PMC:2726924 / 6253-7953 JSONTXT

{"project":"2_test","denotations":[{"id":"19345691-16095615-62520772","span":{"begin":342,"end":344},"obj":"16095615"},{"id":"19345691-12093917-62520772","span":{"begin":342,"end":344},"obj":"12093917"},{"id":"19345691-11125866-62520773","span":{"begin":541,"end":543},"obj":"11125866"},{"id":"19345691-10022824-62520773","span":{"begin":541,"end":543},"obj":"10022824"},{"id":"19345691-18483195-62520773","span":{"begin":541,"end":543},"obj":"18483195"}],"text":"Imaging and classification of amyloid fibril segments\nImages of β2m amyloid fibrils formed at pH 2.5, acquired by negative staining and cryo-EM, are shown in Fig. 1a. The images revealed long, twisted fibrils with a diameter of around 20 nm and a characteristic pattern of cross-over repeats. As found previously for other amyloid fibrils,18,32 β2m fibrils display a marked heterogeneity in repeat length (120–185 nm). To sort the fibrils into more uniform structural classes, each cross-over repeat unit was treated as a single particle.33–35 This approach revealed two main fibril morphologies, termed here as types A and B. Type A fibrils have polarity, as shown by the arrowhead features in the class average view in Fig. 1b, whereas type B fibrils are bipolar (Fig. 1c). Notably, hints of a globular subunit repeat can be seen in the class averages (Fig. 1b and c), which were determined by reference-free alignment and classification, with no repeat information imposed. This subunit repeat is clearly seen in raw negative stain images, whose power spectra show strong layer lines at spacings in the range of 5.2–6.5 nm, consistent with a subunit repeat of this size (Fig. 1d and e). A small population of fibrils with half the thickness of the type A and type B fibrils (10 nm) was also observed, termed type C (Fig. 2a). This finding, together with the observation that the thicker fibrils were occasionally split into two (data not shown), suggests that the fibrils are assembled from independent halves. Fibrils assembled at pH 7 showed morphologies similar to those generated at low pH, but were associated into bundles, precluding their further structural analysis (Supplementary Fig. 1)."}