Gian Gaetano Tartaglia

Amyloid-like Aggregates Sequester Numerous Metastable Proteins with Essential Cellular Functions

Protein aggregation is linked with neurodegeneration and numerous other diseases by mechanisms that are not well understood. Here, we have analyzed the gain-of-function toxicity of artificial β sheet proteins that were designed to form amyloid-like fibrils. Using quantitative proteomics, we found that the toxicity of these proteins in human cells correlates with the capacity of their aggregates to promote aberrant protein interactions and to deregulate the cytosolic stress response. The endogenous proteins that are sequestered by the aggregates share distinct physicochemical properties: They are relatively large in size and significantly enriched in predicted unstructured regions, features that are strongly linked with multifunctionality. Many of the interacting proteins occupy essential hub positions in cellular protein networks, with key roles in chromatin organization, transcription, translation, maintenance of cell architecture and protein quality control. We suggest that amyloidogenic aggregation targets a metastable subproteome, thereby causing multifactorial toxicity and, eventually, the collapse of essential cellular functions.

Prediction of Aggregation-Prone Regions in Structured Proteins

We present a method for predicting the regions of the sequences of peptides and proteins that are most important in promoting their aggregation and amyloid formation. The method extends previous approaches by allowing such predictions to be carried out for conditions under which the molecules concerned can be folded or contain a significant degree of persistent structure. In order to achieve this result, the method uses only knowledge of the sequence of amino acids to estimate simultaneously both the propensity for folding and aggregation and the way in which these two types of propensity compete. We illustrate the approach by its application to a set of peptides and proteins both associated and not associated with disease. Our results show not only that the regions of a protein with a high intrinsic aggregation propensity can be identified in a robust manner but also that the structural context of such regions in the monomeric form is crucial for determining their actual role in the aggregation process.

Metastability of native proteins and the phenomenon of amyloid formation.

An experimental determination of the thermodynamic stabilities of a series of amyloid fibrils reveals that this structural form is likely to be the most stable one that protein molecules can adopt even under physiological conditions. This result challenges the conventional assumption that functional forms of proteins correspond to the global minima in their free energy surfaces and suggests that living systems are conformationally as well as chemically metastable.

ALS/FTD Mutation-Induced Phase Transition of FUS Liquid Droplets and Reversible Hydrogels into Irreversible Hydrogels Impairs RNP Granule Function

Summary The mechanisms by which mutations in FUS and other RNA binding proteins cause ALS and FTD remain controversial. We propose a model in which low-complexity (LC) domains of FUS drive its physiologically reversible assembly into membrane-free, liquid droplet and hydrogel-like structures. ALS/FTD mutations in LC or non-LC domains induce further phase transition into poorly soluble fibrillar hydrogels distinct from conventional amyloids. These assemblies are necessary and sufficient for neurotoxicity in a C. elegans model of FUS-dependent neurodegeneration. They trap other ribonucleoprotein (RNP) granule components and disrupt RNP granule function. One consequence is impairment of new protein synthesis by cytoplasmic RNP granules in axon terminals, where RNP granules regulate local RNA metabolism and translation. Nuclear FUS granules may be similarly affected. Inhibiting formation of these fibrillar hydrogel assemblies mitigates neurotoxicity and suggests a potential therapeutic strategy that may also be applicable to ALS/FTD associated with mutations in other RNA binding proteins.

DnaK Functions as a Central Hub in the E. coli Chaperone Network

Cellular chaperone networks prevent potentially toxic protein aggregation and ensure proteome integrity. Here, we used Escherichia coli as a model to understand the organization of these networks, focusing on the cooperation of the DnaK system with the upstream chaperone Trigger factor (TF) and the downstream GroEL. Quantitative proteomics revealed that DnaK interacts with at least ~700 mostly cytosolic proteins, including ~180 relatively aggregation-prone proteins that utilize DnaK extensively during and after initial folding. Upon deletion of TF, DnaK interacts increasingly with ribosomal and other small, basic proteins, while its association with large multidomain proteins is reduced. DnaK also functions prominently in stabilizing proteins for subsequent folding by GroEL. These proteins accumulate on DnaK upon GroEL depletion and are then degraded, thus defining DnaK as a central organizer of the chaperone network. Combined loss of DnaK and TF causes proteostasis collapse with disruption of GroEL function, defective ribosomal biogenesis, and extensive aggregation of large proteins.

Systematic In Vivo Analysis of the Intrinsic Determinants of Amyloid β Pathogenicity

Protein aggregation into amyloid fibrils and protofibrillar aggregates is associated with a number of the most common neurodegenerative diseases. We have established, using a computational approach, that knowledge of the primary sequences of proteins is sufficient to predict their in vitro aggregation propensities. Here we demonstrate, using rational mutagenesis of the Aβ42 peptide based on such computational predictions of aggregation propensity, the existence of a strong correlation between the propensity of Aβ42 to form protofibrils and its effect on neuronal dysfunction and degeneration in a Drosophila model of Alzheimer disease. Our findings provide a quantitative description of the molecular basis for the pathogenicity of Aβ and link directly and systematically the intrinsic properties of biomolecules, predicted in silico and confirmed in vitro, to pathogenic events taking place in a living organism.

Prediction of aggregation rate and aggregation-prone segments in polypeptide sequences

The reliable identification of β‐aggregating stretches in protein sequences is essential for the development of therapeutic agents for Alzheimers and Parkinsons diseases, as well as other pathological conditions associated with protein deposition. Here, a model based on physicochemical properties and computational design of β‐aggregating peptide sequences is shown to be able to predict the aggregation rate over a large set of natural polypeptide sequences. Furthermore, the model identifies aggregation‐prone fragments within proteins and predicts the parallel or anti‐parallel β‐sheet organization in fibrils. The model recognizes different β‐aggregating segments in mammalian and nonmammalian prion proteins, providing insights into the species barrier for the transmission of the prion disease.

Predicting protein associations with long noncoding RNAs

Supplementary Figure 4 Predictions of interactions between PRC2 protein components and HOTAIR regions Supplementary Table 1 PDB IDs of non-redundant protein-RNA complexes used to train catRAPID Supplementary Table 2 Coefficients associated with protein and RNA properties Supplementary Table 3 Parameters of the interaction matrix I Supplementary Table 4 Composition of the NPInter dataset Supplementary Table 5 Composition of the Protein-binding (Protein BP), DNAbinding, RNA-binding (RNA BP) datasets Supplementary Table 6 Human MRP complex: Comparison between catRAPID predictions and experimental data Supplementary Methods

The role of aromaticity, exposed surface, and dipole moment in determining protein aggregation rates

The mechanisms by which peptides and proteins form ordered aggregates are not well understood. Here we focus on the physicochemical properties of amino acids that favor ordered aggregation and suggest a parameter‐free model that is able to predict the change of aggregation rates over a large set of natural sequences. Furthermore, the results of the parameter‐free model correlate well with the aggregation propensities of a set of peptides designed by computer simulations.

Physicochemical principles that regulate the competition between functional and dysfunctional association of proteins

To maintain protein homeostasis, a variety of quality control mechanisms, such as the unfolded protein response and the heat shock response, enable proteins to fold and to assemble into functional complexes while avoiding the formation of aberrant and potentially harmful aggregates. We show here that a complementary contribution to the regulation of the interactions between proteins is provided by the physicochemical properties of their amino acid sequences. The results of a systematic analysis of the protein–protein complexes in the Protein Data Bank (PDB) show that interface regions are more prone to aggregate than other surface regions, indicating that many of the interactions that promote the formation of functional complexes, including hydrophobic and electrostatic forces, can potentially also cause abnormal intermolecular association. We also show, however, that aggregation-prone interfaces are prevented from triggering uncontrolled assembly by being stabilized into their functional conformations by disulfide bonds and salt bridges. These results indicate that functional and dysfunctional association of proteins are promoted by similar forces but also that they are closely regulated by the presence of specific interactions that stabilize native states.