Human respiratory syncytial virus (HRSV) envelope glycoproteins traffic to assembly sites through the secretory pathway, while nonglycosylated proteins M and N are present in HRSV inclusion bodies but must reach the plasma membrane, where HRSV assembly happens. Little is known about how nonglycosylated HRSV proteins reach assembly sites. Here, we show that HRSV M and N proteins partially colocalize with the Golgi marker giantin, and the glycosylated F and nonglycosylated N proteins are closely located in the trans-Golgi, suggesting their interaction in that compartment. Brefeldin A compromised the trafficking of HRSV F and N proteins and inclusion body sizes, indicating that the Golgi is important for both glycosylated and nonglycosylated HRSV protein traffic. HRSV N and M proteins colocalized and interacted with sorting nexin 2 (SNX2), a retromer component that shapes endosomes in tubular structures. Glycosylated F and nonglycosylated N HRSV proteins are detected in SNX2-laden aggregates with intracellular filaments projecting from their outer surfaces, and VPS26, another retromer component, was also found in inclusion bodies and filament-shaped structures. Similar to SNX2, TGN46 also colocalized with HRSV M and N proteins in filamentous structures at the plasma membrane. Cell fractionation showed enrichment of SNX2 in fractions containing HRSV M and N proteins. Silencing of SNX1 and 2 was associated with reduction in viral proteins, HRSV inclusion body size, syncytium formation, and progeny production. The results indicate that HRSV structural proteins M and N are in the secretory pathway, and SNX2 plays an important role in the traffic of HRSV structural proteins toward assembly sites. IMPORTANCE The present study contributes new knowledge to understand HRSV assembly by providing evidence that nonglycosylated structural proteins M and N interact with elements of the secretory pathway, shedding light on their intracellular traffic. To the best of our knowledge, the present contribution is important given the scarcity of studies about the traffic of HRSV nonglycosylated proteins, especially by pointing to the involvement of SNX2, a retromer component, in the HRSV assembly process.

SARS-CoV-2 is an emerging virus from the Coronaviridae family and is responsible for the ongoing COVID-19 pandemic. In this work, we explored the previously reported SARS-CoV-2 structural membrane protein (M) interaction with human Proliferating Cell Nuclear Antigen (PCNA). The M protein is responsible for maintaining virion shape, and PCNA is a marker of DNA damage which is essential for DNA replication and repair. We validated the M-PCNA interaction through immunoprecipitation, immunofluorescence co-localization, and PLA (Proximity Ligation Assay). In cells infected with SARS-CoV-2 or transfected with M protein, using immunofluorescence and cell fractioning, we documented a reallocation of PCNA from the nucleus to the cytoplasm and the increase of PCNA and γH2AX (another DNA damage marker) expression. We also observed an increase in PCNA and γH2AX expression in the lung of a COVID-19 patient by immunohistochemistry. In addition, the inhibition of PCNA translocation by PCNA I1 and Verdinexor led to a reduction of plaque formation in an in vitro assay. We, therefore, propose that the transport of PCNA to the cytoplasm and its association with M could be a virus strategy to manipulate cell functions and may be considered a target for COVID-19 therapy.

The M2-2 protein from the respiratory syncytial virus (RSV) is a 10 kDa protein expressed by the second ORF of the viral gene M2. During infection, M2-2 has been described as the polymerase cofactor responsible for promoting genome replication. This function was first inferred by infection with a mutant virus lacking the M2-2 ORF, in which viral genome presented delayed accumulation in comparison to wild-type virus. In accordance with this phenotype, it has been recently shown that M2-2 promotes changes in interactions between the polymerase and other viral proteins at early stages of infection. Despite its well-explored role in the regulation of the polymerase activity, little has been made to investigate the relationship of M2-2 with cellular proteins. In fact, a previous report showed poor recruitment of M2-2 to viral structures, with the protein being mainly localized to the nucleus and cytoplasmic granules. To unravel which other functions M2-2 exerts during infection, we expressed the protein in HEK293T cells and performed proteomic analysis of co-immunoprecipitated partners, identifying enrichment of proteins involved with regulation of translation, protein folding and mRNA splicing. In approaches based on these data, we found that M2-2 expression downregulates eiF2α phosphorylation and inhibits stress granules assembly under arsenite induction. In addition, we also verified that M2-2 inhibits translation initiation, and is targeted for proteasome degradation, being localized to granules composed by defective ribosomal products at the cytoplasm. These results suggest that besides its functions in the regulation of genome replication, M2-2 may exert additional functions to contribute to successful RSV infection.

No abstract

SARS-CoV-2 is an emerging virus from the Coronaviridae family and is responsible for the ongoing COVID-19 pandemic. In this work, we explored the previously reported SARS-CoV-2 structural membrane protein (M) interaction with human Proliferating Cell Nuclear Antigen (PCNA). The M protein is responsible for maintaining virion shape, and PCNA is a marker of DNA damage which is essential for DNA replication and repair. We validated the M PCNA interaction through immunoprecipitation, immunofluorescence co-localization, and a PLA assay. In cells infected with SARS-CoV-2 or transfected with M protein, using immunofluorescence and cell fractioning, we documented a reallocation of PCNA from the nucleus to the cytoplasm and the increase of PCNA and γH2AX (another DNA damage marker) expression. We also observed an increase of PCNA and γH2AX expression in the lung of a COVID-19 patient by immunohistochemistry. In addition, the inhibition of PCNA translocation by PCNA I1 and Verdinexor led to a reduction of plaque formation in an in vitro assay. We, therefore, propose that the transport of PCNA to the cytoplasm and its association with M could be a virus strategy to manipulate cell functions and may be considered a target for COVID-19 therapy.

The M2-2 protein from the respiratory syncytial virus (RSV) is a 10 kDa protein expressed by the second ORF of the viral gene M2. During infection, M2-2 has been described as the polymerase cofactor responsible for promoting genome replication, which occurs by the induction of changes in interactions between the polymerase and other viral proteins at early stages of infection. Despite its well-explored role in the regulation of the polymerase activity, little has been made to investigate the relationship of M2-2 with cellular proteins. A previous report showed poor recruitment of M2-2 to viral structures, with the protein being mainly localized to the nucleus and cytoplasmic granules. To unravel which other functions M2-2 exerts during infection, we performed proteomic analysis of co-immunoprecipitated cellular partners, identifying enrichment of proteins involved with regulation of translation, protein folding and mRNA splicing. In approaches based on these data, we found that M2-2 expression downregulates eiF2α phosphorylation and inhibits both translation and stress granules assembly. Finally, we also verified that M2-2 is targeted for proteasome degradation, being localized to granules composed of defective ribosomal products at the cytoplasm. These results suggest that besides its functions in the replicative complex, M2-2 may exert additional functions to contribute to successful RSV infection.