EM shows the fibrils to be straight, unbranching, 70–120 A˚ in diameter and of indeterminate Keywords Alzheimer’s disease; b-helix; cross-b structure; electron microscopy; mature amyloid
Trang 1Structures for amyloid fibrils
O Sumner Makin and Louise C Serpell
Department of Biochemistry, John Maynard Smith Building, School of Life Sciences, University of Sussex, Falmer, East Sussex, UK
Amyloidoses comprise over 20 diseases, including
Alz-heimer’s disease, Creutzfeldt–Jakob Disease and type
II diabetes [1–5] Although amyloid is known to be
toxic [2,6], there is considerable discussion as to its role
in disease It is suggested that the oligomeric
precur-sors to amyloid may be substantially more toxic than
amyloid itself [7,8] Even if this is the case, fibrils are
likely to play an important role, either as reservoirs or
sinks of toxic oligomers In some amyloidoses, no such
doubt exists because the mass of amyloid may exceed
that of the undiseased organ [9,10] Once the amyloid
structure has been determined, the rational design of
new drugs may be possible (e.g peptide mimetics)
[11,12] Amyloid in disease is generally defined to be
extracellular, although intracellular structures sharing
the same core structure are also observed (e.g
a-synuc-lein in Lewy bodies in Parkinson’s disease) [13] As
amyloid-like fibrils can also be formed in vitro from
protein unconnected to the amyloidoses, the distinction
between amyloid and amyloid-like fibrils is blurred [14]
It has been suggested that nearly all proteins have the ability to form amyloid under certain conditions, which has implications for the understanding of pro-tein folding [8] Amyloid precursor propro-teins do not share a common size, sequence or secondary structure, yet the mature fibrils appear to share similar highly organized multimolecular morphology and mechanisms
of toxicity [15]
Amyloid is defined in terms of empirical observa-tions from X-ray fibre diffraction, electron microscopy (EM) and specific chemical staining (Fig 1) The cross-b diffraction pattern has two characteristic sig-nals, a sharp reflection at 4.7 A˚ along the same direc-tion as the fibre and a more diffuse reflecdirec-tion at between 10 and 11 A˚ perpendicular to the fibre direc-tion (Fig 1B) [16] EM shows the fibrils to be straight, unbranching, 70–120 A˚ in diameter and of indeterminate
Keywords
Alzheimer’s disease; b-helix; cross-b
structure; electron microscopy; mature
amyloid fibril; model; solid state nuclear
magnetic resonance; X-ray fibre diffraction
Correspondence
L C Serpell, Department of Biochemistry,
John Maynard Smith Building, School of Life
Sciences, University of Sussex, Falmer,
East Sussex BN1 9QG, UK
Fax: +44 1273678433
Tel: +44 1273877363
E-mail: L.C.Serpell@sussex.ac.uk
(Received 24 June 2005, accepted
7 October 2005)
doi:10.1111/j.1742-4658.2005.05025.x
Alzheimer’s disease and Creutzfeldt–Jakob disease are the best-known examples of a group of diseases known as the amyloidoses They are char-acterized by the extracellular deposition of toxic, insoluble amyloid fibrils Knowledge of the structure of these fibrils is essential for understanding the process of pathology of the amyloidoses and for the rational design of drugs to inhibit or reverse amyloid formation Structural models have been built using information from a wide variety of techniques, including X-ray diffraction, electron microscopy, solid state NMR and EPR Recent advan-ces have been made in understanding the architecture of the amyloid fibril Here, we describe and compare postulated structural models for the mature amyloid fibril and discuss how the ordered structure of amyloid contributes
to its stability
Abbreviations
Ab, amyloid b-peptide; EM, electron microscopy; EPR, electron-paramagnetic resonance; FTIR, Fourier transform infra red; IAPP, islet amyloid polypeptide; PrP, prion protein; TTR, transthyretin; STEM, scanning transmission electron microscopy; ssNMR, solid state NMR.
Trang 2length (Fig 1A) [17] An apple-green colour is
observed in the light microscope through
cross-polaris-ers after staining with Congo red dye [18,19] A shift
in fluorescence after staining with Thioflavine T is also
observed [20] Both CD and Fourier transform infra
red (FTIR) spectroscopy support a high b-sheet
con-tent for amyloid fibrils
Structural studies have led to a better understanding
of the mechanisms by which normally soluble proteins
undergo a conformational change, associated with
aggregation, to form amyloid Amyloid may have
bio-nanotechnology applications [21,22] These include
roles in catalysis, in electronics, as a plastic, for
sup-porting cells or as a therapy for treating animals and
humans [23] It is only by understanding the detailed
structure of fibrils that their properties can be
improved and further applications developed
Much of the high-resolution, detailed structural data
has been obtained from nonphysiological amyloid-like
fibrils assembled from short peptides (either
homolog-ous to regions of disease-related peptides or designed)
These assemblies have been found to be extremely
valuable for yielding high-quality data enabling
detailed structures to be solved (see ‘Recent Advances’
below) These assemblies can serve as model systems
that give greater insight into the internal arrangements
within amyloid fibrils and are likely to be highly
rele-vant to the amyloid core structure
Debate on the structure of the toxic oligomer (or
protofibril) and structural intermediates on the fibril
formation pathway are outside the scope of this review Therefore, we limit our discussion to the struc-ture of the mastruc-ture amyloid fibril
Macromolecular structure of amyloid: protofilaments
EM and atomic-force microscopy have revealed much about the macromolecular structure of amyloid Proto-filaments are fibrillar subunits comprising amyloid fibrils and are clearly visible in micrographs, even before image processing [17,24–26] Type II diabetes-related amyloid fibrils composed of islet amyloid polypeptide (IAPP) are able to form different morpho-logies, depending on the in vitro conditions Transmis-sion EM and atomic force microscopy were used to study fibril formation and showed that the predomin-ant fibrillar structure was composed of two 5 nm diameter protofilaments wound in a left-handed direction [24,27] Cross-sections of ex vivo amyloid fibrils taken from many sources and analysed by single-particle methods resulted in averaged images showing several protofilaments [25] Improved images
of the protofilaments have been revealed by single par-ticle averaging of clearly helical fibrils by the Saibil group [28–30] (Fig 2) The SH3 domain of phosphoti-dylinositol-3¢-kinase forms twisted fibrils in which four protofilaments twist slowly around one another [28] Differing numbers and arrangements of protofilaments may be present under the same experimental
condi-100 nm
4.7 Å
~10 Å
Fibre axis
Fig 1 The characteristics of amyloid fibrils include their appearance in the electron microscope and the cross-b diffraction pattern (A) Elec-tron microscopy (EM) of negatively stained amyloid fibrils formed by islet amyloid polypeptide (IAPP), showing long, unbranching fibrils of
100 A˚ in diameter (B) X-ray fibre diffraction pattern from aligned IAPP amyloid fibrils, showing the positions of the 4.7 A˚ meridional and
10 A˚ equatorial reflections in a cross-b pattern.
Trang 3tions [26,28] For example, the 3D reconstruction of
insulin fibrils revealed fibrils formed with two, four
(Fig 2) and six protofilaments [29], although the size
and shape of the individual protofilaments was the
same Cryo-EM images of ex vivo amyloid fibrils of
Asp67His variant lysozyme showed wavy fibrils, and
image analysis indicated the presence of six
protofila-ments [30]
Internal structure of amyloid
protofilaments
Cross-b structure models for amyloid
The cross-b pattern was first observed by X-ray
dif-fraction from the egg-stalk of the lacewing Chrysopa
[31] The protein chains run orthogonal to the fibril
direction and are hydrogen-bonded, 4.7 A˚ apart, to
form a b-sheet A pseudo-repeat of 6.9 A˚ is evident
along the pleated b-chain (i.e with an axial advance
per peptide unit of 3.4 A˚ arranged with a twofold
heli-cal repeat) The spacing between the b-sheets depends
on the size of the side-chain groups [32,33] An early
model of the protofilament structure arose from
ana-lysis of X-ray fibre diffraction patterns from ex vivo
transthyretin (TTR) Met30 variant amyloid fibrils [34]
in which the b-strands were hydrogen bonded to form
a continuous b-sheet structure In this model, four
b-sheets twisted around a central axis X-ray fibre
dif-fraction from a gallery of ex vivo and synthetic
amy-loid fibrils suggested that they may share this generic
cross-b structure [3,35]
X-ray diffraction studies have commonly examined
amyloid formed by peptides corresponding to
frag-ments of amyloid b-peptide (Ab) [36–43] These serve
as valuable model systems that give information about
the core amyloid structure X-ray diffractograms from
magnetically aligned Ab(11–25) amyloid fibrils were
recorded for three mutually orthogonal beam
direc-tions [43] These showed three different patterns,
indi-cating that the structure was highly ordered with a
preferred orientation A structure was built in which
the 15mer formed an extended b-strand associating to
form cross-b ribbons These sheets were 10.6 A˚ apart,
associated via side-chain contacts The high order of
the fibrils assembled from the short, central peptide of
Ab enabled the collection of high-quality data and led
to a detailed structural model [43] Solid state NMR (ssNMR) studies of Ab(11–25) fibrils at two pH values resulted in models illustrating that the b-strands are able to slip within the protofilament structure, depend-ing on the fibril formation conditions [44] X-ray dif-fraction data from full-length Ab is less detailed than that of Ab(11–25) fibrils A model for Ab(1–40) amy-loid fibrils comprised five or six cross-b cylinders, 28 A˚ wide, and spaced 55 A˚ apart [42] Cross-b models also appeared to fit data collected from synthetic amyloid fibrils formed by other short, disease-related peptides These peptides included the first predicted a-helical region, residues (109–122) of cellular prion protein (PrPc) (H1) [41]; short PrP fragments [45]; 11-residue N-termini of the apoSAA family [46] and IAPP [26] Recent advances in ssNMR have led to the creation
of amyloid structural models [47] This allows the measurement of distances between 13C labels up to
6 A˚ apart, with standard error values of 0.1–0.2 A˚ Additional information is revealed about torsion angles, orientations relative to the applied magnetic field and the amount of order in the structure [48] Recent ssNMR and EPR experiments on synthetic amyloid fibrils formed by full-length Ab have found the b-strands to be parallel and in-register [49–53] A parallel, in-register structure was also found for Ab(10–35) fibrils [50,54–58], whilst studies on other peptides found antiparallel arrangements of b-strands [44,59–63] A recent study, incorporating acylation with octanoic acid into fibrils of Ab(6–22), showed that the amphilicity of a peptide may be associated with the preference to form parallel or antiparallel b-sheet structures [64]
Information derived from ssNMR can be comple-mented by information from scanning transmission
EM (STEM), which allows the determination of mass per unit length of a fibril by comparison with a stand-ard, such as the tobacco mosaic virus [65] ssNMR and STEM measurements for Ab(1–40) amyloid fibrils were consistent with a structure in which the Ab(1–40) peptide is folded once and then these units stacked to form two b-sheets [58] (Fig 3C) The b-sheets are in contact via side-chains This model is consistent with
Fig 2 Helical image reconstruction using single particle analysis of amyloid fibrils composed of insulin shows four individual protofilaments that twist around one another Adapted from [29] The image was generated using PYMOL (http://www.pymol.org) It shows a high-density contour of the four protofilaments in solid white and a lower threshold contour of the fibril as a transparent blue surface.
Trang 4B
C
Fig 3 Structures showing the arrangement of polypeptide chains within amyloid-like fibrils, solved from X-ray or solid state NMR (ssNMR) data Amyloid fibril structures viewed down the fibre axis (A) A 15Q peptide [75] constructed from X-ray fibre diffraction data The structure shows interdigitation of the glutamine side-chains allowing very close packing of the b-sheets (B) KFFEAAAKKFFE [101] constructed from X-ray and electron diffraction data One pair of antiparallel b-strands is shown (two layers, one above the other, into the page) The structure shows interactions between the phenylalanine residues between the b-sheets and also between b-strands within a b-sheet (C) Amyloid b-peptide (Ab) 1-40 molecule [Ab(1–40)] constructed from ssNMR data [58] The structure shows the Ab(1–40) molecule folding into two b-strands and joined by a turn Many molecules stack to form a pair of parallel b-sheets (D) GNNQQNY [103] solved by X-ray crystallography The crystal structure shows two pairs of sheets interacting via interdigitating side-chains with water excluded The adjacent sheets interact via water molecules and a single interaction between the tyrosine side-chain (E) A schematic, side-on view of the b-sheets, showing b-strands that are hydrogen bonded over the length of the fibre This scheme shows parallel b-sheets, although the sheets can also be com-posed of antiparallel b-sheets (as in the models shown in panels A and B) The figure was prepared using Pymol (http://www.pymol.org).
Trang 5protofilament width measurements from electron
micrographs of Ab(1–40) amyloid fibrils
The b-structure within a single fibril has been
visu-alized using cryo-EM of Ab(11–25) fibrils, showing a
strong meridional signal at 4.7 A˚ in the Fourier
transform and striations 4.7 A˚ apart in the image
[66] This clearly indicates that the b-strands run
per-pendicular to the fibre axis The observation of
stria-tions within the image suggests an in-register
arrangement of more than one b-sheet and highlights
the stability of the amyloid fibril A strong reflection
at 4.7 A˚ was also observed in Fourier transforms of
cryo-EM images of full-length IAPP fibrils in ice [26],
supporting the view that this is common to amyloid
fibrils
Modelling studies have suggested that Ure2p [67]
and IAPP [68] amyloid fibrils can be modelled as
‘par-allel superpleated beta’ structures, in which the protein
or peptide folds into ‘serpentines’ linking b-strands
between adjacent b-sheets These units then stack to
form a several b-sheets that gradually twist
Alternative models to the cross-b
structure
b-helix and nanotube
A b-helical or nanotube structure has been suggested
as a possible generic structure for the amyloid fibril In
these models, one or more extended b-sheets wrap
around a hollow core in a helical manner This leads
to an rise of the helix-per-turn The first b-helical
model was a cylindrical antiparallel b-helix with a
radius of 10 A˚, suggested as a model for amyloid
formed from the peptide with sequence
KLKLKLE-LELELG [69] The model was based on X-ray
diffrac-tion, EM and, particularly, the CD spectrum analyzed
by comparing it with that of pectate lyase E [70] A
later study on this peptide using FTIR concluded that
the structure might be an extended b-strand [71]
Anti-parallel b-helix models have also been inferred for
amyloid formed from TTR, Ab and immunoglobulin
light chain [72]
A water-filled nanotube b-helical model was
pro-posed from analysis of data collected from fibrils
assembled by a polyglutamine peptide [73] Twenty
res-idues per turn formed a hollow tube with internal and
external radii of 6 and 16 A˚, respectively [74] This
model was based on the absence of a 10–11 A˚ spacing
on the equator of X-ray diffraction patterns The
apparent similarities between such a structure and that
of carbon nanotubes suggest an underlying elegance,
simplicity and perhaps an understanding of the generic
nature of amyloid However, re-evaluation of the dif-fraction data showed that it was actually consistent with a cross-b arrangement of antiparallel b-hairpins [75] The diffraction data showed an 8.3 A˚ diffraction signal, which was shown to be consistent with closely packed b-sheets involving hydrogen bonding of the glutamine side-chains (Fig 3A) This model shows interdigitation of side-chains as well as hydrogen bond-ing between the glutamines along the fibre axis, result-ing in a highly stable, rigid structure
Some b-helical models are based on the b-helical fold of globular proteins with a triangular rather than
a circular projection along the fibre axis [76,77] These structures show a rise-per-turn consistent with a helix The b-helical protein family includes the pectate lyases, P22 tailspike protein and UDP-N-acetylglucosamine acyltransferase [78–81] Electron crystallography of 2D crystals of scrapie prion fragment used image analysis
to yield low-resolution projection maps [82] The resulting average was compared with calculated projec-tion maps of trimers of left-handed parallel b-helices [83]
Studies based on hydrogen–deuterium exchange and
a proline scan on full-length Ab also proposed ‘b-heli-cal models’ [77,84–87] Exchange protection resulting from hydrogen bonding was present along substantial lengths of the peptide; this differs from the pattern for most globular proteins, which generally have regions several residues long that are exchange-protected and interspersed with shorter unprotected regions This implies either a close-to-ideal b-helix or a very wide b-sheet
Further evidence for b-helical models comes from the suggestion that the predicted intersheet distance signal (usually at 10–11 A˚) may be a dehydration artefact illustrated in X-ray diffraction experiments involving Sup35 fibrils [88] However, this may also
be explained by the reduced quality of diffraction data from fibrils in solution These fibrils will be dis-persed in solution (and not packed as in a dried sample), leading to a low coherence length This, coupled with water scatter, will probably lead to the intersheet reflection being very weak and therefore unobserved
NMR data are inconclusive on this length scale (more than 6 A˚) and unable to clearly differentiate between in-register parallel b-helix (although this does not show a rise-per-turn) and an in-register parallel cross-b structure However, the collapse of such a
‘b-helix’ structure would result in a conformation very similar to the cross-b structure [58] 3D reconstructions
of SH3 fibrils revealed protofilaments with flat cross-sections, inconsistent with a three-sided b-helix [28]
Trang 6Amyloid models retaining native structure
Models composed largely of native structure (i.e
retaining structure that is present in the monomeric,
native form) have been proposed for filaments of the
yeast prion, Ure2p Biochemical evidence suggested
that Ure2p retained much of its native structure with
the filaments and that they may be composed of
asso-ciated monomers [89] An opposing model concludes
that the fibrils have a cross-b core [90,91] In the case
of Ure2p filaments, the experimental conditions are
extremely important because a cross-b structure is
evi-dent after heating [92] It remains unclear whether
Ure2p filaments fit the criteria for amyloid
X-ray fibre diffraction data of TTR fibrils led to the
construction of a model composed of axially arrayed
monomers [93] The crystal structure of a highly
amy-loidogenic TTR triple mutant has been solved and
shows a three-residue slip in one of the b-strands [94]
From this data, a model was constructed in which the
b-slip allows the construction of an infinite b-sheet in
which existing b-strands, present in the native
struc-ture, align The resulting structure is a double helical
arrangement of monomers [94] There is an overlap
repeat of 114.5 A˚, close to the meridional repeat
dis-tance, 115.5 A˚, calculated from fibre diffraction [95]
The diameter of model is 120 A˚, which is close to
130 A˚, measured using EM [96] However, TTR
mono-mers have a pair of b-sheets at an angle to one
another In the largely native TTR fibril model, the
b-strands stack in a more complicated manner than
purely along the fibre axis This is not consistent with
the cross-b model or with X-ray diffraction data A
29 A˚ meridional reflection might be expected from
stacked monomers, but is not observed [93,95]
Prox-imity information from site-directed spin labelling of
TTR fibrils permitted the building of a head-to-head
and tail-to-tail model [97], where the edge-strands of
the TTR monomer are displaced However, although
the interfaces in one sheet were revealed, the
arrange-ment of the second sheet could not be visualized
Therefore, it may be that the TTR conformation is
altered in the fibrils
Recent advances in amyloid structure
As described in the introduction, amyloid has long
been known to be composed of a ladder of b-strands
in a hydrogen-bonded b-sheet It is clear that many
proteins (involved both in disease and in vitro) are able
to access this rather simple and repetitive structure,
indicating that perhaps primary sequence plays a
minor role However, it has become increasingly clear
that primary sequence is important from fibril forma-tion experiments using very short peptides [98] and lar-ger proteins [99] Recent advances in the elucidation of structure of amyloid have enabled a better understand-ing of why this might be
Examination of sequences in disease-related, amy-loidogenic proteins revealed a preponderance of aro-matic groups [100], and suggested the importance of phenyalanine side-chains in p–p stacking This was highlighted in a subsequent structural study in which b-sheets were zipped together via p-stacking and salt bridges [101] A 12mer peptide containing two KFFE motifs separated by an AAAK motif (AAAK) formed amyloid nanocrystals that yielded high-resolution X-ray and electron diffraction data These data were indexed to a unit cell and space group, revealing the symmetry arrangements of the monomeric molecules The peptide associates to form antiparallel b-sheets, and the sheets are associated via a staggered arrange-ment allowing contacts between the side-chains (Fig 3B) Modelling revealed that very few arrange-ments of the phenylalanine residues were possible within the tightly constrained cell, and the structure showed p–p stacking of the phenylalanine residues both between the b-sheets and also between hydrogen-bonded strands The peptide is almost palindromic and has almost identical faces, meaning that the structure was propagated in both the hydrogen bonding and sheet directions [101] This structure presented a key step in understanding the nature of the intersheet association in amyloid fibrils and revealed how side-chains might enable the sheets to zip together, exclu-ding water (Fig 3B)
Similarly, fibrous crystals were grown from an amy-loid fibril forming peptide from Sup35, GNNQQNY [102,103] Electron diffraction and X-ray fibre diffrac-tion from nanocrystals yielded high-resoludiffrac-tion data [102] Subsquent growth of microcrystals allowed col-lection of X-ray crystallography data at the synchro-tron, enabling the structure of this amyloid-like assembly to be solved [103] In contrast to ‘AAAK’, this peptide has different faces and stacks into pairs of parallel b-sheets The crystal structure showed very close packing between two sheets involving interdigi-tated side-chains that exclude water, termed a ‘steric zipper’ (Fig 3D) The other face of the sheet was packed against another sheet with water molecules, and contacts occurred only via a pairing of the tyro-sine side-chain in an edge-to-face arrangement (similar
to the arrangement of Phe in the ‘AAAK’ peptide) ssNMR of fibrils formed by another peptide corres-ponding to a fragment of a yeast prion, HET-s, has supported a two-sheet structure [104] Modelling
Trang 7suggested that the peptide formed four b-strands,
forming two b-strand⁄ turn ⁄ b-strand structural motifs
Importantly, infectivity of the prion was seen to
corre-late with the ability to form the b-sheet structure This
ability was examined by substituting residues within
the predicted b-strand with Pro, which would be
expected to disrupt the b-strand structure Some of
these Pro mutants were unable to form large
aggre-gates and this correlated with their inability to infect
Again, this highlights the importance of the constituent
side-chains for fibril formation
Structural studies have begun to explain the existence
of strains that are particularly relevant to the prion
diseases These are self-perpetuating conformations that
may underlie different phenotypes (e.g the age of
onset, disease progression, ability for cross-species
infectivity) in the absence of primary sequence changes
ssNMR and STEM measurements of amyloid fibrils
formed by Ab(1–40) showed that different
morpholo-gies had different molecular structures [44,105]
Differ-ent morphologies can be influenced by fibril growth
conditions, and these conditions (and different
struc-tures) can yield samples with significantly different
toxi-cities All the fibrils, grown in quiescent or agitating
conditions, contained the cross-b motif; however, they
differed in the number of molecular layers and in the participation of particular residues in the b-strands [105] These different ‘strains’ could be shown to per-petuate themselves in seeding experiments A study involving the yeast prion, Sup35, from two different yeast species, showed that different fibril growth condi-tions could produce different ‘strains’ of fibrils with different abilities to cross the species barrier [106] Structural examination of these different strains using FTIR and EPR suggested that they differed in the resi-dues which participated in the b-strand structure Atomic force microscopy showed that PrP from differ-ent species form particular morphologies [107] that can cross-species seed and maintain their morphology Biophysical studies using fluorescently labelled vari-ants of the NM region of Sup35 have revealed specific interactions between residues within the amyloid core [108] Variations in length of the amyloid and nature
of intermolecular interactions within it have been sug-gested to underlie different ‘strains’ of prion A model structure was suggested, based on the b-helical struc-ture for a protofilament However, the accumulated data for amyloid, described in this review, seems to support a two-sheet b-helix that would show all the structural features of the cross-b fold
Table 1 Summary of the major proposed structures for amyloid fibrils.
electron diffraction
Twisted protofilaments, cross-b
[24,27]
(four sheets, twisted)
[34]
Double helical arrangement
of axially arrayed monomers
[93,94]
Ure2p and IAPP EM, X-ray diffraction, electron
diffraction, modelling
Polyglutamine peptide
(D 2 Q 15 K 2 )
Cross-b arrangement of antiparallel b-hairpins
[75] KFFEAAAKKFFE X-ray and electron diffraction Staggered, antiparallel cross-b
sheets associated via p–p stacking
of Phe groups
[101]
‘steric zipper’
[103]
Ab, amyloid b-peptide; AFM, atomic-force microscopy; EM, electron microscopy; IAPP, islet amyloid polypeptide; ssNMR, solid state NMR; TTR, transthyretin.
Trang 8In the section describing models of b-helices, it is
clear that a b-helix should have a rise-per-turn
consis-tent with a helix (as suggested for the Perutz model for
polyQ) [73] The accumulated structural data presented
here show rather a stacking of b-turn units, where the
repeat distance of the ‘helix’ is no larger than the
hydro-gen bonding distance (i.e the helical pitch is zero)
Conclusions
Recent advances have highlighted the importance of
residue composition and sequence on amyloid fibril
formation It is clear that certain simple primary
sequence motifs have the ability to fit together in a
complementary way, yielding highly ordered aggregates
and thus crystalline arrangements [101,103] In the
prion diseases, it is postulated that this is responsible
for the species barrier [103,105–108] In disease, some
peptides or proteins may have an enhanced ability to
fit together in a complementary way, packing the
side-chains to form highly stable structures This is a
prop-erty of a particular primary sequence However,
several studies [105–108] have now highlighted that
dif-ferent fibril growth conditions can affect the internal
architecture of the fibril, favouring different side-chain
packing arrangements and allowing different parts of
the sequence to participate in the b-structure This can
yield fibrils with outwardly different morphologies and
with differing growth rates, stabilities and the ability
to seed other, related, peptides (strains)
High-resolu-tion structural studies have given some insight into the
incredibly ordered arrangements of side-chains that
could underlie this phenomenon [101,103]
Many models for amyloid structure have been
pos-tulated, including cross-b, b-helix and predominantly
native structures (summarized in Table 1) The cross-b
structure has considerable support, including highly
detailed structures for amyloid fibrils formed from
cer-tain peptides [43,75,101,103] (Fig 3) The evidence for
these structures appears to clearly exclude other
mod-els It may be that some fibrils, which have been
referred to as amyloid-like, are in fact not amyloid but
simply have a sufficiently high b-sheet content to fall
within an over-broad definition It is only by
combi-ning data from many sources, such as X-ray
diffrac-tion, ssNMR and EM, for a wide range of peptides,
that an improved understanding of amyloid structure
can be developed
Acknowledgements
LCS is supported by a Wellcome Trust Research
Career Development fellowship OSM is funded
by BBSRC The authors would like to thank Prof
H Saibil for providing Fig 2, Dr P Sikorski for providing coordinates for the Poly Q structure for Fig 3A and Dr R Tycko for coordinates for Ab(9–40) for Fig 3C
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