Key questions regarding the molecular nature of prions are how different prion strains can be propagated by the same protein and whether they are only protein. Here we demonstrate the protein-only nature of prion strains in a yeast model, the [PSI] genetic element that enhances the read-through of nonsense mutations in the yeast Saccharomyces cerevisiae. Infectious fibrous aggregates containing a Sup35 prion-determining amino-terminal fragment labelled with green fluorescent protein were purified from yeast harbouring distinctive prion strains. Using the infectious aggregates as 'seeds', elongated fibres were generated in vitro from the bacterially expressed labelled prion protein. De novo generation of strain-specific [PSI] infectivity was demonstrated by introducing sheared fibres into uninfected yeast hosts. The cross-sectional morphology of the elongated fibres generated in vitro was indistinguishable from that of the short yeast seeds, as visualized by electron microscopy. Electron diffraction of the long fibres showed the 4.7 A spacing characteristic of the cross-beta structure of amyloids. The fact that the amyloid fibres nucleated in vitro propagate the strain-specific infectivity of the yeast seeds implies that the heritable information of distinct prion strains must be encoded by different, self-propagating cross-beta folding patterns of the same prion protein.

SUMMARY The physiology of N-Methyl-D-aspartate (NMDA) receptors in mammals is fundamental to brain development and function. NMDA receptors are ionotropic glutamate receptors that function as heterotetramers composed mainly of GluN1 and GluN2 subunits. Activation of NMDA receptors requires binding of neurotransmitter agonists to a ligand-binding domain (LBD) and structural rearrangement of an amino terminal domain (ATD). Recent crystal structures of GluN1/GluN2B NMDA receptors in the presence of agonists and an allosteric inhibitor, ifenprodil, represent the allosterically inhibited state. However, how the ATD and LBD move to activate the NMDA receptor ion channel remains unclear. Here, we combine x-ray crystallography, single-particle electron cryomicroscopy, and electrophysiology to show that, in the absence of ifenprodil, the bi-lobed structure of GluN2 ATD adopts an open-conformation accompanied by rearrangement of the GluN1-GluN2 ATD heterodimeric interface, altering subunit orientation in the ATD and LBD and forming an active receptor conformation that gates the ion channel.

Integration of EM, protein–protein interaction, and phenotypic data reveals novel insights into the structure and function of the nuclear pore complex’s ∼600-kD heptameric Nup84 complex.

Background:Fascin is the main actin-bundling protein in filopodia. Results: Biochemical, cryo-electron tomographic, and x-ray crystal structural data reveal the unique actin-binding characteristics of fascin. Conclusion: There are two major actin-binding sites on fascin and there is a concerted conformational change between the actin-binding sites. Significance: These data will advance our understanding of the function of fascin in filopodial formation.