Structural characterisation of islet amyloid polypeptide fibrils

Islet amyloid is found in many patients suffering from type 2 diabetes. Amyloid fibrils found deposited in the pancreatic islets are composed of a 37-residue peptide, known as islet amyloid polypeptide (IAPP) (also known as amylin) and are similar to those found in other amyloid diseases. Synthetic IAPP peptide readily forms amyloid fibrils in vitro and this has allowed fibril formation kinetics and the overall morphology of IAPP amyloid to be studied. Here, we use X-ray fibre diffraction, electron microscopy and cryo-electron microscopy to examine the molecular structure of IAPP amyloid fibrils. X-ray diffraction from aligned synthetic amyloid fibrils gave a highly oriented diffraction pattern with layer-lines spaced 4.7 A apart. Electron diffraction also revealed the characteristic 4.7 A meridional signal and the position of the reflection could be compared directly to the image of the diffracting unit. Cryo-electron microscopy revealed the strong signal at 4.7 A that has been previously visualised from a single Abeta fibre. Together, these data build up a picture of how the IAPP fibril is held together by hydrogen bonded beta-sheet structure and contribute to the understanding of the generic structure of amyloid fibrils.     

Nucleation of alpha 1-antichymotrypsin polymerization

Alpha(1)-antichymotrypsin is an acute phase plasma protein and a member of the serpin superfamily. We show here that wildtype alpha(1)-antichymotrypsin forms polymers between the reactive center loop of one molecule and the beta-sheet A of a second at a rate that is dependent on protein concentration and the temperature of the reaction. The rate of polymerization was accelerated by seeding with polymers of alpha(1)-antichymotrypsin and a complex of alpha(1)-antichymotrypsin with an exogenous reactive loop peptide but not with reactive loop cleaved alpha(1)-antichymotrypsin or with polymers of other members of the serpin superfamily. Sonication of alpha(1)-antichymotrypsin polymers markedly increased the efficacy of seeding such that polymers were able to form under physiological conditions. Taken together, these data provide the first demonstration that serpin polymerization can result from seeding. This mechanism is analogous to the fibrillization of the Abeta(1-42) peptide and may be important in the deposition of alpha(1)-antichymotrypsin in the plaques of Alzheimer's disease.     

Cyclic nucleotide binding proteins in the <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> and <Emphasis Type="Italic">Oryza sativa</Emphasis> genomes

Background  

Cyclic nucleotides are ubiquitous intracellular messengers. Until recently, the roles of cyclic nucleotides in plant cells have proven difficult to uncover. With an understanding of the protein domains which can bind cyclic nucleotides (CNB and GAF domains) we scanned the completed genomes of the higher plants Arabidopsis thaliana (mustard weed) and Oryza sativa (rice) for the effectors of these signalling molecules.     

Proteasomal degradation of tau protein

Filamentous inclusions composed of the microtubule-associated protein tau are a defining characteristic of a large number of neurodegenerative diseases. Here we show that tau degradation in stably transfected and non-transfected SH-SY5Y cells is blocked by the irreversible proteasome inhibitor lactacystin. Further, we find that in vitro, natively unfolded tau can be directly processed by the 20S proteasome without a requirement for ubiquitylation, and that a highly reproducible pattern of degradation intermediates is readily detectable during this process. Analysis of these intermediates shows that 20S proteasomal processing of tau is bi-directional, proceeding from both N- and C-termini, and that populations of relatively stable intermediates arise probably because of less efficient digestion of the C-terminal repeat region. Our results are consistent with an in vivo role for the proteasome in tau degradation and support the existence of ubiquitin-independent pathways for the proteasomal degradation of unfolded proteins.     

Determining the evolutionary potential of a gene

In addition to information for current functions, the sequence of a gene includes potential information for the evolution of new functions. The wild-type ebgA (evolved beta-galactosidase) gene of Escherichia coli encodes a virtually inactive beta-galactosidase, but that gene has the potential to evolve sufficient activity to replace the lacZ gene for growth on the beta-galactoside sugars lactose and lactulose. Experimental evidence, which has suggested that the evolutionary potential of Ebg enzyme is limited o two specific amino acid replacements, is limited to examining the consequences of single base- substitutions. Thirteen beta-galactosidases homologous with the Ebg beta-galactosidase are widely dispersed, being found in gram-negative and gram-positive eubacteria and in a eukaryote. A comparison of Ebg beta-galactosidase with those 13 beta-galactosidases shows that Ebg is part of an ancient clade that diverged from the paralogous lacZ beta- galactosidase over 2 billion years ago. Ebg differs from other members of its clade at only 2 of the 15 active-site residues, and the two mutations required for full Ebg beta-galactosidase activity bring Ebg into conformity with the other members of its clade. We conclude that either these are the only acceptable amino acids at those positions, or all of the single-base-substitution replacements that must arise as intermediates on the way to other acceptable amino acids are so deleterious that they constitute a deep selective valley that has not been traversed in over 2 billion years. The evolutionary potential of Ebg is thus limited to those two replacements.      

Increased Set Shifting Costs in Fasted Healthy Volunteers

We investigated the impact of temporary food restriction on a set shifting task requiring participants to judge clusters of pictures against a frequently changing rule. 60 healthy female participants underwent two testing sessions: once after fasting for 16 hours and once in a satiated state. Participants also completed a battery of questionnaires (Hospital Anxiety and Depression Scale [HADS]; Persistence, Perseveration and Perfectionism Questionnaire [PPPQ-22]; and Eating Disorders Examination Questionnaire [EDE-Q6]). Set shifting costs were significantly increased after fasting; this effect was independent of self-reported mood and perseveration. Furthermore, higher levels of weight concern predicted a general performance decrement under conditions of fasting. We conclude that relatively short periods of fasting can lead to set shifting impairments. This finding may have relevance to studies of development, individual differences, and the interpretation of psychometric tests. It also could have implications for understanding the etiology and maintenance of eating disorders, in which impaired set shifting has been implicated.     

Characterizing the Assembly of the Sup35 Yeast Prion Fragment, GNNQQNY: Structural Changes Accompany a Fiber-to-Crystal Switch

Amyloid-like fibrils can be formed by many different proteins and peptides. The structural characteristics of these fibers are very similar to those of amyloid fibrils that are deposited in a number of protein misfolding diseases, including Alzheimer's disease and the transmissible spongiform encephalopathies. The elucidation of two crystal structures from an amyloid-like fibril-forming fragment of the yeast prion, Sup35, with sequence GNNQQNY, has contributed to knowledge regarding side-chain packing of amyloid-forming peptides. Both structures share a cross-β steric zipper arrangement but vary in the packing of the peptide, particularly in terms of the tyrosine residue. We investigated the fibrillar and crystalline structure and assembly of the GNNQQNY peptide using x-ray fiber diffraction, electron microscopy, intrinsic and quenched tyrosine fluorescence, and linear dichroism. Electron micrographs reveal that at concentrations between 0.5 and 10 mg/mL, fibers form initially, followed by crystals. Fluorescence studies suggest that the environment of the tyrosine residue changes as crystals form. This is corroborated by linear dichroism experiments that indicate a change in the orientation of the tyrosine residue over time, which suggests that a structural rearrangement occurs as the crystals form. Experimental x-ray diffraction patterns from fibers and crystals also suggest that these species are structurally distinct. A comparison of experimental and calculated diffraction patterns contributes to an understanding of the different arrangements accessed by the peptide.     

Membrane and surface interactions of Alzheimer's Aβ peptide--insights into the mechanism of cytotoxicity

Alzheimer's disease is the most common form of dementia and its pathological hallmarks include the loss of neurones through cell death, as well as the accumulation of amyloid fibres in the form of extracellular neuritic plaques. Amyloid fibrils are composed of the amyloid-β peptide (Aβ), which is known to assemble to form 'toxic' oligomers that may be central to disease pathology. Aβ is produced by cleavage from the amyloid precursor protein within the transmembrane region, and the cleaved peptide may retain some membrane affinity. It has been shown that Aβ is capable of specifically binding to phospholipid membranes with a relatively high affinity, and that modulation of the composition of the membrane can alter both membrane-amyloid interactions and toxicity. Various biomimetic membrane models have been used (e.g. lipid vesicles in solution and tethered lipid bilayers) to examine the binding and interactions between Aβ and the membrane surfaces, as well as the resulting permeation. Oligomeric Aβ has been observed to bind more avidly to membranes and cause greater permeation than fibrillar Aβ. We review some of the recent advances in studying Aβ-membrane interactions and discuss their implications with respect to understanding the causes of Alzheimer's disease.