The structure of N184K amyloidogenic variant of gelsolin highlights the role of the H-bond network for protein stability and aggregation properties

Accurate and Efficient SAXS/SANS Implementation Including Solvation Layer Effects Suitable for Molecular Simulations

Small-angle X-ray and neutron scattering (SAXS/SANS) provide valuable insights into the structure and dynamics of biomolecules in solution, complementing a wide range of structural techniques, including molecular dynamics simulations. As contrast-based methods, they are sensitive not only to structural properties but also to solvent-solute interactions. Their use in molecular dynamics simulations requires a forward model that should be as fast and accurate as possible. In this work, we demonstrate the feasibility of calculating SAXS and SANS intensities using a coarse-grained representation consisting of one bead per amino acid and three beads per nucleic acid, with form factors that can be corrected on the fly to account for solvation effects at no additional computational cost. By coupling this forward model with molecular dynamics simulations restrained with SAS data, it is possible to determine conformational ensembles or refine the structure and dynamics of proteins and nucleic acids in agreement with the experimental results. To assess the robustness of this approach, we applied it to gelsolin, for which we acquired SAXS data on its closed state, and to a UP1-microRNA complex, for which we used previously collected measurements. Our hybrid-resolution small-angle scattering (hySAS) implementation, being distributed in PLUMED, can be used with atomistic and coarse-grained simulations using diverse restraining strategies.

The use of pharmacological chaperones in rare diseases caused by reduced protein stability

Rare diseases are most often caused by inherited genetic disorders that, after translation, will result in a protein with altered function. Decreased protein stability is the most frequent mechanism associated with a congenital pathogenic missense mutation and it implies the destabilization of the folded conformation in favour of unfolded or misfolded states. In the cellular context and when experimental data is available, a mutant protein with altered thermodynamic stability often also results in impaired homeostasis, with the deleterious accumulation of protein aggregates, metabolites and/or metabolic by-products. In the last decades, a significant effort has enabled the characterization of rare diseases associated to protein stability defects and triggered the development of innovative therapeutic intervention lines, say, the use of pharmacological chaperones to correct the intracellular impaired homeostasis. Here, we review the current knowledge on rare diseases caused by reduced protein stability, paying special attention to the thermodynamic aspects of the protein destabilization, also focusing on some examples where pharmacological chaperones are being tested.

Investigating the Sequence Determinants of the Curling of Amyloid Fibrils Using Ovalbumin as a Case Study

Highly ordered, straight amyloid fibrils readily lend themselves to structure determination techniques and have therefore been extensively characterized. However, the less ordered curly fibrils remain relatively understudied, and the structural organization underlying their specific characteristics remains poorly understood. We found that the exemplary curly fibril-forming protein ovalbumin contains multiple aggregation prone regions (APRs) that form straight fibrils when isolated as peptides or when excised from the full-length protein through hydrolysis. In the context of the intact full-length protein, however, the regions separating the APRs facilitate curly fibril formation. In fact, a meta-analysis of previously reported curly fibril-forming proteins shows that their inter-APRs are significantly longer and more hydrophobic when compared to straight fibril-forming proteins, suggesting that they may cause strain in the amyloid state. Hence, inter-APRs driving curly fibril formation may not only apply to our model protein but rather constitute a more general mechanism.

A novel hotspot of gelsolin instability triggers an alternative mechanism of amyloid aggregation

Gelsolin comprises six homologous domains, named G1 to G6. Single point substitutions in this protein are responsible for AGel amyloidosis, a hereditary disease causing progressive corneal lattice dystrophy, cutis laxa, and polyneuropathy. Although several different amyloidogenic variants of gelsolin have been identified, only the most common mutants present in the G2 domain have been thoroughly characterized, leading to clarification of the functional mechanism. The molecular events underlying the pathological aggregation of 3 recently identified mutations, namely A551P, E553K and M517R, all localized at the interface between G4 and G5, are here explored for the first time. Structural studies point to destabilization of the interface between G4 and G5 due to three structural determinants: β-strand breaking, steric hindrance and/or charge repulsion, all implying impairment of interdomain contacts. Such rearrangements decrease the temperature and pressure stability of gelsolin but do not alter its susceptibility to furin cleavage, the first event in the canonical aggregation pathway. These variants also have a greater tendency to aggregate in the unproteolysed forms and exhibit higher proteotoxicity in a C. elegans-based assay. Our data suggest that aggregation of G4G5 variants follows an alternative, likely proteolysis-independent, pathway.