Membrane protein oligomeric structure and transport function

The K+-induced apparent heterogeneity of high-affinity nucleotide-binding sites in (Na+ + K+)-ATPase can only be due to the oligomeric structure of the enzyme

K+ induces an apparent heterogeneity among an otherwise homogeneous population of nucleotide-binding sites in (Na+ + K+)-ATPase preparations from pig kidney. With the help of ouabain we show that this heterogeneity cannot be due to a mixture of different and independent sites and conclude that each enzyme molecule must contain two nucleotide site-containing units that show interaction. Na+ induces an apparent heterogeneity among an otherwise homogeneous population of ouabain-binding sites. The argument is, therefore, extended to include one ouabain site on each of the structural units that bind nucleotide. All these structural units are shown to hydrolyse substrate at identical rates. Using the presently available molecular weight data, it is concluded that the enzyme is composed of two subunits each possessing one nucleotide-binding site, one ouabain-binding site, one alpha-peptide and the capacity for hydrolysing ATP and p-nitrophenyl phosphate.

Modulation of substrate transport to the brain

Variations of substrate transport across the cerebral capillary endothelium were examined in response to variations of the substrate demand of the brain tissue, and to variations of substrate concentration in the blood. The substrates examined included glucose and ketone bodies. The transport changes were measured in rats, using an indicator fractionation method modified by the reviewer. Four mechanisms appeared to contribute to the adjustment of substrate transport to variations in substrate demand. The first and least important mechanism was the change of concentration gradient across the endothelium that occurred when the substrate consumption rate changed. The second mechanism was the flow-dependency of the average capillary substrate concentration: the higher the perfusion rate, the higher the average capillary concentration. This mechanism failed to account for the changes of substrate transport observed during marked increases of the metabolic rate. The third and most important mechanism was a change of the capillary diffusion capacity, probably associated with a change of the number of perfused capillaries. The fourth mechanism, not previously described, was an adaptation of transport to permanent changes of substrate concentration in the blood. This mechanism appeared to reflect changes of the concentration (and affinity?) of transport proteins in the plasma membranes of endothelial cells, possibly in association with changes of cellular protein synthesis and gene expression.

Isolation of plasma membrane domains from murine T lymphocytes

Murine T-lymphoma cells have been homogenized in dense sucrose solution and centrifuged under isopycnic conditions for membrane components. Floating fractions banding between 10% and 22.5% sucrose ("light" membranes) and between 22.5% and 35% sucrose ("heavy" membranes) were shown to consist of smooth membrane vesicles. The amounts of cholesterol and phospholipids recovered after chloroform/methanol extraction were similar in both fractions, but heavy membranes contained two to three times more protein than light membranes. The most striking difference between the two membrane fractions was revealed by their labeled surface glycoprotein patterns on polyacrylamide gels, suggesting that (i) the smooth membrane vesicles originated from the plasma membrane and (ii) two distinct segments of the plasma membrane can be recovered in fractions characterized by specific surface glycoproteins. Light membranes were enriched in Thy-1 antigen, whereas Ly-5 antigen and a 170,000-dalton surface glycoprotein were recovered almost exclusively from heavy membranes, as were metabolically labeled protein spots comigrating with the H-2k/d antigen in two-dimensional electrophoresis. The patterns of the unlabeled proteins in light and heavy membranes appeared similar, except for polypeptides of 180,000 and 85,000 daltons that were found preferentially in heavy membranes. These results support the concept of plasma membrane domains by showing that two distinct populations of plasma membrane vesicles can be isolated and that these populations contain different sets of cell surface glycoproteins.

Monosaccharide transporter of the human erythrocyte. Characterization of an improved preparation

The human erythrocyte monosaccharide transporter has been purified through the use of the dialyzable detergent octyl glucoside. It was found that the transporter denatures in the detergent and that the rate of this process could be reduced by increasing the ratio of phospholipid to detergent. The transporter was obtained in higher yield and with a higher specific activity for cytochalasin B binding than has been previously reported. Scatchard plot analysis of cytochalasin B binding to the reconstituted preparations gave a dissociation constant of 1.5 X 10(-7) M, and there were found to be 15.3 nmol of sites/mg of protein. On the basis of a value of 46 000 for the molecular weight of the polypeptide, this specific activity corresponds to 0.70 site/polypeptide chain; and there are reasons to believe that the value of the stoichiometry may be one site per functional transporter polypeptide. The complete amino acid composition and the N- and C-terminal residues of the transporter have been determined. Both the intact transporter and transporter that had been partially depleted of carbohydrate by treatment with endo-beta-galactosidase were found to migrate anomalously upon sodium dodecyl sulfate--polyacrylamide gel electrophoresis, relative to the behavior of standard proteins.

Analysis of valine uptake by Commelina mesophyll cells in a biphasic active and a diffusional component

Valine uptake by isolated Commelina benghalensis L. mesophyll cells was measured over a wide concentration range (10(-6)-4·10(-2) mol l(-1)). The uptake data were subjected to iterative fitting. Experiments with carbonyl cyanide mchlorophenyl hydrazone (CCCP), diethylstilbestrol (DES), and p-chloromercuriphenylsulphonic acid (PCMBS) provided evidence that the biphasic uptake kinetics of valine consists of a diffusional component and a biphasic active uptake. The data from the control experiments, were also best fitted to one diffusional component and two Michaelis-Menten systems. The presence of two carrier systems in the plasmalemma, however, was considered to be virtual for the following reasons: (1) Both phases of active uptake were equally decreased by high concentrations of K(+)-ions. (2) Fusicoccin stimulated the active uptake in both phases to the same extent. (3) Inhibitors of the proton-driven uptake (CCCP, DES, PCMBS) similarly inhibited the active uptake at all concentrations. (4) The active uptake equally responded in both phases to changes in the pH. (5) Light also promoted the active uptake over the whole concentration range. These results strongly indicate that, despite its biphasic character, the active uptake is due to one proton-driven carrier system.

Simple model for the chemical potential change of a transported ion in active transport

The mechanism for active transport of ions across a membrane probably involves two distinct conformational states of the transport protein, in which the binding sites for the transported ion face opposite sides of the membrane. It is likely that the binding affinity for the ion changes in synchrony with the change in site orientation, such that the affinity is high on the uptake side of the membrane and low on the discharge side. A structural model is proposed for the transmembrane portion of such a protein, based on the known multihelical structure of bacteriorhodopsin. This structure is well adapted to a cyclical alternation between two conformations that differ simultaneously in orientation and binding affinity. No unfolding of the helices or other significant alterations in secondary structure is required. The model is explicitly intended as a hypothetical representation of the E1 and E2 states of ATP-driven Na+,K+ and Ca2+ pumps.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Evidence for negative cooperativity in human erythrocyte sugar transport

1. When D-glucose exchange influx is measure over a wide range of concentrations then two affinity constants (2.27 and 26.0 mM) are evident. This is consistent with a transport model (the allosteric pore model) in which there is negative cooperativity between subunits of the transport protein. 2. The equations for the allosteric pore model interacting with two substrates (or a substrate and an inhibitor) have been derived and have been used to analyse data from exchange inhibition and for mixed infinite-trans uptake experiments. 3. The exchange inhibition of tracer 3-O-methyl-D-glucose, D-xylose and D-fructose uptake by D-glucose also shows evidence for negative cooperativity and for two inhibition constants which are approximately equal to the D-glucose equilibrium exchange affinity constants. 4. The uptake of D-glucose into infinite-trans D-glucose or 3-O-methyl-D-glucose gives Km values of 2.6 and 2.33 mM, respectively. The uptake of 3-O-methyl-D-glucose into infinite-trans D-glucose or 3-O-methyl-D-glucose gives Km values of 6.0 and 4.6 mM, respectively. V values are slightly higher when the internal sugar is 3-O-methyl-D-glucose. 5. In cells that are treated with fluorodinitrobenzene the apparent Ki value for D-glucose inhibition of tracer D-fructose uptake is lowered. It is proposed that this is due to a partially selective effect of FDNB on the internal subunit interface stability constant (the internal pore gate).

Molecular considerations relevant to the mechanism of active transport

A small group of closely related proteins is responsible for all active transport in animal cells, and inorganic cations are the only substances transported by these enzymes. They share a common kinetic mechanism in which two fundamental conformations participate, each receiving and dispatching substrates from its unique side of the membrane. During transport, the cations must pass through their enzyme to cross the membrane and intense interest is currently focused on the possibility that the path which they follow lies within the interface between two discrete subunits in a dimeric structure. Although 'half-of-sites' behaviour, consistent with this hypothesis, has been reported, it is now known that systematic errors were responsible for this mistaken conclusion. The number of protomers which comprise a functional unit of active transport has not been determined.

Structure-function relationships in insulin

A new interpretation of structure-function relationships in the insulin molecule is presented. Negative cooperativity is postulated to arise from a dimerization event occurring between two receptor-bound molecules. The receptor-binding surface of insulin can necessarily not involve residues involved in dimerization as has been generally accepted. Support for this interpretation is based on published data.