Voltage-dependent translocation of the asialoglycoprotein receptor across lipid membranes

Hepatocyte culture on carbohydrate-modified star polyethylene oxide hydrogels

We describe the synthesis and in vitro biological characterization of a new class of carbohydrate-modified hydrogels based on radiation-cross-linked star polyethylene oxide (PEO). Hydrogels were synthesized from either of two types of PEO star molecules in order to vary the terminal hydroxyl content of the gels while keeping other gel properties such as molecular weight between cross-links and water content constant. The resulting gels were covalently modified with monosaccharide ligands and the behaviour of primary rat hepatocytes on the modified gels was evaluated under culture conditions. Hepatocytes exhibited a sugar-specific adhesion to the modified gels, adhering to gels bearing galactose but not glucose. Cell spreading was observed on both types of galactose-modified PEO star gels; moreover, the gels supported long-term (6 d) culture and differentiated function of primary hepatocytes. Further, on comparing the cell spreading behaviour observed on the PEO star gels with that reported previously for galactose-modified polyacrylamide, we find that our gels elicit spreading at ligand concentrations lower by an order of magnitude. A simple mechanistic analysis indicates that this enhanced ability of PEO star gels to support spreading of primary hepatocytes on low concentrations of immobilized galactose derives from freedom of the immobilized ligands to come within sufficiently close proximity to mimic a high-affinity branched oligosaccharide.

A transmembrane potential does not affect the vertical location of charged lipid spin labels with respect to the surface of a phosphatidylcholine bilayer

The effect of a transmembrane potential on the vertical location of a charged lipid in a neutral phosphatidylcholine (PC) lipid bilayer has been investigated using negatively and positively charged spin-labeled lipids. A transmembrane potential was generated across extruded large unilamellar vesicles either by using a K+/Na+ ion gradient and a K+ ionophore or by using a pH gradient. Since a transmembrane potential could have opposing effects on lipids in the inner and outer monolayer, some of the acidic spin labels were asymmetrically located in the inner monolayer as a result of a pH gradient. No significant effect on their order parameters was observed upon applying a transmembrane potential. The internal dipole potential of the bilayer was modified by using dialkyl-PC or by incorporating 10 mol% phloretin, or 6-ketocholestanol in the PC, but a transmembrane potential still had no detectable effect on the spin labeled lipids. Therefore, it is concluded that the electrochemical potential across membranes probably does not cause a significant change in the vertical location of charged lipids with respect to the surface of a PC bilayer. This suggests that polar interactions and/or van der Waals interactions between the spin probe and the surrounding lipids stabilize the overall structure of the membranes and these interactions are not disrupted by a selective effect of the transmembrane potential on the charged lipids.

Calcium as a possible modulator of Kupffer cell phagocytic function by regulating liver-specific opsonic activity

Recently we described some properties of organ-specific serum opsonins which differentiate between liver- and spleen-specific opsonic activities, and reported that, on dialysis of serum, its liver opsonic activity is enhanced by 2- to 3-fold, whereas spleen-specific activity is reduced by 20-30% of that of control serum (Moghimi, S.M. and Patel, H.M. (1989) Biochim. Biophys. Acta 984, 379-383). This observation suggests that serum contains dialysable factors which regulate liver- as well as spleen-specific opsonic activities. Our results from EGTA-treated serum suggest that dialysable factor(s) could be divalent cations such as Ca2+, Mn2+, Mg2+ or Co2+, and among them, calcium may be the key regulatory factor for liver-specific opsonic activity. The regulatory mechanism of spleen-specific opsonic activity seems to be complex, since addition of dialysate or calcium or magnesium to the dialysed serum does not restore its activity; probably the removal of divalent cations has induced an irreversible conformational change in spleen-specific opsonin. In conclusion, we propose that the blood calcium concentration may play an important role in modulating hepatic phagocytic function by modifying liver-specific opsonic activity in serum. An increase in the physiological concentration of calcium will suppress and a decrease will enhance this opsonic activity.

Antibodies against the hepatic asialoglycoprotein receptor perfused in situ preferentially attach to periportal liver cells in the rat

Autoantibodies reacting with the galactose-specific hepatic asialoglycoprotein receptor--a liver-specific component expressed on the surfaces of hepatocytes--are often found in patients with chronic active hepatitis of presumed autoimmune origin. As part of an investigation into whether these anti-asialoglycoprotein receptor antibodies might be involved in the development of periportal liver damage in chronic active hepatitis, livers of ether-anesthetized rats were perfused in situ with polyclonal guinea pig anti-rabbit asialoglycoprotein receptor or murine monoclonal anti-human galactose-specific hepatic asialoglycoprotein receptor antibodies in excess at less than 8 degrees C or, as a control, with guinea pig anti-human plasma protein antibodies or normal guinea pig serum. Rapid (1 min) antegrade (by way of portal vein) or retrograde (through hepatic veins by way of vena cava) perfusions were performed in a nonrecirculating (once-through) mode in Ca+(+)-free medium. Blocks of liver tissue were immediately snap-frozen and the distribution of the antibody examined in cryostat sections by using an avidin-biotin immunohistochemical technique. In all of the perfusions with anti-asialoglycoprotein receptor (six antegrade, seven retrograde), the antibodies were found to be prominently and almost exclusively deposited on liver cells in the periportal areas. No deposition of immunoglobulins was detected in livers perfused with the control guinea pig sera. The findings suggest that the asialoglycoprotein receptor is expressed at high density mainly on cells in zone 1 of the hepatic lobule, and this may have implications for the development of periportal liver damage in chronic active hepatitis.

Kinetics of voltage-induced conductance increases in the outer mitochondrial membrane

The kinetics of the increase in conductance in the outer mitochondrial membrane induced by patch-clamping at various negative potentials (pipette inside negative) are reported. The changes are biphasic, a rapid increase is followed by a slowly developing larger change. The results can be predicted by a model in which an initial activation of channels is followed by their assembly into highly conducting channels. The model suggests that five to seven activated subunits form each high-conductance channel.

Luminescence spectroscopic approaches in studying cell surface dynamics

The major elements of membranes, such as proteins, lipids and polysaccharides, are in dynamic interaction with each other (Albertset al.1983). Protein diffusion in the lipid matrix of the membrane, the lipid diffusion and dynamic domain formation below and above their transition temperature from gel to fluid state, have many functional implications. This type of behaviour of membranes is often summarized in one frequently used word membrane fluidity (coined by Shinitzky & Henkart, 1979). The dynamic behaviour of the cell membrane includes rotational, translational and segmental movements of membrane elements (or their domain-like associations) in the plane of, and perpendicular to the membrane. The ever changing proximity relationships form a dynamic pattern of lipids, proteins and saccharide moieties and are usually described as ‘cell-surface dynamics’ (Damjanovichet al.1981). The knowledge about the above defined behaviour originates from experiments performed mostly on cytoplasmic membranes of eukaryotic cells. Nevertheless numerous data are available also on the mitochondrial and nuclear membranes, as well as endo (sarco-)plasmic reticulum (Martonosi, 1982; Slater, 1981; Siekevitz, 1981).

Evidence for a novel voltage-activated channel in the outer mitochondrial membrane

Patch-clamp studies of the outer mitochondrial membrane indicate a voltage-dependent increase in conductance for potentials positive relative to the exterior of the mitochondrion. The time course of the conductance changes is consistent with an activation of channels. Voltage pulse experiments suggest that the activation phenomenon corresponds to assembly of the channels from subunits with disassembly occurring after recovery of the original conductance. Effects of temperature and concanavalin A on the voltage-induced conductances are also consistent with a channel assembly model.

Gating of a voltage-dependent channel (colicin E1) in planar lipid bilayers: translocation of regions outside the channel-forming domain

C-terminal fragments of colicin E1, ranging in mol wt from 14.5 to 20 kD, form channels with voltage dependence and ion selectivity qualitatively similar to those of whole E1, placing an upper limit on the channel-forming domain. Under certain conditions, however, the gating kinetics and ion selectivity of channels formed by these different E1 peptides can be distinguished. The differences in channel behavior appear to be correlated with peptide length. Enzymatic digestion with trypsin of membrane-bound E1 peptides converts channel behavior of longer peptides to that characteristic of channels formed by shorter fragments. Apparently trypsin removes segments of protein N-terminal to the channel-forming region, since gating behavior of the shortest fragment is little affected by the enzyme. The success of this conversion depends on the side of the membrane to which trypsin is added and on the state, open or closed, of the channel. Trypsin modifies only closed channels from the cis side (the side to which protein has been added) and only open channels from the trans side. These results suggest that regions outside the channel-forming domain affect ion selectivity and gating, and they also provide evidence that large protein segments outside the channel-forming domain are translocated across the membrane with channel gating.

Multiple mechanisms of protein insertion into and across membranes

Protein localization in cells is initiated by the binding of characteristic leader (signal) peptides to specific receptors on the membranes of mitochondria or endoplasmic reticulum or, in bacteria, to the plasma membrane. There are differences in the timing of protein synthesis and translocation into or across the bilayer and in the requirement for a transmembrane electrochemical potential. Comparisons of protein localization in these different membranes suggest underlying common mechanisms.

Conditioning hyperpolarization reveals a property of the light-sensitive current in photoreceptors that is modified by cGMP and EGTA

In the dark, an ionic current flows into the outer segments of retinal rods. The absorption of photons by rhodopsin leads to a graded suppression or inactivation of this inward 'light-sensitive' current. The present study shows that following a conditioning hyperpolarization of the rod membrane in the dark, there was a transient increase in total inward current which could no longer be completely suppressed or inactivated by a bright light. The increased inward current following a hyperpolarization had the same reversal potential as the normal inward current but was apparently less sensitive to light. Thus the chain of events between light stimulation and the subsequent membrane current response can be influenced by voltage. Previously, it has been shown that the light-sensitive current is modulated by cGMP and Ca2+, both of which have been proposed as internal messengers that link the absorption of photons to the change in inward membrane current. In the present study, intracellular injection of cGMP or EGTA produced an additional increase in inward current following a conditioning hyperpolarization. On the other hand, these substances antagonized the effect of voltage by permitting complete inactivation of the total inward current by light after a hyperpolarizing step. These results indicate that cyclic nucleotides and calcium can modify the inactivation of an ionic current following a conditioning hyperpolarization. A simple model suggests that these findings are consistent with the notion that this inward conductance in rods has two open states, one sensitive to light and the other insensitive. cGMP or low Ca2+ appears to favor the open state that can be inactivated by light.

Influence of liposome charge on the association of liposomes with Kupffer cells in vitro. Effects of divalent cations and competition with latex particles

We studied the interaction of large unilamellar liposomes carrying different surface charges with rat Kupffer cells in maintenance culture. In addition to 14C-labeled phosphatidylcholine, all liposome preparations contained either 3H-labeled inulin or 125I-labeled bovine serum albumin as a non-degradable or a degradable aqueous space marker, respectively. With vesicles carrying no net charge, intracellular processing of internalized liposomes caused nearly complete release of protein label into the medium in acid-soluble form, while phospholipid label was predominantly retained by the cells, only about one third being released. The presence of the lysosomotropic agent, ammonia, inhibited the release of both labels from the cells. At 4 degrees C, the association and degradation of the vesicles were strongly reduced. These results are very similar to what we reported on negatively charged liposomes (Dijkstra, J., Van Galen, W.J.M., Hulstaert, C.E., Kalicharan, D., Roerdink, F.H. and Scherphof, G.L. (1984) Exp. Cell Res. 150, 161-176). The interaction of both types of vesicles apparently proceeds by adsorption to the cell surface followed by virtually complete internalization by endocytosis. Similar experiments with positively charged vesicles indicated that only about half of the liposomes were taken up by the endocytic route, the other half remaining adsorbed to the cell-surface. Attachment of all types of liposomes to the cells was strongly dependent on the presence of divalent cations; Ca2+ appeared to be required for optimal binding. Neutral liposomes only slightly competed with the uptake of negatively charged vesicles, both at 4 degrees and 37 degrees C, whereas negatively charged small unilamellar vesicles and negatively charged latex beads were found to compete very effectively with the large negatively charged liposomes. Neutral vesicles competed effectively for uptake with positively charged ones. These results suggest that neutral and positively charged liposomes are largely bound by the same cell-surface binding sites, while negatively charged vesicles attach mainly to other binding sites.

Hairpin configuration of H-2Kk in liposomes formed by detergent dialysis

H-2Kk is a transmembrane glycoprotein having the N-terminal region of the heavy chain exposed at the cell surface and the C-terminal region exposed at the cytoplasmic face in its native configuration in the plasma membrane. The configuration of H-2Kk in liposomes formed by detergent dialysis was investigated by using fluorescently labeled H-2 and Co2+ ions to quench fluorescence. H-2Kk was incorporated into sealed lipid vesicles when deoxycholate was removed by dialysis from a mixture of protein and lipid. Including 20 mM carboxyfluorescein (CF) in the mixture prior to dialysis resulted in CF trapped inside the vesicles at concentrations where self-quenching occurred. Vesicles with CF trapped inside were shown to be osmotically active and impermeable to Na+ and Co2+ ions. In order to examine the configuration of H-2Kk in these liposomes, the heavy chain was covalently labeled by using the sulfhydryl reactive fluorescent reagents fluorescein-5-ylmaleimide (NFM) or 5-[[2-[(iodoacetyl)amino]-ethyl]amino]naphthalene-1-sulfonic acid (IAEDANS). In both cases, approximately equal amounts of fluorescent label were incorporated into the N- and C-terminal regions of the protein. Incorporation of the labeled H-2 into liposomes and examination of the effect of Co2+ on the fluorescence showed that all of the label was accessible to quenching by Co2+ and thus exposed on the outside of the liposome. The results demonstrate that the H-2Kk is incorporated into these liposomes in a hairpin configuration, not in the transmembrane configuration found in native membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

The hepatic asialoglycoprotein receptor

Asialo- (i.e., galactose-terminal) glycoproteins are specifically and avidly recognized by a mammalian hepatic parenchymal cell receptor. This receptor, itself a glycoprotein, binds ligand molecules and directs their delivery to lysosomes for catabolism. The receptor is reutilized during this process of receptor-mediated endocytosis. Ligand specificity is conferred by galactose or N-acetyl-galactosamine at the nonreducing termini of the oligosaccharide chains. The receptor appears to be a transmembrane protein and is localized both to the cell surface as well as to several membranous intracellular compartments.

Mechanism of tetanolysin-induced membrane damage: studies with black lipid membranes

Tetanolysin produced similar rates of leakage of K+ and hemoglobin from erythrocytes. When studied by using cholesterol-containing black lipid membranes, this hemolysin induced conductance steps with a broad frequency distribution. These findings are inconsistent with the formation of structural channels and suggest that tetanolysin acts by causing lipid perturbations.

Escherichia coli outer membrane protein K is a porin

Protein K is an outer membrane protein found in pathogenic encapsulated strains of Escherichia coli. We present evidence here that protein K is structurally and functionally related to the E. coli K-12 porin proteins (OmpF, OmpC, and PhoE). Protein K was found to cross-react with antibody to OmpF protein and to share 8 out of 17 peptides in common with the OmpF protein. Strains that are OmpC porin- and OmpF porin- and contain protein K as their major outer membrane protein have increased rates of uptake of nutrients and a faster growth rate relative to the parental porin- strain. The protein K-containing strains are at least 1,000-fold more sensitive to colicins E2 and E3 than is the porin -deficient strain. These data suggest that protein K is a functional porin in E. coli. The porin function of protein K was also demonstrated in vitro, using black lipid membranes. Protein K increased the conductance in these membranes in discrete, uniform steps characteristic of channels with a size of about 2 nS.

Model for transformations of the clathrin lattice in the coated vesicle pathway

Transport of receptors by the coated vesicle pathway entails assembly of clathrin triskelions into a lattice in conjunction with receptors in a membrane. The processes by which the receptors are concentrated, the lattice is assembled, transformed into a cage during vesiculation, and subsequently removed from pinched off vesicles are not understood in regard to mechanism, energetics or control. Tubulin and actin assembly are looked to for analogies applicable to clathrin. The present model supposes that clathrin assembly is energy linked and can be described by kinetic equations of the same general form as those for treadmilling in linear polymers. The coat lattice assembles in a steady state involving the degradation of a high energy form of the clathrin triskelions. Diffuse endocytosis receptors are assumed to be associated with individual triskelions and to be able to trigger clustering and coated pit formation by influencing the assembly kinetics of the bound triskelions. A generalization of the treadmilling scheme is proposed by which the kinetic parameters associated with clathrin polymerization can shift simultaneously for an entire lattice to favor alternatively net assembly or disassembly. This shift is effected by a coordinated conversion of the lattice bound receptors. The conversion of the receptors in turn depends on some global property of the membrane compartments (arguably pH, calcium concentration or transmembrane voltage) which is likely to change as a consequence of vesiculation. Thereby, lattice disassembly can be coordinated with the topological conversion from coated pit to coated vesicle.

Studies of the voltage-dependent polyspermy block using cross-species fertilization of amphibians

Fertilization of frog eggs by frog sperm is inhibited if the egg's membrane potential is positive (N.L. Cross and R.P. Elinson, 1980, Dev. Biol. 75, 187-198); however, fertilization of salamander eggs by salamander sperm does not depend on membrane potential (M. Charbonneau, M. Moreau, B. Picheral, J.P. Vilain, and P. Guerrier, 1983, Dev. Biol. 98, 304-318). Since salamander sperm can fertilize frog eggs, we have investigated whether this cross-fertilization is voltage dependent. If, during insemination with Notophthalmus sperm, Xenopus eggs were voltage clamped between +7 and +20 mV, fertilization proceeded in 7/10 (70%) of the clamped eggs, compared to 38/48 (79%) of the neighboring eggs. In control experiments in which voltage-clamped Xenopus eggs were inseminated with Xenopus sperm, fertilization proceeded in only 1/10 (10%) of the clamped eggs, compared to 59/60 (98%) of the neighbors. Similar results were obtained with cross-fertilization experiments between Notophthalmus sperm and Rana eggs. These experiments indicate that the voltage dependence of fertilization depends on the species of sperm.

Differential regulation of muscarinic and nicotinic receptors by cholinergic stimulation in cultured avian retina cells

Sustained cholinergic stimulation of retina cells grown in primary aggregate and monolayer cultures regulated the concentration of muscarinic but not nicotinic receptors. Muscarinic receptor sites, quantified by the binding of [3H]quinuclidinyl benzilate to membranes and the binding of [3H]N-methyl-scopolamine to intact cells, decreased up to 84% following long-term incubation of cultures in muscarinic agonists. This decrease was blocked by atropine and was not induced by chronic nicotine treatment. The rate of the muscarinic response was biphasic. A rapid binding decrease of 30% occurred within 15 min. The slower phase was half-maximal by 6 h and was complete by 24 h. Neither the fast nor the slow receptor loss was reversed by the guanine nucleotide GppNp. Three different depolarizing agents (gramicidin D, protoveratrine, and ouabain) blocked the cholinergic-induced receptor loss, but the hyperpolarizing ionophore valinomycin had no effect. In contrast to the muscarinic response, nicotinic receptor binding was not altered by chronic receptor stimulation. Exposure to receptor-saturating doses of carbamylcholine or nicotine for 48 h did not change [125I]alpha-bungarotoxin or [3H]bromoacetylcholine binding. Differential regulation of acetylcholine receptors is discussed in relation to the possible physiological role of receptor regulation by receptor activity.

Voltage-dependent orientation of membrane proteins

In order to study the influence of electrostatic forces on the disposition of proteins in membranes, we have examined the interaction of a receptor protein and of a membrane-active peptide with black lipid membranes. In the first study we show that the hepatic asialoglycoprotein receptor can insert spontaneously into lipid bilayers from the aqueous medium. Under the influence of a trans-positive membrane potential, the receptor, a negatively charged protein, appears to change its disposition with respect to the membrane. In the second study we consider melittin, an amphipathic peptide containing a generally hydrophobic stretch of 19 amino acids followed by a cluster of four positively charged residues at the carboxy terminus. The hydrophobic region contains two positively charged residues. In response to trans-negative electrical potential, melittin appears to assume a transbilayer position. These findings indicate that electrostatic forces can influence the disposition, and perhaps the orientation, of membrane proteins. Given the inside-negative potential of most or all cells, we would expect transmembrane proteins to have clusters of positively charged residues adjacent to the cytoplasmic ends of their hydrophobic transmembrane segments, and clusters of negatively charged residues just to the extracytoplasmic side. This expectation has been borne out by examination of the few transmembrane proteins for which there is sufficient information on both sequence and orientation. Surface and dipole potentials may similarly affect the orientation of membrane proteins.

Increase in lipid microviscosity of unilamellar vesicles upon the creation of transmembrane potential

Diffusion potential of potassium ions was found in unilamellar vesicles of phosphatidyl choline. The vesicles, which included potassium sulfate buffered with potassium phosphate were diluted into an analogous salt solution made of sodium sulfate and sodium phosphate. The diffusion potential was created by the addition of the potassium-ionophore, valinomycin. The change in lipid microviscosity, ensuing the formation of membrane potential, was measured by the conventional method of fluorescence depolarization with 1,6-diphenyl-1,3,5-hexatriene as a probe. Lipid microviscosity was found to increase with membrane potential in a nonlinear manner, irrespective of the potential direction. Two tentative interpretations are proposed for this observation. The first assumes that the membrane potential imposes an energy barrier on the lipid flow which can be treated in terms of Boltzmann-distribution. The other interpretation assumes a decrease in lipid-free volume due to the pressure induced by the electrical potential. Since increase in lipid viscosity can reduce lateral and rotational motions, as well as increase exposure of functional membrane proteins, physiological effects induced by transmembrane potential could be associated with such dynamic changes.

Receptor-rich intracellular membrane vesicles transporting asialotransferrin and insulin in liver

A wide range of receptors are located at the blood sinusoidal aspect of the hepatocyte plasma membrane. Many circulating ligands that bind to receptors on the cell surfaces are interiorized along two pathways. Asialoglycoproteins are transferred from the plasma membrane to lysosomes and degraded, whereas immunoglobulin A and bile acids are transported across the hepatocyte interior and released into bile. Asialotransferrin type 3 (ref. 6) follows a further pathway termed diacytosis. After binding to the asialoglycoprotein receptor, asialotransferrin is endocytosed and then returned to blood with a proportion of its carbohydrate side chains resialylated. We now describe in liver the properties of intracellular asialotransferrin-enclosing vesicles (diacytosomes) and show that they differ from Golgi, lysosome and plasma membrane fractions. Furthermore, we show that the asialoglycoprotein binding sites are located on the cytoplasmic (outer) surface of diacytosomes.

Studies of the mechanism of the electrical polyspermy block using voltage clamp during cross-species fertilization

Prevention of polyspermic fertilization in sea urchins (Jaffe, 1976, Nature (Lond.). 261:68-71) and the worm Urechis (Gould-Somero, Jaffe, and Holland, 1979, J. Cell Biol. 82:426-440) involves an electrically mediated fast block. The fertilizing sperm causes a positive shift in the egg's membrane potential; this fertilization potential prevents additional sperm entries. Since in Urechis the egg membrane potential required to prevent fertilization is more positive than in the sea urchin, we tested whether in a cross-species fertilization the blocking voltage is determined by the species of the egg or by the species of the sperm. With some sea urchin (Strongylocentrotus purpuratus) females, greater than or equal to 90% of the eggs were fertilized by Urechis sperm; a fertilization potential occurred, the fertilization envelope elevated, and sometimes decondensing Urechis sperm nuclei were found in the egg cytoplasm. After insemination of sea urchin eggs with Urechis sperm during voltage clamp at +50 mV, fertilization (fertilization envelope elevation) occurred in only nine of twenty trials, whereas, at +20 mV, fertilization occurred in ten of ten trials. With the same concentration of sea urchin sperm, fertilization of sea urchin eggs occurred, in only two of ten trials at +20 mV. These results indicate that the blocking voltage for fertilization in these crosses is determined by the sperm species, consistent with the hypothesis that the fertilization potential may block the translocation within the egg membrane of a positively charged component of the sperm.

Fate of receptor and ligand during endocytosis of asialoglycoproteins by isolated hepatocytes

The endocytosis leading to degradation of 125I-labeled asialo-orosomucoid specifically bound to the surface of freshly isolated hepatocytes was monitored as a function of time at 37 degrees C. Experimental values were determined for the rates of internalization, dissociation of the receptor-ligand complex, and degradation of the labeled ligand. Compartmental analysis and computer modeling revealed that the data were compatible with dissociation of ligand from receptor preceding ligand degradation. The rate coefficient for internalization was calculated to be an order of magnitude greater than that for receptor--ligand dissociation. Ligand internalization did not result in concomitant depletion in the total number of cell surface receptors. Our data are taken to indicate that ligand remains associated with the receptor after internalization, that the complex is dissociated prior to degradation, and that new, unoccupied receptors are promptly returned to the cell surface from an internal pool.

Role for membrane potential in the secretion of protein into the periplasm of Escherichia coli

The leucine-specific binding protein of Escherichia coli is a periplasmic protein that is synthesized as a precursor and subsequently is processed during its secretion into the periplasmic space. The processing of both the leucine-specific binding protein and a plasmid-coded beta-lactamase is inhibited by phenethyl alcohol and by the proton ionophore, carbonylcyanide m-chlorophenylhydrazone (CCCP). The levels of CCCP that inhibit processing also produce significant decreases in the membrane potential. Valinomycin, a potassium ionophore, also inhibits processing of the leucine-specific binding protein in spheroplasts. Processing can be restored in CCCP-treated cells and in valinomycin-treated spheroplasts by dilution of the treated cells in fresh medium. These results suggest a role for membrane potential in the secretion of periplasmic proteins. A model is presented which suggests that membrane potential plays a primary role in the proper orientation of the precursor signal sequence within the membrane, thus promoting processing and secretion.