Fluorescence studies of the carboxyl-terminal domain of smooth muscle calponin effects of F-actin and salts.
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The fluorescence parameters of the environment-sensitive acrylodan, selectively attached to Cys273 in the C-terminal domain of smooth muscle calponin, were studied in the presence of F-actin and using varying salt concentrations. The formation of the F-actin acrylodan labeled calponin complex at 75 mm NaCl resulted in a 21-nm blue shift of the maximum emission wavelength from 496 nm to 474 nm and a twofold increase of the fluorescent quantum yield at 460 nm. These spectral changes were observed at the low ionic strengths (< 110 mm) where the calponin : F-actin stoichiometry is 1 : 1 as well as at the high ionic strengths (> 110 mm) where the binding stoichiometry is a 1 : 2 ratio of calponin : actin monomers. On the basis of previous three-dimensional reconstruction and chemical crosslinking of the F-actin-calponin complex, the actin effect is shown to derive from the low ionic strength interaction of calponin with the bottom of subdomain-1 of an upper actin monomer in F-actin and not from its further association with the subdomain-1 of the adjacent lower monomer which occurs at the high ionic strength. Remarkably, the F-actin-dependent fluorescence change of acrylodan is qualitatively but not quantitatively similar to that earlier reported for the complexes of calponin and Ca2+-calmodulin or Ca2+-caltropin. As the three calponin ligands bind to the same segment of the protein, encompassing residues 145-182, the acrylodan can be considered as a sensitive probe of the functioning of this critical region. A distance of 29 A was measured by fluorescence resonance energy transfer between Cys273 of calponin and Cys374 of actin in the 1 : 1 F-actin-calponin complex suggesting that the F-actin effect was allosteric reflecting a global conformational change in the C-terminal domain of calponin. (+info)
Cysteine-scanning mutagenesis of an eukaryotic pore-forming toxin from sea anemone: topology in lipid membranes.
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Equinatoxin II is a cysteineless pore-forming protein from the sea anemone Actinia equina. It readily creates pores in membranes containing sphingomyelin. Its topology when bound in lipid membranes has been studied using cysteine-scanning mutagenesis. At approximately every tenth residue, a cysteine was introduced. Nineteen single cysteine mutants were produced in Escherichia coli and purified. The accessibility of the thiol groups in lipid-embedded cysteine mutants was studied by reaction with biotin maleimide. Most of the mutants were modified, except those with cysteines at positions 105 and 114. Mutants R144C and S160C were modified only at high concentrations of the probe. Similar results were obtained if membrane-bound biotinylated mutants were tested for avidin binding, but in this case three more mutants gave a negative result: S1C, S13C and K43C. Furthermore, mutants S1C, S13C, K20C, K43C and S95C reacted with biotin only after insertion into the lipid, suggesting that they were involved in major conformational changes occurring upon membrane binding. These results were further confirmed by labeling the mutants with acrylodan, a polarity-sensitive fluorescent probe. When labeled mutants were combined with vesicles, the following mutants exhibited blue-shifts, indicating the transfer of acrylodan into a hydrophobic environment: S13C, K20C, S105C, S114C, R120C, R144C and S160C. The overall results suggest that at least two regions are embedded within the lipid membrane: the N-terminal 13-20 region, probably forming an amphiphilic helix, and the tryptophan-rich 105-120 region. Arg144, Ser160 and residues nearby could be involved in making contacts with lipid headgroups. The association with the membrane appears to be unique and different from that of bacterial pore-forming proteins and therefore equinatoxin II may serve as a model for eukaryotic channel-forming toxins. (+info)
ATP-induced opposite changes in the local environments around Cys(697) (SH2) and Cys(707) (SH1) of the myosin motor domain revealed by the prodan fluorescence.
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To obtain a consistent view of the nucleotide-induced conformational changes around Cys(697) (SH2) and Cys(707) (SH1) in skeletal myosin subfragment-1 (S-1), the two thiols were labeled with the same environmentally sensitive fluorophore, 6-acyl-2-dimethylaminonaphthalene group, using 6-acryloyl-2-dimethylaminonaphthalene (acrylodan, AD) and 6-bromoacetyl-2-dimethylaminonaphthalene (BD), respectively. The resultant fluorescent derivatives, AD-S-1 and BD-S-1, have the same fluorophore at either SH2 or SH1, which was verified by inspections of changes in the ATPases and the localization of fluorescence after tryptic digestion and CNBr cleavage for the two derivatives. Especially, AD was found to be a very useful fluorescent reagent that readily reacts with only SH2 of S-1. Measurements of the nucleotide-induced changes in fluorescence emission spectra of AD-S-1 and BD-S-1 suggested that during ATP hydrolysis the environment around the fluorophore at SH2 is very distinct from that around the fluorophore at SH1, being defined as that the former has the hydrophobic and closed characteristics, whereas the latter has the hydrophilic and open ones. The KI quenching study of the fluorescence of the two S-1 derivatives confirmed these results. The most straightforward interpretation for the present results is that during ATP hydrolysis, the helix containing SH2 is buried in hydrophobic side chains and rather reinforced, whereas the adjacent helix containing SH1 moves away from its stabilizing tertiary structural environment. (+info)
Two-photon fluorescence microscopy observation of shape changes at the phase transition in phospholipid giant unilamellar vesicles.
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Using the sectioning effect of the two-photon fluorescence microscope, we studied the behavior of phospholipid giant unilamellar vesicles (GUVs) composed of pure diacylphosphatidylcholine phospholipids during the gel-to-liquid crystalline phase transition. We used the well-characterized excitation generalized polarization function (GP(ex)) of 6-dodecanoyl-2-dimethylamine-naphthalene (LAURDAN), which is sensitive to the changes in water content in the lipid vesicles, to monitor the phase transition in the GUVs. Even though the vesicles do not show temperature hysteresis at the main phase transition, we observed different behaviors of the vesicle shape, depending on how the GUV sample reaches the main phase transition. During the cooling cycles, we observed an increase in the vesicle diameter at the phase transition ( approximately 0.5-1%), followed by a decrease in the diameter when the vesicle reached the gel phase. During the heating cycles and close to the phase transition temperature, a surprising behavior is observed, showing a sequence of different vesicle shapes as follows: spherical-polygonal-ellipsoidal. We attribute these changes to the effect of lipid domain coexistence on the macroscopic structure of the GUVs. The "shape hysteresis" in the GUVs is reversible and largely independent of the temperature scan rate. In the presence of 30 mol% of cholesterol the events observed at the phase transition in the GUVs formed by pure phospholipids were absent. (+info)
Hydration of lipoplexes commonly used in gene delivery: follow-up by laurdan fluorescence changes and quantification by differential scanning calorimetry.
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Lipoplexes, which are formed spontaneously between cationic liposomes and negatively charged nucleic acids, are commonly used for gene and oligonucleotide delivery in vitro and in vivo. Being assemblies, lipoplexes can be characterized by various physicochemical parameters, including size distribution, shape, physical state (lamellar, hexagonal type II and/or other phases), sign and magnitude of electrical surface potential, and level of hydration at the lipid-DNA interface. Only after all these variables will be characterized for lipoplexes with a broad spectrum of lipid compositions and DNA/cationic lipid (L(+)) mole (or charge) ratios can their relevance to transfection efficiency be understood. Of all these physicochemical parameters, hydration is the most neglected, and therefore the focus of this study. Cationic liposomes composed of DOTAP without and with helper lipids (DOPC, DOPE, or cholesterol) or of DC-Chol/DOPE were complexed with pDNA (S16 human growth hormone) at various DNA(-)/L(+) charge ratios (0.1-3.2). (DOTAP=N-(1-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylammonium chloride; DC-Chol=(3beta-[N-(N',N'-dimethylaminoethane)-carbamoyl]-cholester ol; DOPC=1, 2-dioleoyl-sn-glycero-3-phosphocholine; DOPE=1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine). The hydration levels of the different cationic liposomes and the DNA separately are compared with the hydration levels of the lipoplexes. Two independent approaches were applied to study hydration. First, we used a semi-quantitative approach of determining changes in the 'generalized polarization' (GP) of laurdan (6-dodecanoyl-2-dimethylaminonaphthalene). This method was recently used extensively and successfully to characterize changes of hydration at lipid-water interfaces. Laurdan excitation GP at 340 nm (GP(340)DOTAP. The GP(340) of lipoplexes of all lipid compositions (except those based on DC-Chol/DOPE) was higher than the GP(340) of the cationic liposomes alone and increased with increasing DNA(-)/L(+) charge ratio, reaching a plateau at a charge ratio of 1. 0, suggesting an increase in dehydration at the lipid-water interface with increasing DNA(-)/L(+) charge ratio. Confirmation was obtained from the second method, differential scanning calorimetry (DSC). DOTAP/DOPE lipoplexes with charge ratio 0.44 had 16.5% dehydration and with charge ratio 1.5, 46.4% dehydration. For DOTAP/Chol lipoplexes with these charge ratios, there was 17.9% and 49% dehydration, respectively. These data are in good agreement with the laurdan data described above. They suggest that the dehydration occurs during lipoplex formation and that this is a prerequisite for the intimate contact between cationic lipids and DNA. (+info)
Electrostatic properties of membranes containing acidic lipids and adsorbed basic peptides: theory and experiment.
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The interaction of heptalysine with vesicles formed from mixtures of the acidic lipid phosphatidylserine (PS) and the zwitterionic lipid phosphatidylcholine (PC) was examined experimentally and theoretically. Three types of experiments showed that smeared charge theories (e.g., Gouy-Chapman-Stern) underestimate the membrane association when the peptide concentration is high. First, the zeta potential of PC/PS vesicles in 100 mM KCl solution increased more rapidly with heptalysine concentration (14.5 mV per decade) than predicted by a smeared charge theory (6.0 mV per decade). Second, changing the net surface charge density of vesicles by the same amount in two distinct ways produced dramatically different effects: the molar partition coefficient decreased 1000-fold when the mole percentage of PS was decreased from 17% to 4%, but decreased only 10-fold when the peptide concentration was increased to 1 microM. Third, high concentrations of basic peptides reversed the charge on PS and PC/PS vesicles. Calculations based on finite difference solutions to the Poisson-Boltzmann equation applied to atomic models of heptalysine and PC/PS membranes provide a molecular explanation for the observations: a peptide adsorbing to the membrane in the presence of other surface-adsorbed peptides senses a local potential more negative than the average potential. The biological implications of these "discreteness-of-charge" effects are discussed. (+info)
Two photon fluorescence microscopy of coexisting lipid domains in giant unilamellar vesicles of binary phospholipid mixtures.
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Images of giant unilamellar vesicles (GUVs) formed by different phospholipid mixtures (1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1, 2-dilauroyl-sn-glycero-3-phosphocholine (DPPC/DLPC) 1:1 (mol/mol), and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine/1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPE/DPPC), 7:3 and 3:7 (mol/mol) at different temperatures were obtained by exploiting the sectioning capability of a two-photon excitation fluorescence microscope. 6-Dodecanoyl-2-dimethylamino-naphthalene (LAURDAN), 6-propionyl-2-dimethylamino-naphthalene (PRODAN), and Lissamine rhodamine B 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (N-Rh-DPPE) were used as fluorescent probes to reveal domain coexistence in the GUVs. We report the first characterization of the morphology of lipid domains in unsupported lipid bilayers. From the LAURDAN intensity images the excitation generalized polarization function (GP) was calculated at different temperatures to characterize the phase state of the lipid domain. On the basis of the phase diagram of each lipid mixture, we found a homogeneous fluorescence distribution in the GUV images at temperatures corresponding to the fluid region in all lipid mixtures. At temperatures corresponding to the phase coexistence region we observed lipid domains of different sizes and shapes, depending on the lipid sample composition. In the case of GUVs formed by DPPE/DPPC mixture, the gel DPPE domains present different shapes, such as hexagonal, rhombic, six-cornered star, dumbbell, or dendritic. At the phase coexistence region, the gel DPPE domains are moving and growing as the temperature decreases. Separated domains remain in the GUVs at temperatures corresponding to the solid region, showing solid-solid immiscibility. A different morphology was found in GUVs composed of DLPC/DPPC 1:1 (mol/mol) mixtures. At temperatures corresponding to the phase coexistence, we observed the gel domains as line defects in the GUV surface. These lines move and become thicker as the temperature decreases. As judged by the LAURDAN GP histogram, we concluded that the lipid phase characteristics at the phase coexistence region are different between the DPPE/DPPC and DLPC/DPPC mixtures. In the DPPE/DPPC mixture the coexistence is between pure gel and pure liquid domains, while in the DLPC/DPPC 1:1 (mol/mol) mixture we observed a strong influence of one phase on the other. In all cases the domains span the inner and outer leaflets of the membrane, suggesting a strong coupling between the inner and outer monolayers of the lipid membrane. This observation is also novel for unsupported lipid bilayers. (+info)
A further cohort study of workers employed at a factory manufacturing chemicals for the rubber industry, with special reference to the chemicals 2-mercaptobenzothiazole (MBT), aniline, phenyl-beta-naphthylamine and o-toluidine.
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OBJECTIVES: To investigate mortality and cancer morbidity in workers from a factory manufacturing chemicals for the rubber industry. METHODS: The mortality (1955-96) and cancer morbidity experience (1971-92) of a cohort of 2160 male production workers from a chemical factory in north Wales were investigated. All subjects had at least 6 months employment at the factory and some employment in the period 1955-84. Detailed job histories were abstracted from company computerised records and estimates of individual cumulative exposure to 2-mercaptobenzothiazole (MBT) and its derivatives were obtained, with a job exposure matrix derived by a former factory hygienist. Durations of employment in the aniline, phenyl-beta-naphthylamine (PBN) and o-toluidine departments were also calculated. Two analytical approaches were used, indirect standardisation and Poisson regression. RESULTS: Based on serial rates for the general population of England and Wales, observed mortality for the total cohort was close to expectation for all causes (observed (obs) deaths 1131, expected (exp) deaths 1114.5, standardised mortality ratio (SMR) 101), and for all cancers (obs 305, exp 300.2, SMR 102). There was a significant (p < 0.05) excess mortality from cancer of the bladder in the 605 study subjects potentially exposed to one or more of the four chemicals being investigated (obs 9, exp 3.25, SMR 277, 95% confidence interval (95% CI) 127 to 526). This excess was dependent primarily on deaths occurring > 20 years after first exposure in those who started employment before 1955 (obs 7, exp 1.25, SMR 560, 95% CI 225 to 1154, p < 0.001). There were 30 subjects in the total study cohort who, on the basis of death certificates or cancer registration particulars, had had malignant bladder cancer. In separate analyses of the four exposure history variables (after adjustment for age), Poisson regression showed significant positive trends for risk of notification of bladder cancer increasing with cumulative duration of employment in the PBN (p < 0.001) and o-toluidine departments (p < 0.01); similar findings were not obtained for cumulative exposure to MBT or for duration of employment in the aniline department. In a simultaneous analysis of all four chemical exposure variables, a significant positive trend remained for duration of employment with exposure to PBN (p < 0.05). Further analyses of all cases of bladder cancer (malignant and benign diagnoses) used employment histories lagged by 15 years; similar findings were obtained. CONCLUSIONS: It seems likely that some members of this cohort have had occupational bladder cancer. Confident interpretation is difficult because of small numbers in the exposed subcohorts, relatively crude measures of exposure assessment for the four chemicals under study, and presence of unconsidered potential chemical confounders. The simplest interpretation of the findings about bladder cancer may be that PBN (or a chemical reagent or chemical intermediate associated with its production at this factory in the 1930s and 1940s) is a bladder carcinogen. Priority should be given, however, to obtaining information on the cancer experience of other working populations exposed to PBN or to o-toluidine. (+info)