Amyloids are implicated in neurodegenerative diseases. Fibrillar aggregates of the amyloid-β protein (Aβ) are the main component of the senile plaques found in brains of Alzheimer's disease patients. We present the structure of an Aβ(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-β 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.

The genes of N. pharaonis SRII and the carboxy terminal truncated transducer (1-114) were cloned into a pET27bmod expression vector 24 with a C-terminal £ 7 His tag, respectively. Proteins were expressed in Escherichia coli strain BL21 (DE3), and purified as described 25,26. After removal of imidazol by diethyl-aminoethyl chromatography, SRII-His and HtrII 114-His were mixed in a 1:1 ratio, followed by reconstitution into purple membrane (the bacteriorhodopsin containing membrane patches of H. salinarum) lipids 7 (protein to lipid ratio 1:35). After filtration, the reconstituted proteins were pelleted by centrifugation at 100,000g. For resolubilization, the samples were resuspended in a buffer containing 2% n-octyl-b-D-glucopyranoside and shaken for 16 h at 4 8C in the dark. The resolubilized complex was isolated by centrifugation at 100,000g. Crystallization, structure determination and refinement We added the solubilized complex in crystallization buffer (150 mM NaCl, 25 mM Na/KPi, pH 5.1, 0.8% n-octyl-b-D-glucopyranoside) to the lipidic phase, formed from monovaccenin (Nu-Chek Prep). Precipitant was 1 M salt Na/KPi, pH 5.6. Crystals were grown at 22 8C. X-ray diffraction data were collected at beamline ID14-1 of the European Synchrotron Radiation Facility (ESRF), Grenoble, France, using a Quantum ADSC Q4R CCD (charge-coupled device) detector. Data were integrated using MOSFILM 27 and SCALA 28. Molecular replacement using MOLREP 28 to phase a polyalanine model (from Protein Data Bank accession number 1JGJ (ref. 12)) gave a unique solution (R ¼ 0.568, correlation coefficient C ¼ 0.357) at 2.9 A ˚. After inserting side chains for SRII, the helices of HtrII were found (R ¼ 0.329, C ¼ 0.711). Simulated annealing, positional refinement and temperature factor refinement were performed in CNS 29 ; model rebuilding was carried out in O 30 (Table 1).