Studies on the mechanism of membrane fusion: kinetics of calcium ion induced fusion of phosphatidylserine vesicles followed by a new assay for mixing of aqueous vesicle contents

We describe an assay for following the mixing of aqueous contents during fusion of phospholipid vesicles. Terbium is encapsulated as the Tb(citrate)3(6-) chelation complex in one population of vesicles, dipicolinic acid (DPA) in another. Vesicle fusion results in the formation of the fluorescent Tb(DPA)3(3-) chelation complex. The presence of EDTA (0.1 mM) and Ca2+ (greater than 1 mM) prevents the formation of the Tb/DPA complex in the external medium. We have studied the Ca2+-induced fusion of small or large unilamellar vesicles (SUV or LUV, respectively) composed of phosphatidylserine (PS). In addition, vesicle aggregation was monitored by light scattering, and release of vesicle contents was followed by carboxyfluorescein (CF) fluorescence enhancement. The addition of Ca2+ induced an immediate enhancement in Tb fluorescence with both SUV and LUV, which occurs on the same time scale as aggregation but much faster than the release of CF. The release of contents from LUV occurs with a considerable delay. It is estimated that the initial fusion of SUV is accompanied by 10% leakage of the internal volume per fusion event; in contrast, fusion of LUV is essentially nonleaky. Massive release of vesicle contents appears to be a secondary phenomenon related to the collapse of fused vesicles. The initial rate and the extent of Tb fluorescence enhancement are markedly dependent on the Ca2+ concentration. Threshold Ca2+ concentrations are 1.2 and 2.4 mM for SUV nd LUV, respectively. At saturating Ca2+ concentrations (greater than 10 mM), the rate of fusion of LUV is slightly lower than that of SUV at the same vesicle concentration. At any Ca2+ concentration, the rates of both SUV and LUV fusion are consistent with vesicle aggregation being rate limiting. When measured at a subsaturating Ca2+ concentration, fusion is essentially second order over a wide range of relatively low vesicle concentrations, whereas at higher vesicle concentrations the order is decreased. This suggests that at high vesicle concentrations (and at relatively low Ca2+ concentrations) aggregation may proceed faster than fusion.

Mass action kinetics of phosphatidylserine vesicle fusion as monitored by coalescence of internal vesicle volumes

The kinetics of Ca2+-induced fusion of sonicated phosphatidylserine vesicles is analyzed by means of the mass action model. The results of calculations are shown to simulate the experimental results for the mixing of aqueous vesicle volumes, release of vesicle contents and for the observed increase in light scattering [Wilschut, J., Düzgünes, N., Fraley, R., & Papahadjopoulos, D. (1980) Biochemistry (first of three papers in this issue)]. The calculations give the distribution of vesicle sizes during the initial stages of the fusion process and an estimate for the occurrence of multiple fusion events. It is estimated that during the first few seconds from the beginning of the fusion process in the above systems only a small fraction of the material trapped will leak during each fusion event. The fraction of material which leaks per fusion event is further reduced with increased Ca2+ concentrations. The values of the rates of fusion which describe the above experiments suggest that the rate limiting step of the overall fusion reaction is the aggregation and close approach of vesicles to each other rather than the fusion event per se.

Ca2+-induced fusion of phospholipid vesicles monitored by mixing of aqueous contents

Ca2+ has a central role in various cellular phenomena involving membrane fusion. However, little is known about the mechanisms involved. Model membrane systems such as phospholipid vesicles have been used extensively to study the mechanism of membrane fusion at the molecular level. For example, phosphatidylserine (PS) vesicles have been shown to undergo massive aggregation and structural rearrangements on additon of Ca2+, with eventual formation of large cochleate structures. Although these structures do not retain appreciable internal volume, their formation has been proposed to result from fusion of the initial vesicles. The significance of the PS--Ca2+ system as a model for biological membrane fusion has been questioned recently by Ginsberg. Based on the observation that divalent cations induce the release of contents from PS vesicles but fail to bring about the uptake of a marker from the medium, he proposes that the vesicles are ruptured completely during interaction with divalent cations and reassemble subsequently to form large non-vesicular structures. The present study demonstrates that the question raised by Ginsberg is not particularly relevant to the phenomenon concerned, and that his experimental observations do not allow the exclusive conclusion that Ca2+ induces lysis of PS vesicles rather than fusion.

Biochemical studies on rats with insulin-secreting islet cell tumors induced by streptozotocin: with special reference to physiological response to oral glucose load in the course of and after tumor induction

Pancreatic islet cell tumors were induced in 32 of 49 male Wistar rats (73%) surviving 9 months or longer following treatment with streptozotocin alone, with streptozotocin and nicotinamide, or with streptozotocin and picolinamide. Serial oral glucose tolerance tests in rats treated with streptozotocin and nicotinamide showed that the elevation of blood glucose levels after oral glucose load was depressed significantly 7 months after treatment. Plasma insulin responses were distinctly elevated 9 months after treatment. Blood glucose levels remained lower and plasma insulin levels rose markedly after a glucose load in tumor-bearing rats as compared to the response of tumor-free rats. These findings suggest that pancreatic islet cell tumors induced by streptozotocin with and without combined treatment are insulin-secreting, and that streptozotocin itself has oncogenic effects on the rat pancreas. Mean insulin concentration in islet cell tumors amounted to 401 U/g wet wt, whereas the concentration was 14 U/g wet wt in the pancreatic tissue from tumor-free rats.

Effect of diet restriction on some key enzymes tryptophan-NAD pathway in rats

Dietary intake of rats was restricted by feeding varying amounts of a 20% protein diet. After 6 weeks of feeding, some key enzymes of the tryptophan and nicotinic acid-NAD pathway, liver nicotinamide nucleotide concentration, and urinary metabolites of tryptophan and nicotinic acid were studied. With an increase in diet restriction, liver tryptophan oxygenase (EC 1.13.1.12) activity increased. Quinolinate phosphoribosyltransferase (EC 2.4.2.a) activity, on the other hand, was found to decrease with moderate diet restriction up to 50% restriction, but increased again with more severe diet restriction in rats fed 25% of ad libitum intake. Liver nicotinate phosphoribosyltransferase (EC 2.4.2.11) activity was also observed to decrease with moderate diet restriction and did not further change when the restriction was severe while picolinate carboxylase (EC 4.1.1.45) activity increased significantly only in severe diet restriction. In rats fed 25% of ad libitum intake, urinary quinolinic acid excretion was low whereas N'-methylnicotinamide excretion was elevated. Alterations in the enzyme activities accompanied by changes in the levels of urinary metabolites, observed in the present study, suggest that the potential efficiency of conversion of tryptophan to nicotinamide nucleotides is not constant and is influenced by dietary intake.

Dihydrodipicolinate reductases from Bacillus cereus and Bacillus megaterium

Dyhydrodipicolinate reductases were purified 100-fold from crude extracts of B. cereus and B. megaterium and their properties were compared with those of the reductase from B. subtilis. The molecular weights of the reductases of B. cereus and B. megaterium were fount to be 155,000 and 150,000, respectively. These reductases were shown to be free of flavin, unlike the B. subtilis enzyme, which contains flavin. Both NADPH and NADH acted as coenzymes for these two reductases. NADPH being three or four times more effective than NADH. The Km values for NADPH and dihydrodipicolinate were 8 micrometer and 62 micrometer, respectively, with B. cereus reductase, and 13 micrometer and 59 micrometer with B. megaterium reductase. The pH optima of the enzymes from B. cereus and B. megaterium were pH 7.4 and 7.2, respectively. The reductases were inhibited by dipicolinate noncompetitively with respect to dihydrodipicolinate and the Ki values were 85 micrometer and 140 micrometer, respectively. Lysine and diaminopimelate were not inhibitory. The properties of the reductases from B. cereus and B. megaterium were similar, but they differed considerably from those of the B. subtilis enzyme. However, all three Bacillus reductases were markedly inhibited by dipicolinate, unlike the enzyme from E. coli.

Degree of exposure of membrane proteins determined by fluorescence quenching

Quaternary salts of 4-picoline are shown to act as efficient quenchers of tryptophan fluorescence in membrane proteins. Fluorescence quenching determinations of sarcoplasmic reticulum membranes from rabbit muscle and of human erythrocyte membranes of different cholesterol to phospholipid mole ratios (C/PL) were carried out with quaternary picolinium salts in phosphate-buffered saline (PBS) and in 2,2,2-trifluorethanol (TFE)-water 2:1 (v/v), where the membrane is presumably completely disintegrated. In both solvent systems, the tryptophan quenching characteristics were typical of heterogenous systems and were analyzed as such. The ratio of the fraction of fluorescence intensity available for quenching with N-methylpicolinium perchlorate in PBS and in 2:1 TFE-water, (formula: see text), was taken as an index for the bulk degree of exposure of the membrane proteins to the aqueous surrounding. This value was found to increase with C/PL which is in line with the notion that increase in lipid microviscosity results in increase of exposure of membrane proteins. Analogous experiments were performed with N-hexyl- and N-benzylpicolinium, which can quench tryptophyl residues in both the aqueous phase and the hydrocarbon-water interface, and with N-hexadecylpicolinium which is dissolved in the membrane lipid layer and acts mostly as a static quencher of tryptophan at the hydrocarbon-water interface. With these quenchers the complementary indices (formula: see text) and (formula: see text), which represent the fraction of the protein mass located in the hydrocarbon-water interface and in the hydrocarbon layer, respectively, could be semiquantitatively resolved.

Dihydrodipicolinic acid synthase of Bacillus licheniformis. Quaternary structure, kinetics, and stability in the presence of sodium chloride and substrates

Dihydrodipicolinic acid synthase (L-aspartate-beta-semialdehyde hydro-lyase (adding pyruvate and cyclising), EC 4.2.1.52) obtained from Bacillus licheniformis was purified to homogeneity. Its molecular weight was 108 000 to 117 500, depending on the concentration of NaCl and substrates present, and it contained four subunits of identical molecular weight (28000). The Km values for pyruvate and L-aspartic semialdehyde were approximately 5.3 Km values for pyruvate and L-aspartic semialdehyde were approximately 5.3 and 2.6 mM, respectively. It was previously shown that pyruvate and a high sodium chloride concentration contributed to the stability of the enzyme. The effect of these substances and the other substrate, L-aspartic semialdehyde, on molecular weight was determined. None of these three substances significantly affected the apparent molecular weight. The effect of sodium chloride, pyruvate, and L-aspartic semialdehyde on enzyme structure was studied by determining the effect of their presence on inactivation of the enzyme by several chemical denaturants and heat. Pyruvate dramatically protected against inactivation by all of the denaturants. Sodium chloride protected against inactivation by sodium dodecyl sulfate, guanidine-HCl, urea, and heat, but somewhat facilitated inactivation by ethanol. L-Aspartic semialdehyde had no significant effect on inactivation by sodium dodecyl sulfate and ethanol; it rendered the enzyme slightly more sensitive to inactivation by guanidine-HCl and urea. The thermal melting curve obtained for the enzyme in the presence of L-aspartic semialdehyde was biphasic. The activity was reduced approximately 50% by heating for 30 min at temperatures between 50 and 80 degrees C. Only by heating at temperatures above 80 degrees C did the inactivation become complete. The partially inactivated enzyme could be reactivated by heating after removal of the L-aspartic semialdehyde. Pyruvate prevented the partial inactivation and facilitated reactivation. The only difference detected between the native enzyme and the partially inactivated form of the enzyme was that the latter had a reduced V. It is known that in other spore-formers, dihydrodipicolinate synthase increases in activity late in sporulation. This increase may be important for normal sporulation to occur. The possibility is discussed that the intracellular pool sizes of pyruvate and L-aspartic semialdehyde might have an influence on the level of dihydrodipicolinate synthase activity, by controlling the amount of partial inactivation of the enzyme that occurs in vivo.

Bacterial sporulation and regulation of dihydrodipicolinate synthase in ribonucleic acid polymerase mutants of Bacillus subtilis

The activity of dihydrodipicolinate synthase increased late in sporulation in Bacillus subtilis. Mutants blocked at several stages of sporulation due to having an altered ribonucleic acid polymerase failed to exhibit this increase.

Regulation of dihydrodipicolinate synthase during growth and sporulation of Bacillus cereus

A four- to sixfold increase in specific activity of dihydrodipicolinic acid synthase was observed during sporulation of Bacillus cereus. The enzyme from cells harvested before and after the increase in specific activity appeared to be very similar as judged by pH optima, heat denaturation kinetics, apparent Michaelis constants, chromatography on diethylaminoethyl-cellulose and Sephadex G-200, and polyacrylamide gel electrophoresis. Studies with various combinations of amino acids and one of the enzyme substrates, pyruvate, failed to give evidence for control of the enzyme by activation, inhibition, repression, induction, or stabilization. Omission of calcium from the sporulation medium had no significant effect on the specific activity pattern of the enzyme as a function of age of culture.

Peptidoglycan synthesis in L-phase variants of Bacillus licheniformis and Bacillus subtilis

Stable L-phase variants isolated from Bacillus licheniformis and Bacillus subtilis, when grown in osmotically stabilized media, do not synthesize peptidoglycan but have been found to accumulate the nucleotide precursors of this polymer. The enzymes involved in the synthesis of these precursors and the later membrane-bound stages of peptidoglycan synthesis have been investigated, and the L-phase variants have been shown to contain lesions, which provide a rational explanation for the absence of peptidoglycan and for the nature of the precursor accumulated. The majority of the L-phase variants contained a single enzymic defect, but two strains were isolated with double lesions. Five out of seven strains examined accumulated uridine 5'-diphosphate (UDP)-MurAc-L-ala-D-glu and were unable to synthesize diaminopimelic acid as a consequence of a defect in aspartate-beta-semialdehyde dehydrogenase activity. Two strains were deficient in UDP-MurAc: L-alanine ligase and accumulated UDP-MurAc. One strain accumulated the complete nucleotide precursor UDP-MurAc-L-ala-D-glu-mA2pm-D-ala-D-ala and was deficient in phospho-N-acetylmuramyl pentapeptide translocase. A second strain also had this lesion, together with defective aspartate-beta-semialdehyde dehydrogenase activity. The other enzymes of peptidoglycan synthesis were present in the L-phase variants, with activities similar to those found in the parent bacilli grown under identical conditions. Membrane preparations from certain of the L-phase variants were also capable of synthesizing the secondary polymers poly(glycerol phosphate) teichoic acid and teichuronic acid and also a polymer of N-acetylglucosamine.

Regulation of dihydrodipicolinate synthase and aspartate kinase in Bacillus subtilis

The regulation of dihydrodipicolinate synthase (EC 4.2.1.52) and aspartate kinase (EC 2.7.2.4) was studied in Bacillus subtilis 168. Starvation for lysine gave depression of one aspartate kinase isoenzyme but not of dihydrodipicolinate synthase. Strains resistant to growth inhibition by the lysine analogue thiosine exhibited constitutively derepressed synthesis of one aspartate kinase isoenzyme but had normal levels of dihydrodipicolinate synthase. The data provide strong evidence that lysine is not the signal for derepression of dihydrodipicolinate synthase. Nevertheless, dihydrodipicolinate synthase specific activity increased during sporulation, and it is suggested that this increase may result, in part, from resistance to proteolysis of that enzyme.

Overproduction of lysine by mutant strains of Escherichia coli with defective lysine transport systems

Mutants selected on the basis of their resistance to S-(beta-aminoethyl) cysteine and overproduction of lysine were found to be defective in the lysine transport system. The overproduction of lysine was not due to mutation affecting either of the two regulatory enzymes aspartokinase and dihydrodipicolinic acid synthetase. Uptake of labeled lysine by the lysine-specific transport system was reduced to a negligible level, while uptake by the lysine, ornithine, arginine system was also affected. A hypothesis regarding the nature of these mutations and their effects on the regulation of lysine biosynthesis is discussed.

Picolinic acid hydroxylase of Arthrobacter picolinophilus

Picolinic acid is oxidized by cells and extracts of Arthrobacter picolinophilus to 6-hydroxypicolinic acid. The enzyme involved is a particulate hydroxylase which appears to incorporate the hydroxyl group of water into the substrate. The enzyme was purified about 60-fold. The molecular weight as determined by gel filtration was 230 000 daltons. Activity of the purified enzyme was stimulated by ferrous ions and was inhibited by quinacrine. Flavin mononucleotide reversed the inhibition by quinacrine.