Polymerizing properties of pepstatin A

Introduction and technical survey: protein aggregation and fibrillogenesis

In this chapter we provided the overall background to the subject of protein aggregation and fibrillogenesis in amyloidogenesis, with introduction and brief discussion of the various topics that are included with the coming chapters. The division of the book into basic science and clinical science sections enables correlation of the topics to be made. The many proteins and peptides that have currently been found to undergo fibrillogenesis are tabulated. A broad technical survey is made, to indicate the vast array of techniques currently available to study aspects of protein oligomerization, aggregation and fibrillogenesis. These are split into three groups and tabulated, as the microscopical techniques, the analytical and biophysical methods, and the biochemical and cellular techniques. A few techniques are discussed, but in most cases only a link to relevant recent literature is provided.

Negative staining across holes: application to fibril and tubular structures

The negative staining technique, when used with holey carbon support films, presents superior imaging conditions than is the case when samples are adsorbed to continuous carbon films. A demonstration of this negative staining approach is presented, using ammonium molybdate in combination with trehalose, applied to several fibrillar and tubular samples. Fibrils formed from the amyloid-beta peptide and the protease inhibitor pepstain A spread very well unsupported across holes and the different polymorphic fibril forms can be readily assessed. However, tubular forms of amyloid-beta have a tendency to be flattened, due to surface tension forces prior to and during specimen drying. Sub-fibril assembly forms and D-banded rat tail type 1 collagen fibres are presented. The air-dried collagen images produced are shown to contain almost as much detail as those obtainable by cryo-negative staining. Fragile DNA and DNA-protein nanotubes are also shown to yield superior quality images to those produced on continuous carbon films. The iron-storage protein, frataxin, creates elongated oligomeric assemblies, containing bound ferrihydrite microcrystals. The iron particles within these flexuous oligomers can be defined in the presence of ammonium molybdate, but they are more readily demonstrated if the frataxin is spread across holes in the presence of trehalose alone. The samples used here serve to show the likely benefit obtainable from negative staining across holes for a range of other fibrillar and tubular samples in biology, medicine and nanobiotechnology.

In vitro fibrillogenesis of the amyloid beta 1-42 peptide: cholesterol potentiation and aspirin inhibition

Understanding the formation of extracellular amyloid neurofibrillar bundles/senile plaques and their role in the development of Alzheimer's disease is of considerable interest to neuroscientists and clinicians. Major components of the extracellular neurofibrillar bundles are polymerized amyloid beta (Abeta) peptides (1-40), (1-42) and (1-43), derived in vivo from the soluble amyloid precursor protein (sAPP) by proteolytic (beta- and gamma-secretase) cleavage. The Abeta(1-42) peptide is widely considered to be of greatest significance in relation to the pathogenesis of Alzheimer's disease. A well-defined ultrastructural characteristic within Alzheimer dense plaques is the presence of helical fibrils that are believed to consist of polymerized amyloid beta, together with other associated proteins such as the serum amyloid P protein, apolipoprotein E isoform epsilon 4, alpha1-anti-chymotrypsin, catalase, glycoproteins, proteoglycans, cholesterol and other lipids. The spontaneous in vitro fibrillogenesis of chemically synthesized Abeta(1-42) peptide (rat sequence), following 20h incubation at 37 degrees C, has been assessed from uranyl acetate negatively stained specimens studied by transmission electron microscopy (TEM). Amyloid beta(1-42) peptide fibrillogenesis in the presence of cholesterol has been investigated using aqueous suspensions of microcrystalline cholesterol and cholesteryl acetate, globular particles of cholesteryl oleate, a soluble (micellar) cholesterol derivative (polyoxyethyl cholesteryl sebacate/cholesteryl PEG 600 sebacate), cholesterol-sphingomyelin liposomes and sphingomyelin liposomes. In all these cases, with the exception of cholesteryl oleate, considerable potentiation of long smooth helical fibril formation occurred, compared to 20h 37 degrees C control samples containing the Abeta(1-42) peptide alone. The binding of polyoxyethyl cholesteryl sebacate micelles to helical Abeta fibrils/filaments and the binding of fibrils to the surface of cholesterol and cholesteryl acetate microcrystals, and to a lesser extent on cholesteryl oleate globules, indicates an affinity of the Abeta peptide for cholesterol. This potentiation of Abeta(1-42) polymerization is likely to be mediated at the molecular level via hydrophobic interaction between the amino acid side chains of the peptide and the tetracyclic sterol nucleus. Addition of cupric sulphate (0.1mM) to the Abeta solution produced large disorganized fibril aggregates. Inclusion of 1mM aspirin (sodium acetylsalicylate) in the Abeta peptide alone and as an addition to Abeta peptide solution containing cholesterol, cholesteryl acetate, soluble cholesterol, sphingomyelin and sphingomyelin-cholesterol liposomes, and to 0.1mM cupric sulphate solution, completely inhibited fibrillogenesis. Instead, only non-crystalline diffuse, non-filamentous microaggregates of insoluble Abeta particles were found, free and attached to the sterol particles. The in vitro system presented here provides a way to rapidly monitor at the structural/TEM level other compounds (e.g. chelating agents, drugs, beta-sheet breaking peptides and anti-oxidants) for their effects on amyloid beta peptide fibrillogenesis (and on preformed fibril disassembly) in parallel with in vitro biochemical studies and in vivo studies using animal models of Alzheimer's disease as well as studies on man.

Pepstatin A: polymerization of an oligopeptide

Pepstatin A, a pentapeptide with the molecular weight of 686, is a naturally occurring inhibitor of aspartyl proteases secreted by Streptomyces species. Above a critical concentration of 0.1 mM at low ionic strength and neutral pH, it can polymerize into filaments which may extend over several micrometers. After negative staining, these filaments show a helical substructure with characteristic diameters ranging from 6 to 12 nm. Selected images at higher magnification suggest the filaments are composed of two intertwined 6 nm strands. This is in agreement with the optical diffraction analysis which additionally established a periodic pitch of 25 nm for the helical intertwining. Rotary shadowing of the pepstatin A filaments clearly demonstrated the right-handedness of the helical twist. In physiological salt solution or at higher concentrations of pepstatin A, a variety of higher order structures were observed, including ribbons, sheets and cylinders with both regular and twisted or irregular geometries. Pepstatin A can interact with intermediate filament subunit proteins. These proteins possess a long, alpha-helical rod domain that forms coiled-coil dimers, which through both hydrophobic and ionic interactions form tetramers which, in turn, in the presence of physiological salt concentrations, polymerize into the 10 nm intermediate filaments. In the absence of salt, pepstatin A and intermediate filament proteins polymerize into long filaments with a rough surface and a diameter of 15-17 nm. This polymerization appears to be primarily driven by nonionic interactions between pepstatin A and polymerization-competent forms of intermediate filament proteins, resulting in a composite filament. Polymerization-incompetent proteolytic fragments of vimentin, lacking portions of the head and/or tail domain, failed to copolymerize with pepstatin A into long filaments under these conditions. These peptides, as well as bovine serum albumin, were found to stick to the surface of pepstatin A filaments, ribbons and sheets. Independent evidence for direct association of pepstatin A with intermediate filament subunit proteins was provided not only by electron microscopy but also by UV difference spectra. Pepstatin A loses its ability to inhibit the aspartyl protease of the human immunodeficiency virus type 1 following polymerization into the higher order structures described here. The amazing fact that pepstatin A can spontaneously self-associate to form very large polymers seems to be a more rare event for such small peptides. The other examples of synthetic or naturally occurring oligopeptides discussed in this review which are able to polymerize into higher order structures possess a common property, their hydrophobicity, often manifested by clusters of valine or isoleucine residues.(ABSTRACT TRUNCATED AT 400 WORDS)