A conformational isoform of the mammalian prion protein (PrP Sc ) is the sole component of the infectious pathogen that causes the prion diseases. We have obtained X-ray fiber diffraction patterns from infectious prions that show cross-␤ diffraction: meridional intensity at 4.8 Å resolution, indicating the presence of ␤ strands running approximately at right angles to the filament axis and characteristic of amyloid structure. Some of the patterns also indicated the presence of a repeating unit along the fiber axis, corresponding to four ␤-strands. We found that recombinant (rec) PrP amyloid differs substantially from highly infectious brainderived prions, both in structure as demonstrated by the diffraction data, and in heterogeneity as shown by electron microscopy. In addition to the strong 4.8 Å meridional reflection, the recPrP amyloid diffraction is characterized by strong equatorial intensity at approximately 10.5 Å, absent from brain-derived prions, and indicating the presence of stacked ␤-sheets. Synthetic prions recovered from transgenic mice inoculated with recPrP amyloid displayed structural characteristics and homogeneity similar to those of naturally occurring prions. The relationship between the structural differences and prion infectivity is uncertain, but might be explained by any of several hypotheses: only a minority of recPrP amyloid possesses a replication-competent conformation, the majority of recPrP amyloid has to undergo a conformational maturation to acquire replication competency, or inhibitory forms of recPrP amyloid interfere with replication during the initial transmission.amyloid ͉ protein ͉ neurodegeneration ͉ PrP ͉ ␤-helix

Flexible filamentous viruses make up a large fraction of the known plant viruses, but in comparison with those of other viruses, very little is known about their structures. We have used fiber diffraction, cryo-electron microscopy, and scanning transmission electron microscopy to determine the symmetry of a potyvirus, soybean mosaic virus; to confirm the symmetry of a potexvirus, potato virus X; and to determine the low-resolution structures of both viruses. We conclude that these viruses and, by implication, most or all flexible filamentous plant viruses share a common coat protein fold and helical symmetry, with slightly less than 9 subunits per helical turn.Flexible filamentous plant viruses include at least 19 recognized genera (22), almost all in three families of singlestranded, positive-sense RNA viruses, the Potyviridae, the Flexiviridae, and the Closteroviridae. Members of the family Potyviridae account for almost a third of the total known plant virus species (22) and are responsible for more than half the viral crop damage in the world (37), infecting most economically important crops (32). Members of the family Flexiviridae (2), and particularly of the large genus Potexvirus, are also of considerable significance to agriculture (42). Both families show great potential for biotechnological applications, including protein expression and vaccine production (12, 54). Despite their importance, however, little is known about the structures of any of the flexible filamentous plant viruses, in sharp contrast to the amount of data on the rigid tobamoviruses (48,63) or the icosahedral plant viruses (15); flexibility, instability, and in many cases low levels of expression have made these viruses particularly intractable to structural studies. Structural and evolutionary relationships among the flexible filamentous plant viruses have been suggested (18,47,56,60), but there is very little sequence homology between the coat proteins of viruses in the different families, and there has hitherto been no structural support for such relationships at the level of either viral symmetry or coat protein folding. Indeed, reports of viral symmetry until now appeared to contradict hypotheses of evolutionary relationships.Soybean mosaic virus (SMV) is a potyvirus, that is, a member of the genus Potyvirus, the largest genus in the family Potyviridae (3). SMV is a major pathogen of soybeans, transmitted efficiently through seed and by aphids in a nonpersistent manner; yield losses as high as 35% have been reported (30). Despite dramatic morphological differences, members of the family resemble the icosahedral plant comoviruses and animal picornaviruses in genomic organization and replication strategy (32). Early electron microscopic observations found the potyviruses to be about 7,500 Å long and 120 Å in diameter, with helical pitches of about 34 Å (44, 60). A fiber diffraction study (51) of the tritimovirus wheat streak mosaic virus (WSMV) suggested that WSMV has 6.9 subunits per turn of the viral helix, but there was consider...

Amyloid β protein (Aβ), the principal component of the extracellular plaques found in the brains of Alzheimer’s disease patients, forms fibrils well suited to structural study by X-ray fiber diffraction. Fiber diffraction patterns from the 40-residue form Aβ(1–40) confirm a number of features of a three-fold symmetric Aβ model from solid state NMR, but suggest that the fibrils have a hollow core, not present in the original ssNMR models. Diffraction patterns calculated from a revised hollow three-fold model with a more regular β-sheet structure are in much better agreement with the observed diffraction data than patterns calculated from the original ssNMR model. Refinement of a hollow-core model against ssNMR data led to a revised ssNMR model, similar to the fiber diffraction model.

Background: Prions are aberrantly folded infectious proteins whose biological activity is determined by their distinct folds. Results: The infectious fold of the fungal prion-forming domain HET-s(218 -289) can be nucleated by a noninfectious polymorph. Conclusion: Generic amyloid architectures can seed the formation of infectious prions. Significance: Heterogeneous seeding may be a mechanism of prion strain adaptation and interspecies transmission.

Prions are proteins that adopt self-propagating aberrant folds. The self-propagating properties of prions are a direct consequence of their distinct structures, making the understanding of these structures and their biophysical interactions fundamental to understanding prions and their related diseases. The insolubility and inherent disorder of prions have made their structures difficult to study, particularly in the case of the infectious form of the mammalian prion protein PrP. Many investigators have therefore preferred to work with peptide fragments of PrP, suggesting that these peptides might serve as structural and functional models for biologically active prions. We have used x-ray fiber diffraction to compare a series of different-sized fragments of PrP, to determine the structural commonalities among the fragments and the biologically active, self-propagating prions. Although all of the peptides studied adopted amyloid conformations, only the larger fragments demonstrated a degree of structural complexity approaching that of PrP. Even these larger fragments did not adopt the prion structure itself with detailed fidelity, and in some cases their structures were radically different from that of pathogenic PrP(Sc).