Structural Insight into an Alzheimer’s Brain-Derived Spherical Assembly of Amyloid β by Solid-State NMR

Accumulating evidence suggests that various neuro­degenerative diseases, including Alzheimer’s disease (AD), are linked to cytotoxic diffusible aggregates of amyloid proteins, which are metastable intermediate species in protein misfolding. This study presents the first site-specific structural study on an intermediate called amylo­spheroid (ASPD), an AD-derived neurotoxin composed of oligomeric amyloid-β (Aβ). Electron microscopy and immunological analyses using ASPD-specific “conformational” antibodies established synthetic ASPD for the 42-residue Aβ(1–42) as an excellent structural/morphological analogue of native ASPD extracted from AD patients, the level of which correlates with the severity of AD. ¹³C solid-state NMR analyses of approximately 20 residues and interstrand distances demonstrated that the synthetic ASPD is made of a homogeneous single conformer containing parallel β-sheets. These results provide profound insight into the native ASPD, indicating that Aβ is likely to self-assemble into the toxic intermediate with β-sheet structures in AD brains. This approach can be applied to various intermediates relevant to amyloid diseases

A Physicochemical and Mutational Analysis of Intersubunit Interactions of <i>Escherichia coli</i> Ferritin A

Ferritin A from <i>Escherichia coli</i> (EcFtnA) is 24-meric protein, which forms spherical cagelike structures called nanocages. The nanocage structure is stabilized by the interface around 4-, 3-, and 2-fold symmetric axes. The subunit structure of EcFtnA comprises a four-helix bundle (helices A–D) and an additional helix E, which forms a 4-fold axis. In this study, we examined the contribution of the interface around three symmetric axes. pH-induced dissociation experiments monitored by analytical ultracentrifugation and small-angle X-ray scattering showed that the dimer related by 2-fold symmetry is the most stable unit. Mutations located near the 3-fold axis revealed that the contribution of each interaction was small. A mutant lacking helix E at the 4-fold axis formed a nanocage, suggesting that helix E is not essential for nanocage formation. Further truncation of the C-terminus of helix D abrogated the formation of the nanocage, suggesting that a few residues located at the C-terminus of helix D are critical for this process. These properties are similar to those known for mammalian ferritins and seem to be common principles for nanocage formation. The difference between EcFtnA and mammalian ferritins was that helix E-truncated EcFtnA maintained an iron-incorporating ability, whereas mammalian mutants lost it