Anopheles gambiae salivary gland proteins as putative targets for blocking transmission of malaria parasites

Anopheles gambiae is the primary vector of human malaria in sub-Saharan Africa. Invasion of Anopheles salivary glands by Plasmodium sporozoites is a necessary step in the transmission of malaria and is likely to be mediated by specific receptor-ligand interactions. We are interested in identifying putative an A. gambiae salivary gland receptor or receptors for sporozoite invasion as a possible target for blocking malaria transmission. By using monoclonal antibodies against female-specific A. gambiae salivary gland proteins, two molecules, one of 29 kDa and one of 100 kDa, were identified and characterized with respect to the age and blood-feeding process of mosquitoes. In an in vivo bioassay, the monoclonal antibody against the 100-kDa protein inhibited Plasmodium yoelii sporozoite invasion of salivary glands >/=75%. These results show that A. gambiae salivary gland proteins are accessible to monoclonal antibodies that inhibit sporozoite invasion of the salivary glands and suggest alternate targets for blocking the transmission of malaria by this most competent of malaria vectors.

Comparison of formats for the development of fiber-optic biosensors utilizing sol-gel derived materials entrapping fluorescently-labelled protein

The development of fiber-optic biosensors requires that a biorecognition element and a fluorescent reporter group be immobilized at or near the surface of an optical element such as a planar waveguide or optical fiber. In this study, we examined a model biorecognition element-reporter group couple consisting of human serum albumin that was site-selectively labelled at Cys 34 with iodoacetoxy-nitrobenzoxadiazole (HSA-NBD). The labelled protein was encapsulated into sol-gel derived materials that were prepared either as monoliths, as beads that were formed at the distal tip of a fused silica optical fiber, or as thin films that were dipcast along the length of a glass slide or optical fiber. For fiber-based studies, the entrapped protein was excited using a helium-cadmium laser that was launched into a single optical fiber, and emission was separated from the incident radiation using a perforated mirror beam-splitter, and detected using a monochromator-photomultiplier tube assembly. Changes in fluorescence intensity were generated by denaturant-induced conformational changes in the protein or by iodide quenching. The analytical parameters of merit for the different encapsulation formats, including minimum protein loading level, response time and limit-of-detection, were examined, as were factors such as protein accessibility, leaching and photobleaching. Overall, the results indicated that both beads and films were suitable for biosensor development. In both formats, a substantial fraction of the entrapped protein remained accessible, and the entrapped protein retained a large degree of conformational flexibility. Thin films showed the most rapid response times, and provided good detection limits for a model analyte. However, the entrapment of proteins into beads at the distal tip of fibers provided better signal-to-noise and signal-to-background ratios, and required less protein for preparation. Hence, beads appear to be the most viable method for interfacing of proteins to optical fibers.

Unfolding of acrylodan-labeled human serum albumin probed by steady-state and time-resolved fluorescence methods

Steady-state and time-resolved fluorescence spectroscopy was used to follow the local and global changes in structure and dynamics during chemical and thermal denaturation of unlabeled human serum albumin (HSA) and HSA with an acrylodan moiety bound to Cys34. Acrylodan fluorescence was monitored to obtain information about unfolding processes in domain I, and the emission of the Trp residue at position 214 was used to examine domain II. In addition, Trp-to-acrylodan resonance energy transfer was examined to probe interdomain spatial relationships during unfolding. Increasing the temperature to less than 50 degrees C or adding less than 1.0 M GdHCl resulted in an initial, reversible separation of domains I and II. Denaturation by heating to 70 degrees C or by adding 2.0 M GdHCl resulted in irreversible unfolding of domain II. Further denaturation of HSA by either method resulted in irreversible unfolding of domain I. These results clearly demonstrate that HSA unfolds by a pathway involving at least three distinct steps. The low detection limits and high information content of dual probe fluorescence should allow this technique to be used to study the unfolding behavior of entrapped or immobilized HSA.

Effects of metal binding affinity on the chemical and thermal stability of site-directed mutants of rat oncomodulin

Tryptophan fluorescence was used to study the stability and unfolding behavior of several single tryptophan mutants of the metal-binding protein rat oncomodulin (OM); F102W, Y57W, Y65W and the engineered protein CDOM33 which had the 12 residues of the CD loop replaced with a more potent metal binding site. Both the thermal and the chemical stability were improved upon binding of metal ions with the order apo < Ca2+ < Tb3+. During thermal denaturation, the transition midpoints (T(un)) of Y65W was the lowest, followed by Y57W and F102W. The placement of the Trp residue in the F-helix in F102W made the protein slightly more thermostable, although the fluorescence response was readily affected by chemical denaturants, which acted through the disruption of hydrogen bonds at the C-terminal end of the F-helix. Under both thermal and chemical denaturation, the engineered protein showed the highest stability. This indicated that increasing the number of metal ligating oxygens in the binding site, either by using a metal ion with a higher coordinate number (i.e., Tb3+) which binds more carboxylate ligands, or by providing more ligating groups, as in the CDOM33 replacement, produces notable improvements in protein stability.

Preparation of enantiomerically pure L-7-azatryptophan by an enzymatic method and its application to the development of a fluorimetric activity assay for tryptophanyl-tRNA synthetase

The reaction of D,L-7-azatryptophan (D,L-7AW) with tryptophanyl-tRNA synthetase (TrpRS), adenosine triphosphate (ATP), and Mg2+ in the presence of inorganic pyrophosphatase results in the formation of a highly fluorescent l-7AW-adenylate complex. Detection of this complex is based on its enhanced fluorescence at 315 nm excitation and 360 nm emission after the addition of ATP. This stereoselective reaction was used to develop an activity assay for TrpRS using commercially available racemic D,L-7AW. The assay can be used to determine the activity of TrpRS from samples which contain less than 1 nmol of enzyme in 250 microL of sample. Thus the enzyme activity can be assessed without resorting to a radioactive assay of tRNATrp acylation. A secondary use of the stereoselective assay was for confirming the presence of pure L-7AW, D-7AW, or mixtures of the two enantiomers. D-7AW and L-7AW were prepared by reacting D,L-7AW with chloroacetic anhydride to form N-chloroacetyl-D,L-7AW (ClAc-7AW) followed by stereospecific proteolytic digestion of ClAc-7AW using carboxypeptidase A to produce the free L-7AW. The L-7AW could be separated from unreacted N-chloroacetyl-7AW by reverse-phase HPLC. The TrpRS-based assay was able to unambiguously discriminate between the two enantiomers of 7AW. The assay was then used to identify which enantiomer of 7AW was present in resolved fractions of the tripeptide L-lysyl-D,L-7-azatryptophyl-L-lysine. Digestion of the resolved tripeptides with protease enzymes produced the free L or D enantiomer of 7AW, which was easily identified using the TrpRS assay procedure.

Assembly of antibodies in lipid membranes for biosensor development

An investigation of the incorporation of antibody in lipid films of a composition that has been used for biosensor preparation is reported. IgG that is incorporated into lipid monolayers prepared from 7:3 mixtures of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidic acid is edge-active, and enters and penetrates the fluid region of the mixed-phase system when monolayers are held at low pressure (< 20 mN/m). It was found that there is an "exclusion pressure" observed in pressure-area (pi-A) curves that are collected for monolayers that contain antibody. This term refers to a specific threshold of lateral pressure (which is reached by monolayer compression) that can cause explusion of antibody from the interior of a membrane. Microscopic images of monolayers containing the fluorescent phospholipid nitrobenzoxadiazole dipalmitoyl phosphatidylethanolamine (NBD-PE), or antibody labeled with tetramethylrhodamine isothiocyanate (TRITC), were used to determine the structure of membranes, and the location of effects on structure caused by IgG. Ellipsometric measurements of lipid monolayers that were cast onto silicon wafers by the Langmuir-Blodgett method were used to study the thickness of monolayers and to investigate the structural changes that occurred at the "exclusion pressure." Both the use of fluorescent antigen and ellipsometry indicated that antibody binding activity was present and was dependent on compression pressure. The effects of pH and ionic strength of subphase, antibody concentration, incubation time, and lateral pressure have been examined. The results may indicate the conditions that can be used to improve the incorporation of active IgG for preparation of biosensors that are based on lipid membranes.

Ion permeability through bilayer lipid membranes for biosensor development: control by chemical modification of interfacial regions between phase domains

Based on the results of studies on cystic fibrosis, which implicated hydroxystearic acid (HSA) as a contributing factor in altered biomembrane function, solvent-free bilayer lipid membranes (BLMs) and monolayer films were prepared from a lipid mixture containing (by mass) 34% phosphatidylcholine, 19% dipalmitoylphosphatidyl serine, 47% cholesterol and variable amounts of 10- and 12-HSA (0-50%). Ion currents, resulting from K+ permeation through BLMs that were supported in 0.1 mol dm-3 KCl solutions buffered to pH 7.4, were monitored with use of a d.c. circuit. The structures of monolayer films at the air-water interface of a Langmuir-Blodgett trough were studied by pressure-area correlations and by further correlation with microscopic phase separation as revealed by fluorescence microscopy. In order to elucidate the role of the hydroxyl moieties in ion permeability, the transmembrane ion current was corrected for the effect of the negative surface charge of the carboxylic acid by replacement of the HSA component with stearic acid. The ion current was found to increase with the molar proportion of the HSAs. Two models for ion conduction through BLMs were considered: 'hopping' via hydrophilic sites within the hydrophobic zone of the BLMs, introduced by the hydroxyl moiety of 10- or 12-HSA; and transport through interfacial regions between phase domains that represent areas of low steric density and low structural order within monolayers. Although the two mechanisms are not distinct, the ion permeability results indicate a change in the response of ion current to HSA concentration at 35 mol-%, suggesting a change in the relative proportion of the mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical transduction with fluorescent lipid membranes using selective interactions of acetylcholine receptor with agonist/antagonist and acetylcholine with substrate

Alterations in the physical structure of vesicles and monolayers of phospholipids and soybean lecithin were monitored by measurement on the average fluorescence intensity changes from N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)dipalmitoyl-L-a-phosphatidyl ethanolamine (NBD-PE) located in the lipid matrices. This probe was intimately dispersed at a concentration of 1-2 mol-% in lipid membranes and had an emission sensitive to local environmental structure. Alterations in the structure of soybean lecithin vesicles were induced by the selective interaction of acetylcholine receptor with the agonist carbamylcholine and the antagonist alpha-bungarotoxin. Structural changes in vesicles with a 7:3 mole ratio of dipalmitoylphosphatidyl choline to dipalmitoylphosphatidic acid were observed for selective interactions between acetylcholinesterase and acetylcholine. Enhancement of fluorescence emission from the lipid membranes provided transduction of the selective binding events of the receptor and enzyme. A maximum sensitivity of about a 30% enhancement per micromole of carbamylcholine and a detection limit for the toxin of 10 nM were observed for the receptor. Fluorescence microscopy was used to establish that protein could be incorporated in monolayer lipid membranes and to provide information about potential mechanisms of fluorescence enhancement. These studies show that lipid membranes containing NBD-PE can be used as generic transducers of protein-ligand interactions.