bone marrow as a form of biologic recycling ( 16),
an event that is unlikely to occur in infections.
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We thank T. Nussbaumer for mice husbandry and A. S. Neupane
for technical support. We also thank the staff at the University
of Calgary Live Cell Imaging Facility for their assistance with the
photoactivation experiments and K. Poon at the Nicole Perkins
Microbial Communities Core Labs for assistance with flow
cytometry. This work was supported by Canadian Institutes of
Health Research (CIHR) Banting fellowships (A. T. and J. W.), an
Alberta Cancer Foundation postdoctoral fellowship (M.H.), a CIHR
Lung Group Grant for Chronic Inflammation, and a grant from
the Heart and Stroke Foundation of Canada (P.K.). The data in this
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Materials and Methods
Figs. S1 to S8
References ( 20–27)
Movies S1 to S12
12 February 2017; resubmitted 3 July 2017
Accepted 30 August 2017
Fibril structure of amyloid-b(1– 42)
by cryo–electron microscopy
Lothar Gremer,1,2 Daniel Schölzel,1,2 Carla Schenk,1 Elke Reinartz,2 Jörg Labahn,1,2, 3
Raimond B. G. Ravelli, 4 Markus Tusche,1 Carmen Lopez-Iglesias, 4 Wolfgang Hoyer,1,2
Henrike Heise,1,2 Dieter Willbold,1,2 Gunnar F. Schröder1, 5*
Amyloids are implicated in neurodegenerative diseases. Fibrillar aggregates of the amyloid-b
protein (Ab) are the main component of the senile plaques found in brains of Alzheimer’s
disease patients. We present the structure of an Ab(1– 42) fibril composed of two intertwined
protofilaments determined by cryo–electron microscopy (cryo-EM) to 4.0-angstrom resolution,
complemented by solid-state nuclear magnetic resonance experiments. The backbone of all
42 residues and nearly all side chains are well resolved in the EM density map, including
the entire N terminus, which is part of the cross-b structure resulting in an overall “LS”-shaped
topology of individual subunits. The dimer interface protects the hydrophobic C termini from
the solvent. The characteristic staggering of the nonplanar subunits results in markedly
different fibril ends, termed “groove” and “ridge,” leading to different binding pathways on
both fibril ends, which has implications for fibril growth.
Amyloids are involved in various diseases, most prominently in many neurodegener- ative diseases (1– 3). The amyloid-b protein (Ab) forms fibrils that further aggregate into plaques that are found in the brains
of Alzheimer’s disease patients ( 4). These fibrils
are structurally highly heterogeneous (1, 5–8),
which makes the production of highly ordered
samples and structure determination difficult.
Ab fibrils have been described as protofilaments
intertwined in a helical geometry, existing in sev-
eral polymorphs, with varying width and helical
pitch, different cross-section profiles, and dif-
ferent interactions between the protofilaments
( 5–7, 9, 10). The local arrangement of Ab mole-
cules within the fibril can vary drastically between
different isomorphs, with potential implications
for biological activity ( 3). Data from solid-state
nuclear magnetic resonance (NMR) experiments
has allowed for building models of Ab fibrils at
atomic resolution ( 6, 7, 11–15). Here, we present
the atomic structure of Ab(1– 42) fibrils by cryo–
electron microscopy (cryo-EM) (Figs. 1 and 2 and
table S1). To facilitate structure determination,
we identified conditions [aqueous solution at low
pH containing organic cosolvent; see ( 16)] that
yielded a highly homogeneous sample of fibrils
as shown by EM and atomic-force microscopy
(AFM) [figs. S1 and S2; see ( 16)]. The toxicity of
these fibrils was indistinguishable from fibrils
grown at neutral pH (fig. S3). Micrographs re-
vealed micrometer-long unbranched fibrils, where
about 90% of the fibrils had a rather invariable
diameter of about 7 nm (fig. S1). These fibrils
were used in a helical reconstruction procedure
to compute a three-dimensional (3D) density to
4.0-Å resolution [Figs. 1 and 2 and fig. S4; see
( 16)]. The EM data were augmented by solid-
state NMR and x-ray diffraction experiments,
which were performed on identically produced
fibril samples of recombinant uniformly labeled
[15N/13C]-Ab(1– 42) and show that the EM structure
is representative of the sample. Full site-specific
resonance assignments from 2D and 3D homo-
and heteronuclear correlation spectra could be
obtained by solid-state NMR for all 42 residues
(Fig. 3, A and B; figs. S5 to S7; and tables S2 and
S3). For most amino acid residues, only one set
of resonances was observed, indicative of high
structural homogeneity and order.
The reconstructed fibril density and the atomic
model (Fig. 1) show two twisted protofilaments
composed of Ab(1– 42) molecules stacked in a parallel, in-register cross-b structure. The separation
between the parallel b strands is well visible in
the density (Fig. 1A and fig. S8A). The peripheral
b sheets (residues 1 to 9 and 11 to 21) are tilted
with respect to the fibril axis by 10° (Fig. 2C).
Remarkably, the fibril does not show a C2
symmetry but instead an approximate 21 screw symmetry with a rise of 4. 67 Å, which is in excellent
agreement with the strongest peak in the x-ray
diffraction profile of 4. 65 Å (Fig. 3C and fig. S9).
Owing to this helical symmetry, the subunits are
arranged in a staggered manner (Fig. 4A). The
interaction between the protofilaments is thus
not true dimeric, but the subunits are stepwise
shifted along the fibril axis (fig. S10). Such an
arrangement has also been described recently
for a dimeric tau fibril structure ( 17).
A single Ab(1– 42) subunit forms an LS-shaped
structure, in which the N terminus is L-shaped
and the C terminus is S-shaped (Fig. 1D). The C
terminus (Fig. 2 and fig. S11, A and B) roughly
resembles structures of a different polymorph of
Ab(1– 42) determined recently by solid-state NMR
( 11, 13, 14) alone (fig. S12 and tables S4 to S6),
whereas the dimer interface is completely different
(discussed below). In contrast to those NMR
structures, the current structure shows the N-terminal part of Ab(1– 42) to be fully visible and
part of the cross-b structure of the fibril. Secondary chemical shifts from our NMR experiments
and the corresponding secondary structure calculation correlate well with the EM structure (Fig.
3B). Although we could not assign the long-range
contacts unambiguously, all NMR cross peaks,
which are not due to sequential contacts, are in
agreement with the cryo-EM structure (figs. S6
116 6 OCTOBER 2017 • VOL 358 ISSUE 6359 sciencemag.org SCIENCE
1Institute of Complex Systems, Structural Biochemistry (ICS- 6),
Forschungszentrum Jülich, 52425 Jülich, Germany. 2Institut für
Physikalische Biologie, Heinrich-Heine-Universität Düsseldorf,
40225 Düsseldorf, Germany. 3Centre for Structural Systems
Biology (CSSB), Deutsches Elektronen-Synchrotron (DESY),
22607 Hamburg, Germany. 4The Maastricht Multimodal Molecular
Imaging Institute, Maastricht University, Universiteitssingel 50,
6229 ER Maastricht, Netherlands. 5Physics Department, Heinrich-Heine-Universität Düsseldorf, 40225 Düsseldorf, Germany.
*Corresponding author. Email: email@example.com
(G.F.S.); firstname.lastname@example.org (D. W.)