Spongiform encephalopathies are believed to be transmitted by self-perpetuating conformational conversion of the prion protein. It was shown recently that fundamental aspects of mammalian prion propagation can be reproduced in vitro in a seeded fibrillization of the recombinant prion protein variant Y145Stop (PrP23-144). Here we demonstrate that PrP23-144 amyloids from different species adopt distinct secondary structures and morphologies, and that these structural differences are controlled by one or two residues in a critical region. These sequence-specific structural characteristics correlate strictly with the seeding specificity of amyloid fibrils. However, cross-seeding of PrP23-144 from one species with preformed fibrils from another species may overcome natural sequence-based structural preferences, resulting in a new amyloid strain that inherits the secondary structure and morphology of the template. These data provide direct biophysical evidence that protein conformations are transmitted in PrP amyloid strains, establishing a foundation for a structural basis of mammalian prion transmission barriers.

A total of 57 parabolic‐shaped and 9 approximately circular extended, impact crater related features have been found in Magellan synthetic aperture radar (SAR) and thermal emissivity data covering 92% of the surface of Venus. The parabolic features are, with seven exceptions, oriented E‐W with the apex to the east and the impact crater located just west of the apex. They were first identified in the surface emissivity data derived from Magellan radiometry measurements, but the great majority are only clearly visible in the SAR imagery. The overall sizes of both the parabolic and circular features range from several hundred to about two thousand kilometers and are loosely correlated with the diameters of the “parent” craters. The floors of almost all these craters have high specific radar backscatter cross sections (i.e., they are bright in the SAR imagery) relative to their surroundings and tend to have low emissivities. Approximately one‐third of the impact craters with diameters ≥15 km appear to have bright floors and about half of these have an associated parabolic feature which can be observed in the SAR or emissivity data. No features have been found which overlie the parabolic features, indicating that they are among the youngest features on the surface of the planet. This suggests that radar‐bright floors characterize the freshest impact craters and that modification processes subsequently darken their radar signature. A model for the formation of the parabolic features is developed based on the injection of small particles into the upper atmosphere at the time of impact and their transport to the west by the E‐W zonal winds. Fitting of a small perturbation scattering model to the measured average scattering law for the parabolic features placed an upper limit of about 0.6 cm on the wavelength scale (12.6 cm) surface roughness and, hence, of 1 to 2 cm on the largest particle sizes of interest. Fallout times from 50 km in the Venus atmosphere for particles of this size are about 2 hours, allowing westerly drifts of several hundred kilometers for zonal winds of 50 to 100 m s−1. Measurements of the change in backscatter cross section of features overlaid by these extended ejecta deposits, are consistent with deposit depths of a few centimeters to 1 or 2m.

SUMMARY Mutations that lead to splicing defects can have severe consequences on gene function and cause disease. Here, we explore how human genetic variation affects exon recognition by developing a multiplexed functional assay of splicing using Sort-seq (MFASS). We assayed 27,733 variants in the Exome Aggregation Consortium (ExAC) within or adjacent to 2,198 human exons in the MFASS minigene reporter and found that 3.8% (1,050) of variants, most of which are extremely rare, led to large-effect splice-disrupting variants (SDVs). Importantly, we find that 83% of SDVs are located outside of canonical splice sites, are distributed evenly across distinct exonic and intronic regions, and are difficult to predict a priori. Our results indicate extant, rare genetic variants can have large functional effects on splicing at appreciable rates, even outside the context of disease, and MFASS enables their empirical assessment at scale.

One of the most intriguing disease-related mutations in human prion protein (PrP) is the Tyr to Stop codon substitution at position 145. This mutation results in a Gerstmann-Straussler-Scheinkerlike disease with extensive PrP amyloid deposits in the brain. Here, we provide evidence for a spontaneous conversion of the recombinant polypeptide corresponding to the Y145Stop variant (huPrP23-144) from a monomeric unordered state to a fibrillar form. This conversion is characterized by a protein concentration-dependent lag phase and has characteristics of a nucleation-dependent polymerization. Atomic force microscopy shows that huPrP23-144 fibrils are characterized by an apparent periodicity along the long axis, with an average period of 20 nm. Fourier-transform infrared spectra indicate that the conversion is associated with formation of ␤-sheet structure. However, the infrared bands for huPrP23-144 are quite different from those for a synthetic peptide PrP106 -126, suggesting conformational non-equivalence of ␤-structures in the disease-associated Y145Stop variant and a frequently used short model peptide. To identify the region that is critical for the self-seeded assembly of huPrP23-144 amyloid, experiments were performed by using the recombinant polypeptides corresponding to prion protein fragments 23-114, 23-124, 23-134, 23-137, 23-139, and 23-141. Importantly, none of the fragments ending before residue 139 showed a propensity for conformational conversion to amyloid fibrils, indicating that residues within the 138 -141 region are essential for this conversion.

In humans, the >800 G protein-coupled receptors (GPCRs) are responsible for transducing diverse chemical stimuli to alter cell state, and are the largest class of drug targets. Their myriad structural conformations and various modes of signaling make it challenging to understand their structure and function. Here we developed a platform to characterize large libraries of GPCR variants in human cell lines with a barcoded transcriptional reporter of G-protein signal transduction. We tested 7,800 of 7,828 possible single amino acid substitutions to the beta-2 adrenergic receptor (β2AR) at four concentrations of the agonist isoproterenol. We identified residues specifically important for β2AR signaling, mutations in the human population that are potentially loss of function, and residues that modulate basal activity. Using unsupervised learning, we resolve residues critical for signaling, including all major structural motifs and molecular interfaces. We also find a previously uncharacterized structural latch spanning the first two extracellular loops that is highly conserved across Class A GPCRs and is conformationally rigid in both the inactive and active states of the receptor. More broadly, by linking deep mutational scanning with engineered transcriptional reporters, we establish a generalizable method for exploring pharmacogenomics, structure and function across broad classes of drug receptors.