ciliary membrane tubulin specifically associates with lipids, whereas brain tubulin preferentially forms microtubules.     

Reconstituted membranes of tumour cells (proteoliposomes) induce specific protection to murine lymphoma cells

Summary Antigens presented on cell membranes or on liposomes are usually more immunogenic than antigens in soluble form, this being one of the reasons for the weak immunogenicity of extracted tumour-associated transplantation antigens (TATA). The main objective of this study is to solubilize TATA from tumour cells and to present them on a membrane-like structure to the immune system. Crude tumour cell membranes of SL2 lymphosarcoma cells (a spontaneously arising, weakly immunogenic tumour) were solubilized with octylglucoside or sodium deoxycholate, and reconstituted membranes (proteoliposomes) were prepared by detergent removal. Mice immunized s.c. with reconstituted membranes were protected against an i. p. challenge with tumour cells. Although octylglucoside solubilized only 41% of the membrane proteins, the reconstituted membranes were as immunoprotective as crude membranes. (Glyco)proteins were probably the major membrane components in the reconstituted membranes that induce immunoprotection, as mice immunized with preparations constituted of (glyco)lipids from SL2 cells could not reject SL2 cells. If Freund's complete adjuvant was used with the first immunization injection, no potentiation of the elicited immune responses was observed. Besides the membrane TATA, SL2 cells contained an apparently non-membrane-bound TATA, which was found in the cytoplasm. It is concluded that detergent solubilization of membranes and subsequent preparation of reconstituted membranes can be used to obtain membrane tumour-associated antigens that retain activity for induction of protective tumour immunity. The major advantage of this method is that membrane proteins are solubilized and are subsequently presented on a membrane-like structure that resembles the tumour cell membrane. On theoretical and practical grounds it provides a promising alternative for whole-cell vaccines.     

NADH-ascorbate free radical and -ferricyanide reductase activities represent different levels of plasma membrane electron transport

Plasma membranes isolated from rat liver by two-phase partition exhibited dehydrogenase activities for ascorbate free radical (AFR) and ferricyanide reduction in a ratio of specific activities of 1 : 40. NADH-AFR reductase could not be solubilized by detergents from plasma membrane fractions. NADH-AFR reductase was inhibited in both clathrin-depleted membrane and membranes incubated with anti-clathrin antiserum. This activity was reconstituted in plasma membranes in proportion to the amount of clathrin-enriched supernatant added. NADH ferricyanide reductase was unaffected by both clathrin-depletion and antibody incubation and was fully solubilized by detergents. Also, wheat germ agglutinin only inhibited NADH-AFR reductase. The findings suggest that NADH-AFR reductase and NADH-ferricyanide reductase activities of plasma membrane represent different levels of the electron transport chain. The inability of the NADH-AFR reductase to survive detergent solubilization might indicate the involvement of more than one protein in the electron transport from NADH to the AFR but not to ferricyanide.     

Membranes from both Th1 and Th2 T cell clones stimulate B cell proliferation and prepare B cells for lymphokine-induced differentiation to secrete Ig.

Plasma membranes from the mitogen-activated mouse Th2 cell clone D10.G4.1 have recently been shown to provide the cell contact-dependent signals necessary for the induction of small B cell proliferation. Together with the Th2-derived lymphokines IL-4 and IL-5, these membranes stimulate production of Ig isotypes identical to those produced when B cells were stimulated by intact Th2 cells. In contrast, Th1 clones are poor inducers of Ig production in vitro. This could be solely due to differences in the lymphokines released by Th1 and Th2 cells or to differences in the cell-cell contact signals delivered by activated Th1 and Th2 cells. We report that membranes from three different activated Th1 clones induced strong Ag-independent proliferation of small dense B cells. The level of B cell proliferation was enhanced approximately fourfold by the addition of lymphokine-containing supernatant from Con A-activated Th2 cells and was unaffected by any of the lymphokine-containing supernatants from Con A-activated Th1 clones. As with D10.G4.1 membranes, Th1 membranes alone induced B cell proliferation but not secretion of Ig. However, addition of supernatant from Con A-activated D10.G41 cells, but not any supernatants from Con A-activated Th1 cells, induced Ig secretion of all isotypes. These effects were shown to not simply result from increased B cell numbers after stimulation with Th2 lymphokines. Thus, Th1 cell clones seem to poorly induce antibody responses entirely because of their lymphokine repertoire and not because of differences or deficiencies in the ability of these cells to deliver cell contact-dependent signals to B cells.     

Reconstitution of the solubilized insulin receptor in phospholipid vesicles.

The insulin receptor was solubilized from turkey erythrocyte membranes by extraction with 1% beta-octylglucopyranoside. Insulin binding was enhanced when the solubilized material was reconstituted in phospholipid vesicles. The affinity of the reconstituted vesicles for various insulins was similar to that of the intact membranes: porcine insulin greater than proinsulin greater than desoctapeptide insulin. A curvilinear Scatchard plot was obtained for insulin binding to the reconstituted system at 15 degrees C. A high affinity association constant of 1.4 x 10(9) M-1 was obtained from the Scatchard plot. This is a four-fold increase over the value for the turkey erythrocyte membrane, which contains more highly saturated phospholipids. This suggests that the insulin receptor may be sensitive to the lipid composition of the membranes in which it is embedded.     

Purification of a stilbene sensitive chloride channel and reconstitution of chloride conductivity into phospholipid vesicles.

A protein conferring passive chloride permeability was isolated from a N-octylglucoside solubilized extract of partially purified H(+)-transporting osteoclast cell membranes. Purification was achieved by binding of solubilized protein to an amine-linked 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS) Sepharose 4B column and elution with 50 mM KCl. A major protein, with MR = 60 kD on 10% SDS-PAGE, was obtained, which was further purified to homogeneity by HPLC gel filtration. This protein introduced 36Cl- permeability when reconstituted in phospholipid membranes by equilibrium dialysis. The Cl- transport recovered in reconstituted membranes retained sensitivity to DIDS confirming the identity of the isolated protein as a stilbene-sensitive chloride channel.     

Effects of high-glucose and high-fat diets on concanavalin A binding to rat liver plasma membranes and on the amount and pattern of their glycoprotein carbohydrates.

Purified liver plasma membranes were prepared from rats fed a high-fat, carbohydrate-free diet or a high-glucose, fat-free diet. Membranes from rats fed the high-fat diet bound significantly less 125I-labeled concanavalin A (Con A) than did those from rats fed the fat-free diets. The magnitude of the binding difference increased with increasing concentrations of Con A. Neither association nor dissociation rates of the lectin-receptor complex was affected by diet. The extent of degradation of Con A by liver plasma membrane preparations from rats fed either diet was the same. Chemical analysis of delipidated liver plasma membrane showed that membranes prepared from high-fat diet-adapted rats had significantly lower values for all carbohydrate components measured with the exception of galactose. The results indicate that, in liver cells, a change in plasma membrane glycoproteins is part of the complex adaptation to altered diet composition.     

Effect of divalent metal ions on soluble and membrane-bound alkaline phosphatase activity in suckling rat intestine.

EDTA treatment of intestinal brush border membranes (BBM) and epithelial cell supernatant completely inhibited alkaline phosphatase (AP) activity in suckling rat intestine. AP activity was fully regained upon dialysis of the preparations against Zn2+ and to a lesser extent against Co2+, Ca2+ and Mn2+ ions. Other metal ions (Cd2+ and Mg2+) tested were essentially ineffective in restoring the enzyme activity. Considerable differences were observed in kinetic characteristics of the membrane-bound and soluble AP activities in response to various metal ions. There were apparent differences in Km, Vmax, energy of activation (Ea) and thermal stability of the soluble and membrane-bound AP activities, after metal ion substitutions. Nearly 35% AP activity was solubilized on sodium dodecyl sulphate treatment of brush borders (membrane protein: detergent ratio 1:3; w/w). Dialysis of detergent solubilized BBM against different metal ions reconstituted AP activity in the particulate fraction: the order of effectiveness was Zn greater than Ca greater than Mn greater than Co. The kinetic properties of the reconstituted AP were essentially similar to the non-integrated enzyme activity in response to various divalent metal ions examined. But there were apparent differences in Km, Vmax, Ea and thermal stability of the reconstituted AP activity compared to native brush border enzyme. The results suggest the unique requirement of Zn ions for stability and catalytic activity of the soluble and membrane-bound AP activity in suckling rat intestine.     

Analysis of Con A-binding glycoproteins in synaptosomal membranes

Concanavalin A (Con A)-binding proteins obtained from solubilized synaptosomal membranes of bovine brain were analyzed by two-dimensional electrophoresis (2DE), and were identified by peroxidase conjugated Con A (Con A-peroxidase staining), after transfer from 2DE gel to nitrocellulose paper. The Con A-binding proteins were resolved up to 40 spots, ranging in isoelectric points (pI) from 4.5 to 8.0 and molecular weight (MW) from 10 kDa to 120 kDa. Most of the Con A-binding proteins were streaked across a pH gradient and/or exhibited as multiple spots, indicating broad charge and molecular weight heterogeneity. The presence of protein groups that showed high affinities for Con A were revealed. Most interesting group (named GP51), which consisted of seven spots separated horizontally in charge heterogeneity (pI5.85-7.5) with MW 51kDa, was characterized by its binding to an immobilized protein A gel. This implies that GP51 is related to immunoglobulins and/or GP51 may be a new member of the immunoglobulin supergene family.     

Role of low density lipoprotein receptor-dependent and -independent sites in binding and uptake of chylomicron remnants in rat liver

The role of the low density lipoprotein (LDL) receptor in the binding of chylomicron remnants to liver membranes and in their uptake by hepatocytes was assessed using a monospecific polyclonal antibody to the LDL receptor of the rat liver. The anti-LDL receptor antibody inhibited the binding and uptake of chylomicron remnants and LDL by the poorly differentiated rat hepatoma cell HTC 7288C as completely as did unlabeled lipoproteins. The antireceptor antibody, however, decreased binding of chylomicron remnants to liver membranes from normal rats by only about 10%. This was true for intact membranes and for solubilized reconstituted membranes and with both a crude membrane fraction as well as with purified sinusoidal membranes. Further, complete removal of the LDL receptor from solubilized membranes by immunoprecipitation with antireceptor antibody only decreased remnant binding to the reconstituted supernatant by 10% compared to solubilized, nonimmunoprecipitated membranes. Treatment of rats with ethinyl estradiol induced an increase in remnant binding by liver membranes. All of the increased binding could be inhibited by the antireceptor antibody. The LDL receptor-independent remnant binding site was not EDTA sensitive and was not affected by ethinyl estradiol treatment. LDL receptor-independent remnant binding was competed for by beta-VLDL = HDLc greater than rat LDL greater than human LDL (where VLDL is very low density lipoprotein, and HDL is high density lipoprotein). There was weak and incomplete competition by apoE-free HDL, probably due to removal of apoE from the remnant. The LDL receptor-independent remnant-binding site was also present in membranes prepared from isolated hepatocytes and had the same characteristics as the site on membranes prepared from whole liver. In contrast, when chylomicron remnants were incubated with a primary culture of rat hepatocytes, the anti-LDL receptor antibody prevented specific cell association by 84% and degradation of chylomicron remnants completely. Based on these studies, we conclude that although binding of chylomicron remnants to liver cell membranes is not dependent on the LDL receptor, their intact uptake by hepatocytes is.     

The requirements for antigen multivalency in class I antigen recognition and triggering of primed precursor cytolytic T lymphocytes

In vitro generation of a secondary cytolytic T lymphocyte (CTL) response to Class I alloantigen requires two signals: recognition of the Class I antigen by precursor CTL (Signal 1), and subsequent interaction with lymphokine(s) (Signal 2). Previous work using subcellular antigen stimulation has demonstrated that the required lymphokine(s) is produced as a result of adherent cell uptake, processing, and Ia-restricted presentation of alloantigen to helper T cells. This pathway could be bypassed by addition to the cultures of supernatant from Con A-stimulated rat spleen cells. When an optimal level of lymphokine(s) is provided by addition of Con A supernatant, the magnitude of the CTL response obtained is dependent on the effectiveness of alloantigen recognition and triggering of the primed precursor CTL (pCTL). By using this approach, we examined the cellular and molecular requirements for generation of Signal 1. Previous results had indicated that pCTL were able to directly recognize subcellular antigen, and that cellular presentation of the antigen to pCTL was not required. Further evidence for this was provided by the finding that pulsing of the responder population for short times with liposomes containing purified H-2Kk resulted in effective stimulation of the response. Exposure of cells to antigen for 1 to 2 hr at 4 degrees C generated responses of comparable magnitude to those obtained when antigen was continuously present in the cultures. Experiments were also done to directly examine the ability of alloantigen-pulsed splenic adherent cells (SAC) to deliver Signal 1. Although the antigen-pulsed SAC were very effective in presenting to helper T cells to result in factor production, they were found to be very ineffective in providing Signal 1 to the pCTL. Having obtained strong evidence for triggering of pCTL occurring via direct recognition of the subcellular alloantigen, we then examined the role of antigen multivalency in recognition and triggering. Purified H-2Kk was prepared in a variety of forms of differing multivalency, ranging from monovalent papain cleavage product to large, highly multivalent liposomes and plasma membranes. The magnitude of the CTL responses obtained was found to be critically dependent on the multivalency of the antigen preparation. Examination of the antigen dose-response curves and maximal responses obtained suggests that valency of the antigen may be important both in determining the avidity of interaction between the pCTL and the antigen-bearing structure, and in determining the extent to which localized receptor cross-linking occurs on the cell surface to result in triggering.     

Solubilization and reconstitution of dopamine D1 receptor from bovine striatal membranes: Effects of agonist and antagonist pretreatment

The bovine striatal dopamine D₁ receptor was solubilized with a combination of sodium cholate and NaCl in the presence of phospholipids, following treatment of membranes with a dopaminergic agonist (SKF-82526-J) or antagonist (SCH-23390). The solubilized receptors were subsequently reconstituted into lipid vesicles by gel-filtration. A comparison of ligand-binding properties shows that the solubilized and reconstituted receptors bound [³H]SCH-23390 to a homogeneous site in a saturable, stereospecific and reversible manner with a K_d of 0.95 and 1.1 nM and a B_max of 918 and 885 fmol/mg protein respectively for agonist- and antagonist-pretreated preparations. These values are very similar to those obtained for membrane-bound receptors. The competition of antagonists for [³H]SCH-23390 binding exhibited a clear D₁ dopaminergic order in the reconstituted preparation obtained from either agonist or antagonist-pretreated membranes, except that (+)butaclamol was about four-fold more potent thancis-flupentixol in displacing [³H]SCH-23390 binding in preparation obtained from agonist-pretreated membranes compared to antagonist-pretreated membranes. The agonist/[³H]SCH-23390 competition studies revealed the presence of a highaffinity component of agonist binding in both the reconstituted receptor preparations. The number of high-affinity agonist binding sites, however, is 40–80% higher in reconstituted preparation obtained from antagonist-treated membrane compared to that obrained from the agonist-treated membrane. In both the preparations, 100 M guanylylimidodiphosphate (Gpp(NH)p) completely abolished the high-affinity component of agonist binding compared to partial abolition in the native membranes, indicating a close association of a G-protein with the solubilized receptors. Whether the receptor was solubilized following agonist or antagonist preincubation of the membranes, the receptor-detergent complex eluted from a steric-exclusion HPLC column with an apparent molecular size of 360,000. Preincubation of the solubilized preparations with Gpp(NH)p had virtually no effect on the elution profile suggesting a lack of guanine nucleotide-dependent dissociation of G-protein receptor complex.     

Single-molecule spectroscopic characterization of light-harvesting 2 complexes reconstituted into model membranes

The spectroscopic properties of the light-harvesting 2 complexes (LH2) from the purple bacterium Rhodopseudomonas acidophila (strain 10050) in detergent micelles and reconstituted into lipid membranes have been studied by single-molecule spectroscopy. When LH2 complexes are solubilized from their host biological membranes by nondenaturing detergents, such as LDAO, there is a small 2-nm spectral shift of the B850 absorption band in the ensemble spectrum. This is reversed when the LH2 complexes are put back into phospholipid vesicles, i.e., into a more native-like environment. The spectroscopic properties on the single-molecule level of the detergent-solubilized LH2 complexes were compared with those reconstituted into the lipid membranes to see if their detailed spectroscopic behavior was influenced by these small changes in the position of the B850 absorption band. A detailed analysis of the low-temperature single-molecule fluorescence-excitation spectra of the LH2 complexes in these two different conditions showed no significant differences. In particular, the distribution of the spectral splitting between the circular k = +/-1 exciton states of the B850 absorption band and the distribution of the mutual angle between the k = +/-1 exciton states are identical in both cases. It can be concluded, therefore, that the LH2 complexes from Rps. acidophila are equally stable when solubilized in detergent micelles as they are when membrane reconstituted. Moreover, when they are solubilized in a suitable detergent and spin coated onto a surface for the single-molecule experiments they do not display any more structural disorder than when in a phospholipid membrane.     

5'-Nucleotidase activity of lymphocyte plasma membranes. Effect of concanavalin A]

The 5'-nucleotidase properties of isolated lymphocyte plasma membranes from young pig mesenteric nodes are described; nucleosides-5'-monophosphates are the substrates of this specific enzyme. Concanavalin A inhibits this enzyme; on the same membranes this mitogen does not affect alkaline phosphatase and activates the membrane bound (Ca2+) ATPase. The 5'-nucleotidase inhibition is due to a specific interaction of Con A with carbohydrate groups of the membrane; its high positive cooperativity suggests that the lectin promotes reorganization of the membrane bound 5'-nucleotidase. Solubilization of the 5'-nucleotidase does not prevent the effect of Con A and the solubilized enzyme is firmly bound by Con A-Sepharose 4B; these results suggest that Con A inhibits the enzyme by a direct interaction and that 5'-nucleotidase can be considered as an eventual receptor for the lectin.