Functional benefit of structural disorder for the replication of measles, Nipah and Hendra viruses

Virus-like particles of retroviral origin in protein aggregation and neurodegenerative diseases

A wide range of human diseases are associated with protein misfolding and amyloid aggregates. Recent studies suggest that in certain neurological disorders, including Amyotrophic Lateral Sclerosis (ALS), Frontotemporal Dementia (FTD) and various tauopathies, protein aggregation may be promoted by virus-like particles (VLPs) formed by endogenous retroviruses (ERVs). The molecular mechanisms by which these VLPs contribute to protein aggregation, however, remain enigmatic. Here, we discuss possible molecular mechanisms of ERV-derived VLPs in the formation and spread of protein aggregates. An intriguing possibility is that liquid-like condensates may facilitate the formation of both protein aggregates and ERV-derived VLPs. We also describe how RNA chaperoning, and the encapsulation and trafficking of misfolded proteins, may contribute to protein homeostasis through the elimination of protein aggregates from cells. Based on these insights, we discuss future potential therapeutic opportunities.

A conserved motif in Henipavirus P/V/W proteins drives the fibrillation of the W protein from Hendra virus

The Hendra (HeV) and Nipah (NiV) viruses are high-priority, biosafety level-4 pathogens that cause fatal neurological and respiratory disease. Their P gene encodes not only the P protein, an essential polymerase cofactor, but also the virulence factors V and W. We previously showed that the W protein of HeV (WHeV) forms amyloid-like fibrils and that one of its subdomains, PNT3, fibrillates in isolation. However, the fibrillation kinetics is much faster in the case of the full-length WHeV compared to PNT3, suggesting that another WHeV region contributes to the fibrillation process. In this work, we identified the region spanning residues 2-110 (PNT1) as the crucial region implicated in WHeV fibrillation. Through site-directed mutagenesis, combined with thioflavin T binding experiments and negative-staining transmission electron microscopy, we showed that a predicted cryptic amyloidogenic region (CAR) within PNT1 is the main driver of fibrillation and deciphered the underlying molecular mechanism. Using FTIR, we showed that PNT1 fibrils are enriched in cross β-sheets. Sequence alignment revealed conservation of the CAR across the Henipavirus genus and enabled the identification of a hitherto never reported pro-amyloidogenic motif. The ability to form fibrils was experimentally shown to be a common property shared by Henipavirus PNT1 proteins. Overall, this study sheds light on the molecular mechanisms underlying WHeV fibrillation and calls for future studies aimed at exploring the relevance of the newly identified pro-amyloidogenic motif as a valuable target for antiviral approaches.

Unraveling the molecular grammar and the structural transitions underlying the fibrillation of a viral fibrillogenic domain

Hendra virus (HeV) is a biosafety level 4 human pathogen belonging to the Henipavirus genus within the Paramyxoviridae family. In HeV, the phosphoprotein-encoding gene also drives the synthesis of the V and W proteins that are two major players in the host innate immune response evasion. These three proteins share a common intrinsically disordered N-terminal domain (NTD) and have distinct C-terminal domains. We recently reported the ability of a short region (i.e., PNT3), located within the shared NTD, to form fibrils. We subsequently identified a PNT3 motif (EYYY) critically involved in fibrillation and deciphered the contribution of each tyrosine to the process. Herein, we combined mutational studies with various biochemical and biophysical approaches to further investigate the molecular mechanisms underlying PNT3 fibrillation. The results show that (i) lysine residues play a critical role in driving fibrillation, (ii) hydrophobic residues affect the nucleation step, and (iii) charge distribution strongly affects the fibrillation propensities. Vibrational Raman spectroscopy data further validated the role of lysine residues in promoting fibrillation and enabled documenting the formation of cross-β amyloid structures. Altogether, these results illuminate the molecular mechanisms involved in fibril formation and pave the way towards the rational design of inhibitors.

Dissecting Henipavirus W proteins conformational and fibrillation properties: contribution of their N- and C-terminal constituent domains

The Nipah and Hendra viruses are severe human pathogens. In addition to the P protein, their P gene also encodes the V and W proteins that share with P their N-terminal intrinsically disordered domain (NTD) and possess distinct C-terminal domains (CTDs). The W protein is a key player in the evasion of the host innate immune response. We previously showed that the W proteins are intrinsically disordered and can form amyloid-like fibrils. However, structural information on W CTD (CTDW) and its potential contribution to the fibrillation process is lacking. In this study, we demonstrate that CTDWS are disordered and able to form dimers mediated by disulfide bridges. We also show that the NTD and the CTDW interact with each other and that this interaction triggers both a gain of secondary structure and a chain compaction within the NTD. Finally, despite the lack of intrinsic fibrillogenic properties, we show that the CTDW favors the formation of fibrils by the NTD both in cis and in trans. Altogether, the results herein presented shed light on the molecular mechanisms underlying Henipavirus pathogenesis and may thus contribute to the development of targeted therapies.

Transient Non-local Interactions Dominate the Dynamics of Measles Virus NTAIL

The RNA genome of measles virus is encapsidated by the nucleoprotein within a helical nucleocapsid that serves as template for both transcription and replication. The intrinsically disordered domain of the nucleoprotein (NTAIL), partly protruding outward from the nucleocapsid, is essential for binding the polymerase complex responsible for viral transcription and replication. As for many IDPs, binding of NTAIL occurs through a short molecular recognition element (MoRE) that folds upon binding, with the majority of NTAIL remaining disordered. Though NTAIL regions far from the MoRE influence the binding affinity, interactions between them and the MoRE have not been investigated in depth. Using an integrated approach, relying on photo-induced electron transfer (PET) experiments between tryptophan and cysteine pairs placed at different positions in the protein under varying salt and pH conditions, combined with simulations and analytical models, we identified transient interactions between two disordered regions distant in sequence, which dominate NTAIL dynamics, and regulate the conformational preferences of both the MoRE and the entire NTAIL domain. Co-evolutionary analysis corroborates our findings, and suggests an important functional role for the same intramolecular interactions. We propose mechanisms by which these non-local interactions may regulate binding to the phosphoprotein, polymerase recruitment, and ultimately viral transcription and replication. Our findings may be extended to other IDPs, where non-local intra-protein interactions affect the conformational preferences of intermolecular binding sites.

Viral amyloids: New opportunities for antiviral therapeutic strategies

Amyloidoses are an array of diseases associated with the aggregation of proteins into fibrils. While it was previously thought that amyloid fibril-forming proteins are exclusively host-cell encoded, recent studies have revealed that pathogenic viruses can form amyloid-like fibrils too. Intriguingly, viral amyloids are often composed of virulence factors, known for their contribution to cell death and disease progression. In this review, we survey the literature about viral proteins capable of forming amyloid-like fibrils. The molecular and cellular mechanisms underlying the formation of viral amyloid-like aggregates are explored. In addition, we discuss the functional implications for viral amplification and the complex interplay between viral amyloids, biological functions, virulence, and virus-induced pathologies.

Host–Pathogen Interactions Influencing Zoonotic Spillover Potential and Transmission in Humans

Emerging infectious diseases of zoonotic origin are an ever-increasing public health risk and economic burden. The factors that determine if and when an animal virus is able to spill over into the human population with sufficient success to achieve ongoing transmission in humans are complex and dynamic. We are currently unable to fully predict which pathogens may appear in humans, where and with what impact. In this review, we highlight current knowledge of the key host–pathogen interactions known to influence zoonotic spillover potential and transmission in humans, with a particular focus on two important human viruses of zoonotic origin, the Nipah virus and the Ebola virus. Namely, key factors determining spillover potential include cellular and tissue tropism, as well as the virulence and pathogenic characteristics of the pathogen and the capacity of the pathogen to adapt and evolve within a novel host environment. We also detail our emerging understanding of the importance of steric hindrance of host cell factors by viral proteins using a “flytrap”-type mechanism of protein amyloidogenesis that could be crucial in developing future antiviral therapies against emerging pathogens. Finally, we discuss strategies to prepare for and to reduce the frequency of zoonotic spillover occurrences in order to minimize the risk of new outbreaks.

Molecular Determinants of Fibrillation in a Viral Amyloidogenic Domain from Combined Biochemical and Biophysical Studies

The Nipah and Hendra viruses (NiV and HeV) are biosafety level 4 human pathogens classified within the Henipavirus genus of the Paramyxoviridae family. In both NiV and HeV, the gene encoding the Phosphoprotein (P protein), an essential polymerase cofactor, also encodes the V and W proteins. These three proteins, which share an intrinsically disordered N-terminal domain (NTD) and have unique C-terminal domains (CTD), are all known to counteract the host innate immune response, with V and W acting by either counteracting or inhibiting Interferon (IFN) signaling. Recently, the ability of a short region within the shared NTD (i.e., PNT3) to form amyloid-like structures was reported. Here, we evaluated the relevance of each of three contiguous tyrosine residues located in a previously identified amyloidogenic motif (EYYY) within HeV PNT3 to the fibrillation process. Our results indicate that removal of a single tyrosine in this motif significantly decreases the ability to form fibrils independently of position, mainly affecting the elongation phase. In addition, we show that the C-terminal half of PNT3 has an inhibitory effect on fibril formation that may act as a molecular shield and could thus be a key domain in the regulation of PNT3 fibrillation. Finally, the kinetics of fibril formation for the two PNT3 variants with the highest and the lowest fibrillation propensity were studied by Taylor Dispersion Analysis (TDA). The results herein presented shed light onto the molecular mechanisms involved in fibril formation.

Shapeshifting proteins: the role of structural disorder and conformational plasticity in physiology and disease

Intrinsically disordered proteins (IDPs) defy the conventional structure–function paradigm and do not autonomously fold up into unique 3D structures for carrying out functions. They exist as rapidly interconverting conformational ensembles and are thought to expand the functional repertoire of proteins. Such shapeshifting proteins are associated with a multitude of biological functions and a wide range of human diseases. The thematic issue on ‘Shapeshifting Proteins’ in Essays in Biochemistry includes some exciting and emerging aspects of this class of proteins. Articles in this issue provide current trends and contemporary views on various intriguing features of these proteins involving their unique structural and dynamical characteristics, misfolding and aggregation behavior, and their phase transitions into biomolecular condensates. I hope that this thematic issue will be of considerable interest to the practitioners in protein biochemistry and biophysics as well as to the researchers in other allied areas involving cell and molecular biology, neuroscience, virology, pathophysiology, and so forth.