Binding of bovine cytochrome b5 to phosphatidycholine liposomes Characterization of the reconstituted lipid-protein vesicles

Cytochrome $\text{b}{}_5$ was extracted and purified from beef liver by a detergent method (cytochrome $\text{d-b}{}_5$). The hydrophilic moiety which carries the heme group (cytochrome $\text{t-b}{}_5$) was prepared by trypsin action upon pure cytochrome $\text{d-b}{}_5$.Single-shelled lecithin liposomes form complexes with cytochromes $\text{d-b}{}_5$ up to a molar ratio of one protein for 35 phospholipids. The lipid-protein complexes were isolated by gel filtration on Sepharose 4B. They are hollow vesicles in which [³H]-glucose can be trapped. Their diameter is greater than that of the initial liposomes.Cytochrome $\text{t-b}{}_5$ does not interact with the vesicles. These results show that the hydrophobic tail is necessary for the binding and that the hydrophilic part of the protein is located on the outer face of the vesicles. This asymmetry is also proved by the action of reducing agents.Experiments with saturated phosphatidylcholines show that the protein interacts with the lipids both below the transition temperature $\text{T}{}_{\mathrm{M}}$. i.e. when the aliphatic chains are in a crystalline state, and above $\text{T}{}_{\mathrm{M}}$, when the alipathic chain are in a fluid state.¹H NMR spectra show that even at the maximum cytochrome $\text{d-b}{}_5$ concentration the presence of the proteins does not markedly change the dynamics to the phospholipid molecules. An asymmetric single-shelled vesicle structure is proposed for the complex.     

On the hydrophobic part of aminopeptidase and maltases which bind the enzyme to the intestinal brush border membrane.

The intestinal brush border aminopeptidase and unfractionated maltases M2+M3 are composed of a hydrophilic, sugar containing and enzymatically active part, and a smaller hydrophobic part presumably binding the catalytic part of the lipid matrix of the membrane. Hydrophobic parts detaced by trypsin from the detergent forms of aminopeptidase and the maltases were purified and shown to have molecular weights ranging from 8000 to 10000. All are rich in hydrophobic residues and contain no disulfide bridges. However, their overall amino acid composition is different. The hydrophobic parts appear to be N-terminal in the detergent forms of the enzymes.     

Major intrinsic polypeptide of lens membrane: Biochemical and immunological characterization of the major cyanogen bromide fragment

A protein of $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 26 000 has been shown to be the major component of eye-lens junctions, which are similar but not identical to the gap junctions of liver and other tissues. Cyanogen bromide cleavage of the $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 26 000 polypeptide from bovine lenses yields a major fragment of $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 15 000 (fragment 1). However, if the junctions are first treated with trypsin or carboxypeptidase Y, cyanogen bromide treatment yields a fragment of reduced molecular weight. Since protease treatment has been shown to cleave residues almost exclusively from the carboxy-terminal end of the $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 26 000 polypeptide, it follows that fragment 1 represents the carboxy-terminal half of this molecule, part of which is exposed to proteolytic attack outside the membrane. This latter result is corroborated by the fact that antisera which recognize both the $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 26 000 polypeptide and fragment 1 fail to do so after preadsorption with intact membranes. In addition, comparative amino acid and partial sequence analyses of the $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 26 000 polypeptide and fragment 1 indicate that fragment 1 is more hydrophilic in character, suggesting that much of the amino-terminal half of the $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 26 000 polypeptide is buried within the lipid bilayer.     

The hydrophobic nature of the pig intrinsic factor receptor in the intestine

The pig intestinal intrinsic factor receptor has been isolated and dissociated into its α and β subunits. The β subunit was found to be more hydrophobic than the α subunit. In a detergent solution only the α subunit was accessible to digestion with papain. The whole isolated receptor was introduced into artificial single bilayer liposomes where is apparently was randomly oriented. Liposomes containing the receptor were digested with papain and the polypeptide segments that stayed in the lipid fraction were extracted and analyzed using sodium dodecyl sulfate polyacrylamide gel electrophoresis. Four species were found with $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ values of 23 000, 45 000, 70 000 and 86 000.     

A one-step procedure for the isolation of fructose 1,6-bisphosphatase and fructose 1,6-bisphosphate aldolase from rabbit liver

Human ceruloplasmin, which is usually cleaved by limited proteolysis into three major fragments during preparation ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}\text{? 18,650}$, 50,000, and 70,000) was isolated in good yield as an undegraded single-chain protein ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}\text{? 135,00}$). The cryosupernatant from fresh frozen plasma (100 liters) was fractionated with polyethylene glycol (PEG 4000) at + 5°C yielding a ceruloplasmin-enriched fraction in the 20% PEG supernatant. Three steps of chromatography on DEAE-Sephacel, hydroxyapatite, and Sephadex G-200 produced a homogeneous protein with maximal enzymatic activity and the $\text{A}{}_{610}\text{A}{}_{280}$ ratio of 0.046 corresponding to 98–100% purity. Two forms of ceruloplasmin having this absorbance ratio were obtained; Form I was predominant and was studied further. The procedure separated both forms from apoceruloplasmin and degraded ceruloplasmin. The single-chain ceruloplasmin (Form I) had an NH₂-terminal sequence of Lys-Glu-Lys-His-Tyr-Tyr-Ile-, the same as for the 70,000 fragment, and is suitable for structural study by sequence analysis and physicochemical methods.     

Interaction of lymphocytes and macrophage cell line cells (M1 cells). I. Functional maturation and appearance of Fc receptors im M1 cells

M₁ cells, which are cell line cells established from myeloid leukemia cells of the SL strain mouse, can differentiate from blast cells ($\text{M}{}_1{}^{\mathrm{?}}$) to mature macrophages ($\text{M}{}_1{}^{\mathrm{+}}$) within 48 hr, when they are cultured with conditioned medium (CM) obtained from murine embryonic fibroblasts. While $\text{M}{}_1{}^{\mathrm{?}}$ cells have no phagocytic activity nor Fc receptor (FcR), $\text{M}{}_1{}^{\mathrm{+}}$ cells possess both characteristics. The appearance of FcR is temperature-dependent and inhibited by a metabolic inhibitor, cycloheximide. FcR on $\text{M}{}_1{}^{\mathrm{+}}$ cells is resistant to trypsin and pronase. $\text{M}{}_1{}^{\mathrm{+}}$ cells improve the viability of macrophage-depleted SL splenic lymphocytes and restore the in vitro secondary plaque forming cell response of macrophage-depleted spleen cells to particulate and soluble antigens. $\text{M}{}_1{}^{\mathrm{?}}$ cells lack this macrophage-substituting capacity. Mm₁ cells, mutant cells from M₁ cells, having FcR and higher phagocytic activity than $\text{M}{}_1{}^{\mathrm{+}}$ cells, are also devoid of this capacity.     

Lipid-protein interactions in membrane models fluorescence polarization study of cytochrome b5-phospholipids complexes

According to previous authors, cytochrome $\text{b}{}_5$, when extracted from bovine liver by a detergent method, is called cytochrome $\text{d-b}{}_5$. On the other hand, the protein obtained after trypsin action, which eliminates an hydrophobic peptide of about 54 residues, is called cytochrome $\text{t-b}{}_5$.Fluorescence polarization of the dansyl phosphatidylethanolamine probe inserted into phospholipid vesicles is very senstive to the binding of proteins, and so is a useful method to study lipid-protein interactions.The chromophore mobility, $\text{R}$, decreases markedly when dipalmitoyl phosphatidylcholine vesicles are incubated with cytochrome $\text{d-b}{}_5$, whereas $\text{R}$ does not change for cytochrome $\text{c}$ and cytochrome $\text{t-b}{}_5$. This can be interpreted as a strengthening of the bilayer, only due to the interaction of the hydrophobic peptide tail.Interaction of dipalmitoyl phosphatidylcholine vesicles with cytochrome $\text{d-b}{}_5$ occurs either below or above the melting temperature of the aliphatic chains (41 °C). Even for a high protein to lipid molar ratio (1 molecule of protein for 40 phospholipid molecules), the melting temperature is apparently unaffected.Phosphatidylserine and phosphatidylinositol do not interact at pH 7.7 with cytochrome $\text{d-b}{}_5$, because electrostatic forces prevent formation of complexes. At low pH, the interaction with the protein occurs, but the binding is mainly of electrostatic nature.     

The concept of high- and low-affinity reactions in bovine cytochrome c oxidase steady-state kinetics

(1) Analysis of the data from steady-state kinetic studies shows that two reactions between cytochrome c and cytochrome c oxidase sufficed to describe the concave Eadie-Hofstee plots ($\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{? 1 · 10}{}^{\mathrm{?8}}\text{M}$ and $\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{? 2 · 10}{}^{\mathrm{?5}}\text{M}$). It is not necessary to postulate a third reaction of $\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{? 10}{}^{\mathrm{?6}}\text{M}$. (2) Change of temperature, type of detergent and type of cytochrome c affected both reactions to the same extent. The presence of only a single catalytic cytochrome c interaction site on the oxidase could explain the kinetic data. (3) Our experiments support the notion that, at least under our conditions (pH 7.8, low-ionic strength), the dissociation of ferricytochrome c from cytochrome c oxidase is the rate-limiting step in the steady-state kinetics. (4) A series of models, proposed to describe the observed steady-state kinetics, is discussed.     

Structural studies of a french bean storage protein: phaseolin

Molecular weights and sedimentation coefficients have been measured for different oligomeric forms of phaseolin, the major storage protein in seeds of Phaseolus vulgaris L. The results indicate that phaseolin is a trimer ($\text{M}{}_{\mathrm{r}}\text{= 150000}$) at neutral pH which aggregates further to a dodecamer form ($\text{M}{}_{\mathrm{r}}\text{= 596000}$) at pH 4.5. The subunit size is in good agreement with the recently determined sequence molecular weight, if allowance is made for bound oligosaccharide and phytic acid moieties. The trimeric nature at neutral pH has been confirmed by chemical crosslinking studies using dimethylsuberimidate and dithiobis(succinimidylpropionate). Analyses of optical rotatory dispersion and circular dichroism data have been used to examine the corformation of phaseolin. In common with other seed globulins, a low proportion of α-helix ($\text{～ 10\%}$) coupled with a high level of β-sheet ($\text{～50\%}$) is predicted. These data are compared with a structural analysis based on the amino acid sequence of a phaseolin subunit polypeptide. The predicted level of α-helix is increased ($\text{～20\%}$) when phaseolin is heated in sodium dodecyl sulphate, but not when the detergent is added at room temperature.     

Tryptic peptide map analysis of the major human blood platelet membrane glycoproteins separated by two-dimensional polyacrylamide gel electrophoresis

Washed platelets were surface-labelled by lactoperoxidase catalyzed iodination and either the platelets or membranes were solubilized in detergent and applied to a wheat germ agglutinin-Sepharose column and a Lens culinaris lectin Sepharose column coupled sequentially. The glycoproteins eluted from the lectin columns were separated by two-dimensional gel electrophoresis. Alternatively, labelled whole platelets or membranes were solubilized and then directly separated by two-dimensional polyacrylamide gel electrophoresis. Spots corresponding to specific glycoproteins identified by apparent isoelectric point ($\text{p}\text{I}$), apparent molecular weight ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}$), staining and labelling characteristics were cut from the gels and analyzed by tryptic peptide mapping. The maps of the individual glycoproteins (GP) Ia, Ib, IIa, IIb, GP_132–135^4–4.5 IIIa, IIIb and IIIc were all different. Glycoproteins with the same $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ but different $\text{p}\text{I}$ were distinct with the exception of regions of GP Ib. There were minor differences in the maps of glycoproteins separated in the reduced or non-reduced state. Tryptic peptide maps provide a valuable additional parameter for the identification and characterization of platelet glycoproteins.     

Regulatory interaction between calmodulin and ATP on the red cell Ca2+ pump

The interactions between calmodulin, ATP and Ca²⁺ on the red cell Ca²⁺ pump have been studied in membranes stripped of native calmodulin or rebound with purified red cell calmodulin. Calmodulin stimulates the maximal rate of $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ by 5–10-fold and the rate of Ca²⁺-dependent phosphorylation by at least 10-fold. In calmodulin-bound membranes ATP activates $\text{(}\text{Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ along a biphasic concentration curve ($\text{K}{}_{\mathrm{<mtext>m</mtext><mtext>1</mtext>}}\text{≈ 1.4 μ}\text{M}\text{, K}{}_{\mathrm{<mtext>m</mtext><mtext>2</mtext>}}\text{≈ 330 μ}\text{M}$), but in stripped membranes the curve is essentially hyperbolic ($\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{≈ 7 μ}\text{M}$). In calmodulin-bound membranes Ca²⁺ activates $\text{(}\text{Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-ATPase}$ at low concentrations ($\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{< 0.28 μ}\text{M}$) in stripped membranes the apparent Ca²⁺ affinities are at least 10-fold lower.The results suggest that calmodulin (and perhaps ATP) affect a conformational equilibrium between E₂ and E₁ forms of the Ca²⁺ pump protein.     

The cell cycle time of the haemocytes in the insect Calliphora vicina

The cell cycle time of Calliphora vicina prohaemocytes was examined using the labelled mitoses method after the administration of a pulse of H³-thymidine. The total cycle time occupied 9.1 hr, while $\text{G}{}_1\text{+}\text{1}\text{2}\text{M}$, S and $\text{G}{}_2\text{+}\text{1}\text{2}\text{M}$ occupied 1.6 hr, 2.7 hr and 4.8 hr respectively.     

Sodium dependence of maximum flux, JM, and Km of amino acid transport in Ehrlich ascites cells

The Michaelis-Menten parameters, $\text{J}{}_{\mathrm{<mtext>M</mtext>}}$ and $\text{K}{}_{\mathrm{m}}$ of the initial 1-min fluxes of uptake of l-phenylalanine and of α-aminoisobutyric acid were determined for extracellular concentrations of Na⁺ ranging from 0.5 to 110 mequiv/l for Ehrlich ascites tumor cells. The maximal initial flux, $\text{J}{}_{\mathrm{<mtext>M</mtext>}}$, decreased with decrease in extracellular Na⁺ for both α-aminoisobutyric acid and phenylalanine but the $\text{K}{}_{\mathrm{m}}$ for α-aminoisobutyric acid increased markedly as the Na⁺ concentration fell whereas the $\text{K}{}_{\mathrm{m}}$ for phenylalanine decreased. Cycloleucine behaved like phenylalanine.The data provides strong evidence that the Na⁺-independent flux of phenylalanine is an exchange diffusion flux that can be varied by changing the intracellular level of amino acids such as phenylalanine. For phenylalanine, cyclolcucine, and methionine this exchange diffusion flux appears to be additive with the Na⁺-dependent initial flux. α-Aminoisobutyric acid also has an exchange diffusion that is Na⁺-independent but it has a high $\text{K}{}_{\mathrm{m}}$ and is not additive with the Na⁺-dependent flux.     

High-molecular-weight carrier ampholytes for isoelectric focusing of peptides

A method is described for the synthesis of high-molecular-weight carrier ampholytes for preparative isoelectric focusing of peptides. A giant polyethylene imine ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}\text{40 000–60 000}$) is mixed with a linear gradient of acrylic acid in a flow-through system and let to react at 80°C for 70 h. Giant carrier ampholytes ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}\text{range}\text{50 000–90 000}$) are thus obtained. These compounds interact very strongly among themselves, probably not by hydrogen bonds or hydrophobic interactions but ionic bonds. In fact, the aggregates are split by high salt (NaCl) or by zwitterionic compounds (Gly, taurine) or at acidic or alkaline pHs. They appear to interact only weakly and reversibly with proteins and no interactions are apparent with model dipeptides (His-Ser, His-Met, His-Phe and His-Lys).     

A stable traveling-wave solution of a model of cellular control processes with positive feedback

Edelstein's model

$\text{?E}\text{?τ}\text{=F(M, E)}$

,

$\text{?M}\text{?τ}\text{=G(M, E)+D}\text{?}{}^2\text{M}\text{?s}{}^2$

,

$\text{M(s,0)=?(s)}$

,

$\text{E(s,0)=ψ(s)}$

, where τ ? 0 and ?∞<s<∞, $\text{F(M, E>) = (K}{}_1\text{+M}{}^{\mathrm{m}}\text{)}\text{(K}{}_2\text{+M}{}^{\mathrm{m}}\text{)?k}{}_1\text{E, G(M, E)}\text{= k}{}_1\text{E ? k}{}_2\text{M,}$ m ? 2, describes the behavior of two basic chemical species during the cellular differentiation in a linear ensemble of the same cell type. We prove the existence and uniqueness of a travelling-wavefront solution. We also demonstrate one kind of stability for this solution.     

Transport of [14C]Gly-Pro in a proline peptidase mutant of Salmonella typhimurium

The transport of [¹⁴C]Gly-Pro was examined using a mutant of Salmonella typhimurium (strain TN87) deficient in an X-Pro dipeptidase and an X-Pro-Y iminopeptidase. The dipeptide was taken up by one saturable transport system having a $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ of $\text{5.3 · 10}{}^{\mathrm{?7}}\text{M}$ and a $\text{V}$ of 1.4 nmol/mg dry wt cell per min. The uptake of Gly-Pro was not inhibited by amino acids or tripeptides and the transport system exhibited a rather broad side chain specificity for dipeptides. Dipeptides containing hydrophobic residues were the most potent inhibitors of this dipeptide transport system exhibiting $\text{K}{}_{\mathrm{<mtext>i</mtext>}}$ values between 10^?8 and 10^?7 M. In contrast, dipeptides containing glycine residues were particularly weak inhibitors. Finally, Gly-Pro was found to be in the intact form inside the cell and was concentrated more than 1000-fold.     

Purification of squid rhodopsin and reassembly into lipid bilayers

Rhodopsin from squid photoreceptor membranes was solubilized in octyl glucoside and purified to a single band on SDS-polyacrylamide gels of $\text{M}{}_{\mathrm{<mtext>r</mtext>}}$ 46 000. Purified rhodopsin was recombined with phospholipids to form vesicles by detergent dialysis. Spectroscopic analysis of the rhodopsin-lipid vesicles showed that the interconversion between acid and basic metarhodopsin had a $\text{p}\text{K}$ of 8. Furthermore, rhodopsin in the vesicles could be photoregenerated from metarhodopsin in solutions of either neutral or alkaline pH. These two spectroscopic properties are comparable to those for rhodopsin in photoreceptor membranes. The results indicate that the native conformation of rhodopsin is preserved during purification and after recombination with phospholipids into vesicles. This preparation is, therefore, an active starting point for functional reconstitution studies.