Virginia Castillo

Amyloidogenic Regions and Interaction Surfaces Overlap in Globular Proteins Related to Conformational Diseases

Protein aggregation underlies a wide range of human disorders. The polypeptides involved in these pathologies might be intrinsically unstructured or display a defined 3D-structure. Little is known about how globular proteins aggregate into toxic assemblies under physiological conditions, where they display an initially folded conformation. Protein aggregation is, however, always initiated by the establishment of anomalous protein-protein interactions. Therefore, in the present work, we have explored the extent to which protein interaction surfaces and aggregation-prone regions overlap in globular proteins associated with conformational diseases. Computational analysis of the native complexes formed by these proteins shows that aggregation-prone regions do frequently overlap with protein interfaces. The spatial coincidence of interaction sites and aggregating regions suggests that the formation of functional complexes and the aggregation of their individual subunits might compete in the cell. Accordingly, single mutations affecting complex interface or stability usually result in the formation of toxic aggregates. It is suggested that the stabilization of existing interfaces in multimeric proteins or the formation of new complexes in monomeric polypeptides might become effective strategies to prevent disease-linked aggregation of globular proteins.

Prediction of the aggregation propensity of proteins from the primary sequence: Aggregation properties of proteomes

In the cell, protein folding into stable globular conformations is in competition with aggregation into non‐functional and usually toxic structures, since the biophysical properties that promote folding also tend to favor intermolecular contacts, leading to the formation of β‐sheet‐enriched insoluble assemblies. The formation of protein deposits is linked to at least 20 different human disorders, ranging from dementia to diabetes. Furthermore, protein deposition inside cells represents a major obstacle for the biotechnological production of polypeptides. Importantly, the aggregation behavior of polypeptides appears to be strongly influenced by the intrinsic properties encoded in their sequences and specifically by the presence of selective short regions with high aggregation propensity. This allows computational methods to be used to analyze the aggregation properties of proteins without the previous requirement for structural information. Applications range from the identification of individual amyloidogenic regions in disease‐linked polypeptides to the analysis of the aggregation properties of complete proteomes. Herein, we review these theoretical approaches and illustrate how they have become important and useful tools in understanding the molecular mechanisms underlying protein aggregation.

The in Vivo and in Vitro Aggregation Properties of Globular Proteins Correlate With Their Conformational Stability: The SH3 Case

Protein misfolding and deposition underlie an increasing number of debilitating human disorders and constitute a problem of major concern in biotechnology. In the last years, in vitro studies have provided valuable insights into the physicochemical principles underlying protein aggregation. Nevertheless, information about the determinants of protein deposition within the cell is scarce and only a few systematic studies comparing in vitro and in vivo data have been reported. Here, we have used the SH3 domain of alpha-spectrin as a model globular protein in an attempt to understand the relationship between protein aggregation in the test-tube and in the more complex cellular environment. The investigation of the aggregation in Escherichia coli of this domain and a large set of mutants, together with the analysis of their sequential and conformational properties allowed us to evaluate the contribution of different polypeptidic factors to the cellular deposition of globular proteins. The data presented here suggest that the rules that govern in vitro protein aggregation are also valid in in vivo contexts. They also provide relevant insights into intracellular protein deposition in both conformational diseases and recombinant protein production.

AGGRESCAN: method, application, and perspectives for drug design.

Protein aggregation underlies the development of an increasing number of conformational human diseases of growing incidence, such as Alzheimers and Parkinsons diseases. Furthermore, the accumulation of recombinant proteins as intracellular aggregates represents a critical obstacle for the biotechnological production of polypeptides. Also, ordered protein aggregates constitute novel and versatile nanobiomaterials. Consequently, there is an increasing interest in the development of methods able to forecast the aggregation properties of polypeptides in order to modulate their intrinsic solubility. In this context, we have developed AGGRESCAN, a simple and fast algorithm that predicts aggregation-prone segments in protein sequences, compares the aggregation properties of different proteins or protein sets and analyses the effect of mutations on protein aggregation propensities.

Designing out disulfide bonds of leech carboxypeptidase inhibitor: implications for its folding, stability and function

Leech carboxypeptidase inhibitor (LCI) is a 67-residue, tight-binding metallocarboxypeptidase inhibitor composed of a compact domain with a five-stranded beta-sheet and a short alpha-helix that are strongly stabilized by four disulfide bonds. In this study, we investigated the contribution of each particular disulfide to the folding, stability and function of LCI by constructing a series of single and multiple mutants lacking one to four disulfide bonds. The results allow a better understanding of how individual disulfide bonds shape and restrict the conformational space that LCI must explore before attaining its native conformation. The work also dissected the role played by intramolecular rearrangements of disulfides during LCI folding, providing a new kinetic scheme in which the 2S ensemble suffers a non-specific oxidation into the 3S ensemble. These 3-disulfide-bonded species reshuffle to preferentially form III-A and III-B, two major native-like folding intermediates that need structural rearrangements through the formation of scrambled isomers to finally render native LCI. The designed multiple mutants of LCI are unable to fold correctly, displaying a highly unstructured conformation and a very low inhibitory capability, which indicates the importance of disulfide bonds in LCI for both correct folding and achievement of a functional structure. In contrast, the elimination of a single disulfide bond in LCI only results in a significant reduction of conformational stability, but the mutations have a rather moderate impact on carboxypeptidase inhibition, allowing the possibility to target the intrinsic stability and specific activity of LCI independently. In this way, the findings reported provide a basis for the design of novel variants of the molecule with improved therapeutic properties.

The aggregation properties of Escherichia coli proteins associated with their cellular abundance

Proteins are key players in most cellular processes. Therefore, their abundances are thought to be tightly regulated at the gene-expression level. Recent studies indicate, however, that steady-state cellular-protein concentrations correlate better across species than the levels of the corresponding mRNAs; this supports the existence of selective forces to maintain precise cellular-protein concentrations and homeostasis, even if gene-expression levels diverge. One of these forces might be the avoidance of protein aggregation because, in the cell, the folding of proteins into functional conformations might be in competition with anomalous aggregation into non-functional and usually toxic structures in a concentration-dependent manner. The data in the present work provide support for this hypothesis because, in E. coli, the experimental solubility of proteins correlates better with the cellular abundance than with the gene-expression levels. We found that the divergence between protein and mRNAs levels is low for high-abundance proteins. This suggests that because abundant proteins are at higher risk of aggregation, cellular concentrations need to be stringently regulated by gene expression.

Deciphering the role of the thermodynamic and kinetic stabilities of SH3 domains on their aggregation inside bacteria

The formation of insoluble deposits by globular proteins underlies the onset of many human diseases. Recent studies suggest a relationship between the thermodynamic stability of proteins and their in vivo aggregation. However, it has been argued that, in the cell, the occurrence of irreversible aggregation might shift the system from equilibrium, in such a way that it could be the rate of unfolding and associated kinetic stability instead of the conformational stability that controls protein deposition. This is an important but difficult to decipher question, because kinetic and thermodynamic stabilities appear usually correlated. Here we address this issue by comparing the in vitro folding kinetics and stability features of a set of non‐natural SH3 domains with their aggregation properties when expressed in bacteria. In addition, we compare the in vitro stability of the isolated domains with their effective stability in conditions that mimic the cytosolic environment. Overall, the data argue in favor of a thermodynamic rather than a kinetic control of the intracellular aggregation propensities of small globular proteins in which folding and unfolding velocities largely exceed aggregation rates. These results have implications regarding the evolution of proteins.

The N-terminal Helix Controls the Transition between the Soluble and Amyloid States of an FF Domain

Background Protein aggregation is linked to the onset of an increasing number of human nonneuropathic (either localized or systemic) and neurodegenerative disorders. In particular, misfolding of native α-helical structures and their self-assembly into nonnative intermolecular β-sheets has been proposed to trigger amyloid fibril formation in Alzheimer’s and Parkinson’s diseases. Methods Here, we use a battery of biophysical techniques to elucidate the conformational conversion of native α-helices into amyloid fibrils using an all-α FF domain as a model system. Results We show that under mild denaturing conditions at low pH this FF domain self-assembles into amyloid fibrils. Theoretical and experimental dissection of the secondary structure elements in this domain indicates that the helix 1 at the N-terminus has both the highest α-helical and amyloid propensities, controlling the transition between soluble and aggregated states of the protein. Conclusions The data illustrates the overlap between the propensity to form native α-helices and amyloid structures in protein segments. Significance The results presented contribute to explain why proteins cannot avoid the presence of aggregation-prone regions and indeed use stable α-helices as a strategy to neutralize such potentially deleterious stretches.

Trifluoroethanol Modulates Amyloid Formation by the All α-Helical URN1 FF Domain

Amyloid fibril formation is implicated in different human diseases. The transition between native α-helices and nonnative intermolecular β-sheets has been suggested to be a trigger of fibrillation in different conformational diseases. The FF domain of the URN1 splicing factor (URN1-FF) is a small all-α protein that populates a molten globule (MG) at low pH. Despite the fact that this conformation maintains most of the domain native secondary structure, it progressively converts into β-sheet enriched and highly ordered amyloid fibrils. In this study, we investigated if 2,2,2-trifluoroethanol (TFE) induced conformational changes that affect URN1-FF amyloid formation. Despite TFE having been shown to induce or increase the aggregation of both globular and disordered proteins at moderate concentrations, we demonstrate here that in the case of URN1-FF it reinforces its intrinsic α-helical structure, which competes the formation of aggregated assemblies. In addition, we show that TFE induces conformational diversity in URN1-FF fibrils, in such a way that the fibrils formed in the presence and absence of the cosolvent represent different polymorphs. It is suggested that the effect of TFE on both the soluble and aggregated states of URN1-FF depends on its ability to facilitate hydrogen bonding.