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J C Owicki

Publications and source records attributed to J C Owicki.

33 records · Page 2Linked to original sources
PubMed1989

Concentration effects on reactions in membranes: rhodopsin and transducin.

The reaction rate of two laterally-diffusing species in a biological membrane shows a maximum at some concentration of reactants, because an increase in the concentration of reactants tends to increase the reaction rate by the law of mass action but decreases the diffusion rate of the reactants. The activation of transducin by rhodopsin in the disk membrane of the rod outer segment is described in terms of a steady-state diffusion model with concentration-dependent diffusion coefficients. The optimum concentrations of reactants are obtained from contour plots of the reaction rate as a function of rhodopsin and transducin concentrations, and the sensitivity of the results to the assumed values of the variables is examined. To determine whether the observed concentrations are in fact those yielding the maximum reaction rate, several variables must be known more accurately.

Diffusion↗
PubMed1989

Mutual diffusion of interacting membrane proteins.

The generalized Stokes-Einstein equation is used, together with the two-dimensional pressure equation, to analyze mutual diffusion in concentrated membrane systems. These equations can be used to investigate the role that both direct and hydrodynamic interactions play in determining diffusive behavior. Here only direct interactions are explicitly incorporated into the theory at high densities; however, both direct and hydrodynamic interactions are analyzed for some dilute solutions. We look at diffusion in the presence of weak attractions, soft repulsions, and hard-core repulsions. It is found that, at low densities, attractions retard mutual diffusion while repulsions enhance it. Mechanistically, attractions tend to tether particles together and oppose the dissipation of gradients or fluctuations in concentration, while repulsions provide a driving force that pushes particles apart. At higher concentrations, changes in the structure of the fluid enhance mutual diffusion even in the presence of attractions. It is shown that the theoretical description of postelectrophoresis relaxation and fluorescence correlation spectroscopy experiments must be modified if interacting systems are studied. The effects of interactions on mutual diffusion coefficients have probably already been seen in postelectrophoresis relaxation experiments.

Diffusion↗
PubMed1989

Self diffusion of interacting membrane proteins.

A two-dimensional version of the generalized Smoluchowski equation is used to analyze the time (or distance) dependent self diffusion of interacting membrane proteins in concentrated membrane systems. This equation provides a well established starting point for descriptions of the diffusion of particles that interact through both direct and hydrodynamic forces; in this initial work only the effects of direct interactions are explicitly considered. Data describing diffusion in the presence of hard-core repulsions, soft repulsions, and soft repulsions with weak attractions are presented. The effect that interactions have on the self-diffusion coefficient of a real protein molecule from mouse liver gap junctions is also calculated. The results indicate that self diffusion is always inhibited by direct interactions; this observation is interpreted in terms of the caging that will exist at finite protein concentration. It is also noted that, over small distance scales, the diffusion coefficient is determined entirely by the very strong Brownian forces; therefore, as a function of displacement the self-diffusion coefficient decays (rapidly) from its value at infinite dilution to its steady-state interaction-averaged value. The steady-state self-diffusion coefficient describes motion over distance scales that range from approximately 10 nm to cellular dimensions and is the quantity measured in fluorescence recovery after photobleaching experiments. The short-ranged behavior of the diffusion coefficient is important on the interparticle-distance scale and may therefore influence the rate at which nearest-neighbor collisional processes take place. The hard-disk theoretical results presented here are in excellent agreement with lattice Monte-Carlo results obtained by other workers. The concentration dependence of experimentally measured diffusion coefficients of antibody-hapten complexes bound to the membrane surface is consistent with that predicted by the theory. The variation in experimental diffusion coefficients of integral membrane proteins is greater than that predicted by the theory, and may also reflect protein-induced perturbations in membrane viscosity.

Animals↗
PubMed1988

Theoretical comparison of the self diffusion and mutual diffusion of interacting membrane proteins.

Self diffusion and mutual diffusion in two-dimensional membrane systems are analyzed. It is shown that interprotein interactions can produce markedly different density-dependent changes in the diffusion coefficients describing these two processes; the qualitative differences are illustrated by using a theoretical formalism valid for dilute solutions. Results are obtained for three analytical potentials: hard-core repulsions, soft repulsions, and soft repulsions with weak attractions. Self diffusion is inhibited by all three interactions. In contrast, mutual diffusion is inhibited by attractions but is enhanced by repulsions. It is shown that such interaction-dependent differences in self diffusion and mutual diffusion could underlie, among other things, the disparity in protein diffusion coefficients extracted from fluorescence recovery after photobleaching and postelectrophoresis relaxation data.

Algorithms↗
PubMed1987

Lateral interactions among membrane proteins. Valid estimates based on freeze-fracture electron microscopy.

We consider the lateral distribution of intrinsic membrane proteins from the viewpoint of the statistical-mechanical theory of liquids. We connect the information in freeze-fracture electron micrographs--positions of proteins but not lipids or aqueous species--to a well developed theory of liquid mixtures. An algorithm, based on the Born-Green-Yvon integral equation, is presented for deducing forces between proteins from correlations among protein positions that are observed in micrographs. The algorithm is tested on simulated micrographs, obtained by Monte-Carlo methods, where forces between proteins are known analytically. We conclude that valid estimates of such forces, both attractions and repulsions, can be obtained from the positions of a few thousand proteins.

Algorithms↗
PubMed1987

Lateral interactions among membrane proteins. Implications for the organization of gap junctions.

We have studied the relationship between interprotein forces and the lateral distribution of proteins in disordered mouse liver gap junctions. Data on protein positions are obtained from freeze-fracture electron micrographs. Short-ranged correlations in observed positions are characteristic of interacting particles in a fluid state. An analysis derived from statistical mechanics allows the determination of the magnitude and functional form of interprotein forces. We find that jap junction proteins are mutually repulsive, in a manner consistent with electrostatics and excluded volume. This dictates that long-ranged protein aggregation into jap junction plaques cannot arise solely from interparticle interactions. An alternative is the balance of lateral pressures between the junction and the surrounding glycocalyx. This idea is quantified into a model. Junctional pressure arises from protein-protein interactions and is computed from a pressure equation based on the force and a radial distribution function describing order. The pressure from the glycocalyx is assumed to arise from mixing, electrostatic, and elastic interactions of sugar residues, and is described with terms from Flory-Krigbaum and McMillan-Mayer theories. The results of this modeling are in reasonable agreement with available experimental data.

Animals↗
PubMed1985

Synthesis and characterization of a highly fluorescent peptidyl-phosphatidylethanolamine.

The synthesis of a fluorescent lipid for use in studies of immune recognition of model membranes is described. The molecule has the basic structure HAPTEN-SPACER-LIPID, where fluorescein is the hapten, an oligopeptide (triglycine) is the spacer, and dipalmitoylphosphatidylethanolamine (DPPE) is the lipid. The spacer, which is necessary for immunological reactivity, is first linked via a peptide bond to DPPE. The free N-terminus of the peptidyl-DPPE is then reacted with 5-dichlorotriazinylaminofluorescein (DCTAF) to yield fluoresceinchlorotriazinyltriglycyl-DPPE (FG3P). The structure is confirmed by mass spectrometry and Fourier transform NMR. When FG3P is incorporated into phospholipid vesicles it retains the brilliant fluorescence and high-affinity immunological reactivity of fluorescein. The general synthesis scheme may prove useful in other membrane and lipoprotein applications.

Fluorescein↗
PubMed1984

Interaction of antibodies with liposomes bearing fluorescent haptens.

Kinetic and equilibrium aspects of the recognition of antigenic model membranes by antibodies have been studied. Monoclonal anti-fluorescein IgG and its monovalent Fab fragment were allowed to interact with a fluorescein-lipid hapten that was incorporated into phospholipid vesicles. The binding was assayed in the nanomolar hapten concentration range by monitoring the quenching of hapten fluorescence by antibody. The rate and strength of the binding depended on the lipid composition of the vesicles; cholesterol enhanced both. The biphasic binding kinetics observed at high antibody concentrations for some compositions, plus additional spectroscopic evidence, led us to hypothesize that the hapten existed in a composition-dependent equilibrium between at least two conformations: (1) extended away from the membrane surface, available for binding, and (2) sequestered at or in the surface, unavailable for binding. The rate and strength of IgG binding were always greater than those of Fab, indicating bivalent binding by the IgG. This binding was intra-vesicular, since no agglutination of the vesicles was detected.

Antigen-Antibody Complex↗
PubMed1984

Location and dynamics of a membrane-bound fluorescent hapten. A spectroscopic study.

In the preceding paper (Petrossian, A. and Owicki, J.C. (1984), Biochim. Biophys. Acta 776, 217-227), we describe the binding of a monoclonal anti-fluorescein antibody to a membrane bound fluorescein-lipid hapten. Those results suggest that some of the hapten fluorescein moiety is extended away from the membrane surface and is available for antibody binding, while some of the hapten is sequestered and not immediately available for antibody binding. In this paper, we carry out a spectroscopic study of the membrane-bound hapten and show that there is more than one physically distinct fluorophore environment, with the sequestered hapten associated with the phospholipid headgroup region. The amount of membrane-associated fluorophore depends upon the membrane lipid composition: most of the fluorophore is associated when the lipid is unsaturated or branched-chain phosphatidylcholines (PC), whereas the hapten is largely extended for PC/cholesterol mixtures. The effect of cholesterol on the availability of membrane-bound hapten to antibody binding is not unique to this system. The conversion between sequestered and extended hapten is slow (minutes).

Antigen-Antibody Complex↗
PubMed1980

Dynamic properties of binary mixtures of phosphatidylcholines and cholesterol.

We have observed the paramagnetic resonance spectra of a head group spin-labeled phosphatidylethanolamine (L-alpha-dipalmitoylphosphatidyl-N-ethanolamine) as a function of temperature and cholesterol concentration in binary mixtures of cholesterol and dimyristoylphosphatidylcholine. These spectra bear on two interrelated topics involving mixtures of phosphatidylcholine and cholesterol: (1) lipid phase equilibria and the lateral ordering and diffusion of lipid molecules and (2) model membrane immunochemistry using spin-label lipid haptens.

Cholesterol↗
PubMed1980

Lateral diffusion in inhomogeneous membranes. Model membranes containing cholesterol.

The problem of lateral diffusion in inhomogeneous membranes is illustrated by a theoretical calculation of the lateral diffusion of a fluorescent lipid probe in binary mixtures of phosphatidylcholine and cholesterol under conditions of temperature and composition such that this lipid mixture consists of alternating parallel domains of fluid and solid lipid, having separations that are small compared with the distance scale employed in photobleaching experiments. The theoretical calculations clearly illustrate how inhomogeneities in membrane composition affecting the lateral motion of membrane components on a small (10-100 nm) distance scale can give complex diffusive responses in experiments such as fluorescence photobleaching that employ comparatively macroscopic distances (10-100 micrometers) for the measurement of diffusive recovery. The theoretical calculations exhibit the unusual dependence of the apparent lateral diffusion coefficient of a fluorescent lipid probe on lipid composition in binary mixtures of cholesterol and phosphatidylcholines as reported by Rubenstein et al. (1979, Proc. Natl. Acad. Sci. U.S.A., 76:15-18).

Cholesterol↗
PubMed1979

Theory of protein-lipid and protein-protein interactions in bilayer membranes.

A model for protein-lipid interactions in bilayer membranes where the proteins are very dilute is extended to higher protein concentration, where appreciable lipid-mediated protein-protein interactions occur. It is found that proteins may change the lipid phase transition temperature and that they weaken the phase transition. There exists a critical protein concentration above which the sharp lipid phase transition is abolished. The model also qualitatively reproduces several experimental observations on the physical behavior of bilayers formed from mixtures of cholesterol and phosphatidylcholines.

Cholesterol↗
PubMed1978

Theoretical study of protein--lipid interactions in bilayer membranes.

An analysis is given for the perturbation of the order and composition of lipid bilayers near an intrinsic membrane protein. Two cases are examined: the protein influences the lipid order (i.e., "fluidity"), and the protein associates with one component of a lipid mixture preferentially. The order perturbation is studied as a function of temperature and lateral pressure by using Landau--de Gennes theory and a variational procedure. It is concluded that, for a given lateral pressure, the greatest amount of boundary lipid is present at the lipid phase-transition temperature. A critical point for the phase transition occurs, near which the amount of boundary lipid increases dramatically. The composition perturbation is modeled in a binary lipid mixture by using a simple regular solution theory. The perturbation is found not to extent much beyond the directly bound layer of lipids unless the solution is near a critical mixing point.

Chemical Phenomena↗
PubMedDate not supplied

How a gap junction maintains its structure.

In gap junctions, identical membrane proteins are linked up in pairs (dyads) that bridge the extracellular space between two apposed cell membranes. Typically, several thousand of these dyads are aggregated in the plane of the membranes and form a junctional plaque with a distinct boundary. The question thus arises as to what maintains the dyads in an aggregated state. From a statistical mechanical analysis of the positions of dyads in a freeze-fracture electron micrograph, we report here that the aggregates are not maintained by an attractive force between pairs of dyads, but probably by the minimization of the repulsive force between apposed membranes. On the basis of this analysis we present a model for the structure of mature gap junctions as well as certain aspects of the formation and disassembly of gap junctions.

Animals↗