Dimers of Aβ (amyloid β-protein) are believed to play an important role in Alzheimer's disease. In the absence of sufficient brain-derived dimers, we studied one of the only possible dimers that could be produced in vivo, [Aβ](DiY) (dityrosine cross-linked Aβ). For comparison, we used the Aβ monomer and a design dimer cross-linked by replacement of Ser²⁶ with cystine [AβS26C]₂. We showed that similar to monomers, unaggregated dimers lack appreciable structure and fail to alter long-term potentiation. Importantly, dimers exhibit subtly different structural propensities from monomers and each other, and can self-associate to form larger assemblies. Although [Aβ](DiY) and [AβS26C]₂ have distinct aggregation pathways, they both populate bioactive soluble assemblies for longer durations than Aβ monomers. Our results indicate that the link between Aβ dimers and Alzheimer's disease results from the ability of dimers to further assemble and form synaptotoxic assemblies that persist for long periods of time.

Spider silks are among the toughest known materials and thus provide models for renewable, biodegradable, and sustainable biopolymers. However, the entirety of their diversity still remains elusive, and silks that exceed the performance limits of industrial fibers are constantly being found. We obtained transcriptome assemblies from 1098 species of spiders to comprehensively catalog silk gene sequences and measured the mechanical, thermal, structural, and hydration properties of the dragline silks of 446 species. The combination of these silk protein genotype-phenotype data revealed essential contributions of multicomponent structures with major ampullate spidroin 1 to 3 paralogs in high-performance dragline silks and numerous amino acid motifs contributing to each of the measured properties. We hope that our global sampling, comprehensive testing, integrated analysis, and open data will provide a solid starting point for future biomaterial designs.

The β-sheet is the key structure underlying the excellent mechanical properties of spider silk. However, the comprehensive mechanism underlying β-sheet formation from soluble silk proteins during the transition into insoluble stable fibers has not been elucidated. Notably, the assembly of repetitive domains that dominate the length of the protein chains and structural features within the spun fibers has not been clarified. Here we determine the conformation and dynamics of the soluble precursor of the repetitive domain of spider silk using solution-state NMR, far-UV circular dichroism and vibrational circular dichroism. The soluble repetitive domain contains two major populations: ~65% random coil and ~24% polyproline type II helix (PPII helix). The PPII helix conformation in the glycine-rich region is proposed as a soluble prefibrillar region that subsequently undergoes intramolecular interactions. These findings unravel the mechanism underlying the initial step of β-sheet formation, which is an extremely rapid process during spider silk assembly.

The tiny spider makes dragline silk fibers with unbeatable toughness, all under the most innocuous conditions. Scientists have persistently tried to emulate its natural silk spinning process using recombinant proteins with a view toward creating a new wave of smart materials, yet most efforts have fallen short of attaining the native fiber’s excellent mechanical properties. One reason for these shortcomings may be that artificial spider silk systems tend to be overly simplified and may not sufficiently take into account the true complexity of the underlying protein sequences and of the multidimensional aspects of the natural self-assembly process that give rise to the hierarchically structured fibers. Here, we discuss recent findings regarding the material constituents of spider dragline silk, including novel spidroin subtypes, nonspidroin proteins, and possible involvement of post-translational modifications, which together suggest a complexity that transcends the two-component MaSp1/MaSp2 system. We subsequently consider insights into the spidroin domain functions, structures, and overall mechanisms for the rapid transition from disordered soluble protein into a highly organized fiber, including the possibility of viewing spider silk self-assembly through a framework relevant to biomolecular condensates. Finally, we consider the concept of “biomimetics” as it applies to artificial spider silk production with a focus on key practical aspects of design and evaluation that may hopefully inform efforts to more closely reproduce the remarkable structure and function of the native silk fiber using artificial methods.

Lactococcus lactis is a promising host for (membrane) protein overproduction. Here, we describe a protocol for incorporation of selenomethionine (SeMet) into proteins expressed in L. lactis. Incorporation efficiencies of SeMet in the membrane protein complex OpuA (an ABC transporter) and the soluble protein OppA, both from L. lactis, were monitored by mass spectrometry. Both proteins incorporated SeMet with high efficiencies (>90%), which greatly extends the usefulness of the expression host L. lactis for X-ray crystallography purposes. The crystal structure of ligand-free OppA was determined at 2.4 Å resolution by a semiautomatic approach using selenium single-wavelength anomalous diffraction phasing.