Solution and interface aggregation states of Crotalus atrox venom phospholipase A2 by two-photon excitation fluorescence correlation spectroscopy

A Review of Rattlesnake Venoms

Venom components are invaluable in biomedical research owing to their specificity and potency. Many of these components exist in two genera of rattlesnakes, Crotalus and Sistrurus, with high toxicity and proteolytic activity variation. This review focuses on venom components within rattlesnakes, and offers a comparison and itemized list of factors dictating venom composition, as well as presenting their known characteristics, activities, and significant applications in biosciences. There are 64 families and subfamilies of proteins present in Crotalus and Sistrurus venom. Snake venom serine proteases (SVSP), snake venom metalloproteases (SVMP), and phospholipases A2 (PLA2) are the standard components in Crotalus and Sistrurus venom. Through this review, we highlight gaps in the knowledge of rattlesnake venom; there needs to be more information on the venom composition of three Crotalus species and one Sistrurus subspecies. We discuss the activity and importance of both major and minor components in biomedical research and drug development.

NAD(H)-mediated tetramerization controls the activity of Legionella pneumophila phospholipase PlaB

The virulence factor PlaB promotes lung colonization, tissue destruction, and intracellular replication of Legionella pneumophila, the causative agent of Legionnaires' disease. It is a highly active phospholipase exposed at the bacterial surface and shows an extraordinary activation mechanism by tetramer deoligomerization. To unravel the molecular basis for enzyme activation and localization, we determined the crystal structure of PlaB in its tetrameric form. We found that the tetramer is a dimer of identical dimers, and a monomer consists of an N-terminal α/β-hydrolase domain expanded by two noncanonical two-stranded β-sheets, β-6/β-7 and β-9/β-10. The C-terminal domain reveals a fold displaying a bilobed β-sandwich with a hook structure required for dimer formation and structural complementation of the enzymatic domain in the neighboring monomer. This highlights the dimer as the active form. Δβ-9/β-10 mutants showed a decrease in the tetrameric fraction and altered activity profiles. The variant also revealed restricted binding to membranes resulting in mislocalization and bacterial lysis. Unexpectedly, we observed eight NAD(H) molecules at the dimer/dimer interface, suggesting that these molecules stabilize the tetramer and hence lead to enzyme inactivation. Indeed, addition of NAD(H) increased the fraction of the tetramer and concomitantly reduced activity. Together, these data reveal structural elements and an unprecedented NAD(H)-mediated tetramerization mechanism required for spatial and enzymatic control of a phospholipase virulence factor. The allosteric regulatory process identified here is suited to fine tune PlaB in a way that protects Legionella pneumophila from self-inflicted lysis while ensuring its activity at the pathogen-host interface.

Morphological and functional alteration of erythrocyte ghosts and giant unilamellar vesicles caused by Vipera latifi venom

Snake bites are an endemic public health problem in Iran, both in rural and urban area. Viper venom as a hemolytic biochemical "cocktail" of toxins, primarily cause to the systemic alteration of blood cells. In the sixties and seventies, human erythrocytes were extensively studied, but the mechanical and chemical stresses commonly exerted on red blood cells continue to attract interest of scientists for the study of membrane structure and function. Here, we monitor the effect of Vipera latifi venom on human erythrocytes ghost membranes using phase contrast and fluorescent microscopy and changes in ATPase activity under snake venom influence in vitro. The ion pumps [Na⁺,K⁺]-ATPase and (Ca²⁺+Mg²⁺)-ATPase plays a pivotal role in the active transport of certain cations and maintenance of intracellular electrolyte homeostasis. We also describe the interaction of Vipera latifi (VL) venom with giant unilamellar vesicles (GUVs) composed of the native phospholipid mixtures visualized by the membrane fluorescence probe, ANS, used to assess the state of membrane and specifically mark the phospholipid domains.

Oligomerization inhibits Legionella pneumophila PlaB phospholipase A activity

The intracellularly replicating lung pathogen Legionella pneumophila consists of an extraordinary variety of phospholipases, including at least 15 different phospholipases A (PLA). Among them, PlaB, the first characterized member of a novel lipase family, is a hemolytic virulence factor that exhibits the most prominent PLA activity in L. pneumophila. We analyzed here protein oligomerization, the importance of oligomerization for activity, addressed further essential regions for activity within the PlaB C terminus, and the significance of PlaB-derived lipolytic activity for L. pneumophila intracellular replication. We determined by means of analytical ultracentrifugation and small angle x-ray scattering analysis that PlaB forms homodimers and homotetramers. The C-terminal 5, 10, or 15 amino acids, although the individual regions contributed to PLA activity, were not essential for protein tetramerization. Infection of mouse macrophages with L. pneumophila wild type, plaB knock-out mutant, and plaB complementing or various mutated plaB-harboring strains showed that catalytic activity of PlaB promotes intracellular replication. We observed that PlaB was most active in the lower nanomolar concentration range but not at or only at a low level at concentration above 0.1 μm where it exists in a dimer/tetramer equilibrium. We therefore conclude that PlaB is a virulence factor that, on the one hand, assembles in inactive tetramers at micromolar concentrations. On the other hand, oligomer dissociation at nanomolar concentrations activates PLA activity. Our data highlight the first example of concentration-dependent phospholipase inactivation by tetramerization, which may protect the bacterium from internal PLA activity, but enzyme dissociation may allow its activation after export.

Sucrose monoester micelles size determined by Fluorescence Correlation Spectroscopy (FCS)

One of the several uses of sucrose detergents, as well as other micelle forming detergents, is the solubilization of different membrane proteins. Accurate knowledge of the micelle properties, including size and shape, are needed to optimize the surfactant conditions for protein purification and membrane characterization. We synthesized sucrose esters having different numbers of methylene subunits on the substituent to correlate the number of methylene groups with the size of the corresponding micelles. We used Fluorescence Correlation Spectroscopy (FCS) and two photon excitation to determine the translational D of the micelles and calculate their corresponding hydrodynamic radius, R_h. As a fluorescent probe we used LAURDAN (6-dodecanoyl-2-dimethylaminonaphthalene), a dye highly fluorescent when integrated in the micelle and non-fluorescent in aqueous media. We found a linear correlation between the size of the tail and the hydrodynamic radius of the micelle for the series of detergents measured.

Human apolipoprotein A-I-derived amyloid: its association with atherosclerosis

Amyloidoses constitute a group of diseases in which soluble proteins aggregate and deposit extracellularly in tissues. Nonhereditary apolipoprotein A-I (apoA-I) amyloid is characterized by deposits of nonvariant protein in atherosclerotic arteries. Despite being common, little is known about the pathogenesis and significance of apoA-I deposition. In this work we investigated by fluorescence and biochemical approaches the impact of a cellular microenvironment associated with chronic inflammation on the folding and pro-amyloidogenic processing of apoA-I. Results showed that mildly acidic pH promotes misfolding, aggregation, and increased binding of apoA-I to extracellular matrix elements, thus favoring protein deposition as amyloid like-complexes. In addition, activated neutrophils and oxidative/proteolytic cleavage of the protein give rise to pro amyloidogenic products. We conclude that, even though apoA-I is not inherently amyloidogenic, it may produce non hereditary amyloidosis as a consequence of the pro-inflammatory microenvironment associated to atherogenesis.

Fluorescence fluctuation spectroscopy: ushering in a new age of enlightenment for cellular dynamics

Originally developed for applications in physics and physical chemistry, fluorescence fluctuation spectroscopy is becoming widely used in cell biology. This review traces the development of the method and describes some of the more important applications. Specifically, the methods discussed include fluorescence correlation spectroscopy (FCS), scanning FCS, dual color cross-correlation FCS, the photon counting histogram and fluorescence intensity distribution analysis approaches, the raster scanning image correlation spectroscopy method, and the Number and Brightness technique. The physical principles underlying these approaches will be delineated, and each of the methods will be illustrated using examples from the literature.

Insights into the role of oligomeric state on the biological activities of crotoxin: crystal structure of a tetrameric phospholipase A2 formed by two isoforms of crotoxin B from Crotalus durissus terrificus venom

Crotoxin B (CB or Cdt PLA(2)) is a basic Asp49-PLA(2) found in the venom of Crotalus durissus terrificus and it is one of the subunits that constitute the crotoxin (Cro). This heterodimeric toxin, main component of the C. d. terrificus venom, is completed by an acidic, nontoxic, and nonenzymatic component (crotoxin A, CA or crotapotin), and it is related to important envenomation effects such as neurological disorders, myotoxicity, and renal failure. Although Cro has been crystallized since 1938, no crystal structure of this toxin or its subunits is currently available. In this work, the authors present the crystal structure of a novel tetrameric complex formed by two dimers of crotoxin B isoforms (CB1 and CB2). The results suggest that these assemblies are stable in solution and show that Ser1 and Glu92 of CB1 and CB2, respectively, play an important role in the oligomerization. The tetrameric and dimeric conformations resulting from the association of the isoforms may increase the neurotoxicity of the toxin CB by the creation of new binding sites, which could improve the affinity of the molecular complexes to the presynaptic membrane.

Yeast ribosomal stalk heterogeneity in vivo shown by two-photon FCS and molecular brightness analysis

The stalk of Saccharomyces cerevisiae ribosomes contains, on average, five distinct proteins, namely P0 and four acidic proteins, P1alpha, P1beta, P2alpha, and P2beta. Each ribosome contains only one copy of P0, but the distribution of the acidic proteins among the ribosome population in vivo has not been determined. Using two-photon fluorescence correlation spectroscopy and scanning FCS, on cells expressing EGFP-tagged P0, P1, and P2 proteins, we show, with brightness analysis, that individual yeast ribosomes in vivo are compositionally heterogeneous in regard to P1alpha, P1beta, P2alpha, and P2beta. These results are relevant to the hypothesis, based on in vitro studies, that the overall cellular pattern of expressed proteins can be determined by the distribution of the stalk proteins among the ribosome population.

Fluorescence Measurements of Duplex DNA Oligomers under Conditions Conducive for Forming M−DNA (a Metal−DNA Complex)

M-DNA (a metal complex of DNA with millimolar concentrations of Zn2+, Co2+, or Ni2+ and basic pH) has been proposed to undergo electron transfer over long distances along the helix and has generated interest as a potential building block for nanoelectronics. We show that DNA aggregates form under solvent conditions favorable for M-DNA (millimolar zinc and pH = 8.6) by fluorescence correlation spectroscopy. We have performed steady-state Förster resonance energy transfer (FRET) experiments with DNA oligomers conjugated with 6-carboxyfluorescein and tetramethylrhodamine to the opposite ends of double-stranded DNA (dsDNA) molecules. Enhanced acceptor emission is observed for distances larger than expected for identical DNA molecules with no zinc. To avoid intermolecular FRET, the fluorescently labeled dsDNA is diluted with a 100-fold excess of unlabeled dsDNA. The intramolecular FRET efficiency increases 25-fold for a 30-mer doubly labeled duplex DNA molecule upon addition of millimolar concentrations of zinc ions. Without zinc, this oligomer has less than 1% FRET efficiency. This dramatic increase in the FRET efficiency points to either significant changes in the Förster radius or fraying of the ends of the DNA helices. The latter hypothesis is supported by our experiments with a 9-mer that show dissociation of the duplex by zinc ions.

Interaction of high density lipoprotein particles with membranes containing cholesterol

In this study, free cholesterol (FC) efflux mediated by human HDL was investigated using fluorescence methodologies. The accessibility of FC to HDL may depend on whether it is located in regions rich in unsaturated phospholipids or in domains containing high levels of FC and sphingomyelin, known as "lipid rafts." Laurdan generalized polarization and two-photon microscopy were used to quantify FC removal from different pools in the bilayer of giant unilamellar vesicles (GUVs). GUVs made of POPC and FC were observed after incubation with reconstituted particles containing apolipoprotein A-I and POPC [78A diameter reconstituted high density lipoprotein (rHDL)]. Fluorescence correlation spectroscopy data show an increase in rHDL size during the incubation period. GUVs made of two "raft-like" mixtures [DOPC/DPPC/FC (1:1:1) and POPC/SPM/FC (6:1:1)] were used to model liquid-ordered/liquid-disordered phase coexistence. Through these experiments, we conclude that rHDL preferentially removes cholesterol from the more fluid phases. These data, and their extrapolation to in vivo systems, show the significant role that phase separation plays in the regulation of cholesterol homeostasis.

Detection and analysis of protein aggregation with confocal single molecule fluorescence spectroscopy

The misfolding and aggregation of proteins is a common phenomenon both in the cell, in in vitro protein refolding, and the corresponding biotechnological applications. Most importantly, it is involved in a wide range of diseases, including some of the most prevalent neurodegenerative disorders. However, the range of methods available to analyze this highly heterogeneous process and the resulting aggregate structures has been very limited. Here we present an approach that uses confocal single molecule detection of FRET-labeled samples employing four detection channels to obtain information about diffusivity, anisotropy, fluorescence lifetimes and Förster transfer efficiencies from a single measurement. By combining these observables, this method allows the separation of subpopulations of folded and misfolded proteins in solution with high sensitivity and a differentiation of aggregates generated under different conditions. We demonstrate the versatility of the method with experiments on rhodanese, an aggregation-prone two-domain protein.

A micelle nucleation model for the interaction of dodecyl sulphate with Lys49–phospholipases A2

Bothropstoxin-I (BthTx-I) is a Lys49-PLA(2) from the venom of Bothrops jararacussu that lacks detectable catalytic activity, yet causes rapid Ca(2+)-independent membrane damage. With the aim of understanding the interaction between BthTx-I and amphiphilic molecules, we have studied the interaction of sodium dodecyl sulphate (SDS) with the protein. Circular dichroism and attenuated total reflection Fourier-transform infrared spectra of BthTx-I reveal changes in the alpha-helical organization of the protein at an SDS/BthTx-I molar ratio of 20-25. At SDS/BthTx-I ratios of 40-45 the alpha-helices return to a native-like conformation, although fluorescence emission anisotropy measurements of 2-amino-N-hexadecyl-benzamide (AHBA) demonstrate that the total SDS is below the critical micelle concentration when this transition occurs. These results may be interpreted as the result of SDS accumulation by the BthTx-I homodimer and the formation of a pre-micelle SDS/BthTx-I complex, which may subsequently be released from the protein surface as a free micelle. Similar changes in the alpha-helical organization of BthTx-I were observed in the presence of dipalmitoylphosphatidylcholine liposomes, suggesting that protein structure transitions coupled to organization changes of bound amphiphiles may play a role in the Ca(2+)-independent membrane damage by Lys49-PLA(2)s.

Lipid--protein interactions revealed by two-photon microscopy and fluorescence correlation spectroscopy

Cellular processes involve a multitude of chemical reactions that must be kept in delicate equilibrium to maintain cellular homeostasis. Powerful biophysical techniques are needed to measure the localization and concentration of target molecules as well as to quantify complex molecular processes in model and in vivo systems. Two-photon microscopy and fluorescence correlation spectroscopy (FCS) can measure association and dynamics of appropriate molecules under equilibrium conditions. FCS provides information on motility (diffusion coefficients), concentration (number of particles), association (molecular brightness), and localization (image) of the target molecules. All of this information, in conjunction with computational modeling techniques, can help us to better understand the network of complex molecular interactions, which are at the basis of cellular processes. Fluorescence imaging techniques add the beauty of visualization to the scientific information. Photons emitted by a fluorescent dye are digitized, and the associated spatial information and intensity can be translated into different colors and shades providing the researcher not only with quantitative intensity information but also with spatial resolution and visual comprehension of two- or three-dimensional images. In this Account, we review the use of two-photon excitation microscopy and FCS in the study of lipid-protein interactions. We discuss these new methodologies and techniques, and we present examples of different complexity from qualitative to quantitative, from simple model systems to studies in living cells.

Direct Evidence That Neural Cell Adhesion Molecule (NCAM) Polysialylation Increases Intermembrane Repulsion and Abrogates Adhesion

Molecular force measurements quantified the impact of polysialylation on the adhesive properties both of membrane-bound neural cell adhesion molecule (NCAM) and of other proteins on the same membrane. These results show quantitatively that NCAM polysialylation increases the range and magnitude of intermembrane repulsion. The repulsion is sufficient to overwhelm both homophilic NCAM and cadherin attraction at physiological ionic strength, and it abrogates the protein-mediated intermembrane adhesion. The steric repulsion is ionic strength dependent and decreases substantially at high monovalent salt concentrations with a concomitant increase in the intermembrane attraction. The magnitude of the repulsion also depends on the amount of polysialic acid (PSA) on the membranes, and the PSA-dependent attenuation of cadherin adhesion increases with increasing PSA-NCAM:cadherin ratios. These findings agree qualitatively with independent reports based on cell adhesion studies and reveal the likely molecular mechanism by which NCAM polysialylation regulates cell adhesion and intermembrane space.

Application of fluorescence correlation spectroscopy to hapten-antibody binding

Two-photon fluorescence correlation spectroscopy 2P-FCS has received a large amount of attention over the past ten years as a technique that can monitor the concentration, the dynamics, and the interactions of molecules with single molecule sensitivity. In this chapter, we explain how 2P-FCS is carried out for a specific ligand-binding problem. We briefly outline considerations for proper instrument design and instrument calibration. General theory of autocorrelation analysis is explained and straightforward equations are given to analyze simple binding data. Specific concerns in the analytical methods related to IgG, such as the presence of two equivalent sites and fractional quenching of the bound hapten-fluorophore conjugate, are explored and equations are described to account for these issues. We apply these equations to data on two antibody-hapten pairs: antidigoxin IgG with fluorescein-digoxin and antidigitoxin IgG with Alexa488-digitoxin. Digoxin and digitoxin are important cardio glycoside drugs, toxic at higher levels, and their blood concentrations must be monitored carefully. Clearly, concentration assays based on IgG rely on accurate knowledge of the hapten-IgG binding strengths. The protocols for measuring and determining the dissociation constants for both IgG-hapten pairs are outlined and discussed.

Crystal structure of an acidic platelet aggregation inhibitor and hypotensive phospholipase A2 in the monomeric and dimeric states: insights into its oligomeric state

Phospholipases A2 belong to the superfamily of proteins which hydrolyzes the sn-2 acyl groups of membrane phospholipids to release arachidonic acid and lysophospholipids. An acidic phospholipase A2 isolated from Bothrops jararacussu snake venom presents a high catalytic, platelet aggregation inhibition and hypotensive activities. This protein was crystallized in two oligomeric states: monomeric and dimeric. The crystal structures were solved at 1.79 and 1.90 angstroms resolution, respectively, for the two states. It was identified a Na+ ion at the center of Ca2+-binding site of the monomeric form. A novel dimeric conformation with the active sites exposed to the solvent was observed. Conformational states of the molecule may be due to the physicochemical conditions used in the crystallization experiments. We suggest dimeric state is one found in vivo.

Tubulin equilibrium unfolding followed by time-resolved fluorescence and fluorescence correlation spectroscopy

The pathway for the in vitro equilibrium unfolding of the tubulin heterodimer by guanidinium chloride (GdmCl) has been studied using several spectroscopic techniques, specifically circular dichroism (CD), two-photon Fluorescence Correlation Spectroscopy (FCS), and time-resolved fluorescence, including lifetime and dynamic polarization. The results show that tubulin unfolding is characterized by distinct processes that occur in different GdmCl concentration ranges. From 0 to 0.5 M GdmCl, a slight alteration of the tubulin heterodimer occurs, as evidenced by a small, but reproducible increase in the rotational correlation time of the protein and a sharp decrease in the secondary structure monitored by CD. In the range 0.5-1.5 M GdmCl, significant decreases in the steady-state anisotropy and average lifetime of the intrinsic tryptophan fluorescence occur, as well as a decrease in the rotational correlation time, from 48 to 26 nsec. In the same GdmCl range, the number of protein molecules (labeled with Alexa 488), as determined by two-photon FCS measurements, increases by a factor of two, indicating dissociation of the tubulin dimer into monomers. From 1.5 to 4 M GdmCl, these monomers unfold, as evidenced by the continual decrease in the tryptophan steady-state anisotropy, average lifetime, and rotational correlation time, concomitant with secondary structural changes. These results help to elucidate the unfolding pathway of the tubulin heterodimer and demonstrate the value of FCS measurements in studies on oligomeric protein systems.

Chemical denaturation of a homodimeric lysine-49 phospholipase A2: a stable dimer interface and a native monomeric intermediate

Bothropstoxin I (4BthTx-I) is a homodimeric lysine-49 (Lys49) phospholipase A(2) isolated from Bothrops jararacussu venom, which damages liposome membranes via a Ca(2+)-independent mechanism. The stability of the BthTx-I homodimer was evaluated by equilibrium chemical denaturation with guanidinium hydrochloride monitored by changes in the intrinsic tryptophan fluorescence anisotropy, far-UV circular dichroism, dynamic light scattering, and 1-anilinonaphthalene-8-sulfonate binding. Unfolding of the BthTx-I dimer proceeds via a monomeric intermediate with native-like structure, with Gibbs free energy (DeltaG(0)) values of 10.0 and 7.2 kcal mol(-1) for the native dimer-to-native monomer and native-to-denatured monomer transitions, respectively. The experimentally determined DeltaG(0) value for the dimer-to-native monomer transition is higher than the value expected for an interaction dominated by hydrophobic forces, and suggests that an unusually high propensity of hydrogen-bonded side chains found at the BthTx-I homodimer interface make a significant contribution to dimer stability.

A two-photon view of an enzyme at work: Crotalus atrox venom PLA2 interaction with single-lipid and mixed-lipid giant unilamellar vesicles

We describe the interaction of Crotalus atrox-secreted phospholipase A2 (sPLA2) with giant unilamellar vesicles (GUVs) composed of single and binary phospholipid mixtures visualized through two-photon excitation fluorescent microscopy. The GUV lipid compositions that we examined included 1-palmitoyl-2-oleoyl-phosphatidylcholine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) (above their gel-liquid crystal transition temperatures) and two well characterized lipid mixtures, 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE):DMPC (7:3) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)/1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC) (1:1) equilibrated at their phase-coexistence temperature regime. The membrane fluorescence probes, 6-lauroyl-2-(dimethylamino) napthalene, 6-propionyl-2-(dimethylamino) naphthalene, and rhodamine-phosphatidylethanolamine, were used to assess the state of the membrane and specifically mark the phospholipid domains. Independent of their lipid composition, all GUVs were reduced in size as sPLA2-dependent lipid hydrolysis proceeded. The binding of sPLA2 was monitored using a fluorescein-sPLA2 conjugate. The sPLA2 was observed to associate with the entire surface of the liquid phase in the single phospholipid GUVs. In the mixed-lipid GUV's, at temperatures promoting domain coexistence, a preferential binding of the enzyme to the liquid regions was also found. The lipid phase of the GUV protein binding region was verified by the introduction of 6-propionyl-2-(dimethylamino) naphthalene, which partitions quickly into the lipid fluid phase. Preferential hydrolysis of the liquid domains supported the conclusions based on the binding studies. sPLA2 hydrolyzes the liquid domains in the binary lipid mixtures DLPC:DAPC and DMPC:DMPE, indicating that the solid-phase packing of DAPC and DMPE interferes with sPLA2 binding, irrespective of the phospholipid headgroup. These studies emphasize the importance of lateral packing of the lipids in C. atrox sPLA2 enzymatic hydrolysis of a membrane surface.