ER functions are exploited by viruses to support distinct stages of their life cycle

The astrovirus N-terminal nonstructural protein anchors replication complexes to the perinuclear ER membranes

An essential aspect of positive-sense RNA virus replication is anchoring the replication complex (RC) to cellular membranes. Positive-sense RNA viruses employ diverse strategies, including co-translational membrane targeting through signal peptides and co-opting cellular membrane trafficking components. Often, N-terminal nonstructural proteins play a crucial role in linking the RC to membranes, facilitating the early association of the replication machinery. Astroviruses utilize a polyprotein strategy to synthesize nonstructural proteins, relying on subsequent processing to form replication-competent complexes. This study provides evidence for the perinuclear ER membrane association of RCs in five distinct human astrovirus strains. Using tagged recombinant classical human astrovirus 1 and neurotropic MLB2 strains, we establish that the N-terminal domain guides the ER membrane association. We identified di-arginine motifs responsible for the perinuclear ER retention and formation of functional RCs through mutational analysis of the N-terminal domain in replicon and reverse genetics systems. In addition, we demonstrate the association of key components of the astrovirus replication complex: double-stranded RNA, RNA-dependent RNA polymerase, protease, and N-terminal protein. Our findings highlight the intricate virus-ER interaction mechanism employed by astroviruses, potentially leading to the development of novel antiviral intervention strategies.

Autophagy of the ER: the secretome finds the lysosome

Lysosomal degradation of the endoplasmic reticulum (ER) and its components through the autophagy pathway has emerged as a major regulator of ER proteostasis. Commonly referred to as ER-phagy and ER-to-lysosome-associated degradation (ERLAD), how the ER is targeted to the lysosome has been recently clarified by a growing number of studies. Here, we summarize the discoveries of the molecular components required for lysosomal degradation of the ER and their proposed mechanisms of action. Additionally, we discuss how cells employ these machineries to create the different routes of ER-lysosome-associated degradation. Further, we review the role of ER-phagy in viral infection pathways, as well as the implication of ER-phagy in human disease. In sum, we provide a comprehensive overview of the current field of ER-phagy.

The REEP5/TRAM1 complex binds SARS-CoV-2 NSP3 and promotes virus replication

IMPORTANCE

Generation of virus-host protein-protein interactions (PPIs) maps may provide clues to uncover SARS-CoV-2-hijacked cellular processes. However, these PPIs maps were created by expressing each viral protein singularly, which does not reflect the life situation in which certain viral proteins synergistically interact with host proteins. Our results reveal the host-viral protein-protein interactome of SARS-CoV-2 NSP3, NSP4, and NSP6 expressed individually or in combination. Furthermore, REEP5/TRAM1 complex interacts with NSP3 at ROs and promotes viral replication. The significance of our research is identifying virus-host interactions that may be targeted for therapeutic intervention.

Cellular Stress: Modulator of Regulated Cell Death

Cellular stress response activates a complex program of an adaptive response called integrated stress response (ISR) that can allow a cell to survive in the presence of stressors. ISR reprograms gene expression to increase the transcription and translation of stress response genes while repressing the translation of most proteins to reduce the metabolic burden. In some cases, ISR activation can lead to the assembly of a cytoplasmic membraneless compartment called stress granules (SGs). ISR and SGs can inhibit apoptosis, pyroptosis, and necroptosis, suggesting that they guard against uncontrolled regulated cell death (RCD) to promote organismal homeostasis. However, ISR and SGs also allow cancer cells to survive in stressful environments, including hypoxia and during chemotherapy. Therefore, there is a great need to understand the molecular mechanism of the crosstalk between ISR and RCD. This is an active area of research and is expected to be relevant to a range of human diseases. In this review, we provided an overview of the interplay between different cellular stress responses and RCD pathways and their modulation in health and disease.

Reticulons promote formation of ER-derived double-membrane vesicles that facilitate SARS-CoV-2 replication

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the etiologic agent for the global COVID-19 pandemic, triggers the formation of endoplasmic reticulum (ER)-derived replication organelles, including double-membrane vesicles (DMVs), in the host cell to support viral replication. Here, we clarify how SARS-CoV-2 hijacks host factors to construct the DMVs. We show that the ER morphogenic proteins reticulon-3 (RTN3) and RTN4 help drive DMV formation, enabling viral replication, which leads to productive infection. Different SARS-CoV-2 variants, including the delta variant, use the RTN-dependent pathway to promote infection. Mechanistically, our results reveal that the membrane-embedded reticulon homology domain (RHD) of the RTNs is sufficient to functionally support viral replication and physically engage NSP3 and NSP4, two viral non-structural membrane proteins known to induce DMV formation. Our findings thus identify the ER morphogenic RTN3 and RTN4 membrane proteins as host factors that help promote the biogenesis of SARS-CoV-2-induced DMVs, which can act as viral replication platforms.

The V2 Protein from the Geminivirus Tomato Yellow Leaf Curl Virus Largely Associates to the Endoplasmic Reticulum and Promotes the Accumulation of the Viral C4 Protein in a Silencing Suppression-Independent Manner

Viruses are strict intracellular parasites that rely on the proteins encoded in their genomes for the effective manipulation of the infected cell that ultimately enables a successful infection. Viral proteins have to be produced during the cell invasion and takeover in sufficient amounts and in a timely manner. Silencing suppressor proteins evolved by plant viruses can boost the production of viral proteins; although, additional mechanisms for the regulation of viral protein production likely exist. The strongest silencing suppressor encoded by the geminivirus tomato yellow leaf curl virus (TYLCV) is V2: V2 suppresses both post-transcriptional and transcriptional gene silencing (PTGS and TGS), activities that are associated with its localization in punctate cytoplasmic structures and in the nucleus, respectively. However, V2 has been previously described to largely localize in the endoplasmic reticulum (ER), although the biological relevance of this distribution remains mysterious. Here, we confirm the association of V2 to the ER in Nicotiana benthamiana and assess the silencing suppression activity-independent impact of V2 on protein accumulation. Our results indicate that V2 has no obvious influence on the localization of ER-synthesized receptor-like kinases (RLKs) or ER quality control (ERQC)/ER-associated degradation (ERAD), but dramatically enhances the accumulation of the viral C4 protein, which is co-translationally myristoylated, possibly in proximity to the ER. By using the previously described V2_C84S/86S mutant, in which the silencing suppression activity is abolished, we uncouple RNA silencing from the observed effect. Therefore, this work uncovers a novel function of V2, independent of its capacity to suppress silencing, in the promotion of the accumulation of another crucial viral protein.

A specific EMC subunit supports Dengue virus infection by promoting virus membrane fusion essential for cytosolic genome delivery

Dengue virus (DENV) represents the most common human arboviral infection, yet its cellular entry mechanism remains unclear. The multi-subunit endoplasmic reticulum membrane complex (EMC) supports DENV infection, in part, by assisting the biosynthesis of viral proteins critical for downstream replication steps. Intriguingly, the EMC has also been shown to act at an earlier step prior to viral protein biogenesis, although this event is not well-defined. Here we demonstrate that the EMC subunit EMC4 promotes fusion of the DENV and endosomal membranes during entry, enabling delivery of the viral genome into the cytosol which is then targeted to the ER for viral protein biosynthesis. We also found that EMC4 mediates ER-to-endosome transfer of phosphatidylserine, a phospholipid whose presence in the endosome facilitates DENV-endosomal membrane fusion. These findings clarify the EMC-dependent DENV early entry step, suggesting a mechanism by which an ER-localized host factor can regulate viral fusion at the endosome.

ER-Phagy and Microbial Infection

The endoplasmic reticulum (ER) is an essential organelle in cells that synthesizes, folds and modifies membrane and secretory proteins. It has a crucial role in cell survival and growth, thus requiring strict control of its quality and homeostasis. Autophagy of the ER fragments, termed ER-phagy or reticulophagy, is an essential mechanism responsible for ER quality control. It transports stress-damaged ER fragments as cargo into the lysosome for degradation to eliminate unfolded or misfolded protein aggregates and membrane lipids. ER-phagy can also function as a host defense mechanism when pathogens infect cells, and its deficiency facilitates viral infection. This review briefly describes the process and regulatory mechanisms of ER-phagy, and its function in host anti-microbial defense during infection.

Endoplasmic Reticulum Chaperones in Viral Infection: Therapeutic Perspectives

Viruses are intracellular parasites that subvert the functions of their host cells to accomplish their infection cycle. The endoplasmic reticulum (ER)-residing chaperone proteins are central for the achievement of different steps of the viral cycle, from entry and replication to assembly and exit. The most abundant ER chaperones are GRP78 (78-kDa glucose-regulated protein), GRP94 (94-kDa glucose-regulated protein), the carbohydrate or lectin-like chaperones calnexin (CNX) and calreticulin (CRT), the protein disulfide isomerases (PDIs), and the DNAJ chaperones. This review will focus on the pleiotropic roles of ER chaperones during viral infection. We will cover their essential role in the folding and quality control of viral proteins, notably viral glycoproteins which play a major role in host cell infection. We will also describe how viruses co-opt ER chaperones at various steps of their infectious cycle but also in order to evade immune responses and avoid apoptosis. Finally, we will discuss the different molecules targeting these chaperones and the perspectives in the development of broad-spectrum antiviral drugs.

Know your enemy and know yourself - the case of SARS-CoV-2 host factors

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the etiologic agent that causes Coronavirus Disease 2019 (COVID-19) pandemic, is a newly emerging respiratory RNA virus with exceptional transmissibility and pathogenicity. Numerous COVID-19 related studies have been fast-tracked, with the ultimate goal to end the pandemic. Here we review the major stages of SARS-CoV-2 infection cycle in cells, with specific emphasis on essential host factors. Insights into the cell biology of SARS-CoV-2 infection have accelerated the development of host-directed therapeutics, as shown by dozens of clinical trials evaluating COVID-19 treatments using host-targeting compounds.

RNA-Binding Proteins at the Host-Pathogen Interface Targeting Viral Regulatory Elements

Viral RNAs contain the information needed to synthesize their own proteins, to replicate, and to spread to susceptible cells. However, due to their reduced coding capacity RNA viruses rely on host cells to complete their multiplication cycle. This is largely achieved by the concerted action of regulatory structural elements on viral RNAs and a subset of host proteins, whose dedicated function across all stages of the infection steps is critical to complete the viral cycle. Importantly, not only the RNA sequence but also the RNA architecture imposed by the presence of specific structural domains mediates the interaction with host RNA-binding proteins (RBPs), ultimately affecting virus multiplication and spreading. In marked difference with other biological systems, the genome of positive strand RNA viruses is also the mRNA. Here we focus on distinct types of positive strand RNA viruses that differ in the regulatory elements used to promote translation of the viral RNA, as well as in the mechanisms used to evade the series of events connected to antiviral response, including translation shutoff induced in infected cells, assembly of stress granules, and trafficking stress.