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The interaction between influenza A virus (IAV) and host proteins is an important process that greatly influences viral replication and pathogenicity. PB2 protein is a subunit of viral ribonucleoprotein (vRNP) complex playing distinct roles in viral transcription and replication. BAG6 (BCL2-associated athanogene 6) as a multifunctional host protein participates in physiological and pathological processes. Here, we identify BAG6 as a new restriction factor for IAV replication through targeting PB2. For both avian and human influenza viruses, overexpression of BAG6 reduced viral protein expression and virus titers, whereas deletion of BAG6 significantly enhanced virus replication. Moreover, BAG6-knockdown mice developed more severe clinical symptoms and higher viral loads upon IAV infection. Mechanistically, BAG6 restricted IAV transcription and replication by inhibiting the activity of viral RNA-dependent RNA polymerase (RdRp). The co-Immunoprecipitation assays showed BAG6 specifically interacted with the N-terminus of PB2 and competed with PB1 for RdRp complex assembly. The ubiquitination assay indicated that BAG6 promoted PB2 ubiquitination at K189 residue and targeted PB2 for K48-linked ubiquitination degradation. The antiviral effect of BAG6 necessitated its N-terminal region containing a ubiquitin-like (UBL) domain (17-92aa) and a PB2-binding domain (124-186aa), which are synergistically responsible for viral polymerase subunit PB2 degradation and perturbing RdRp complex assembly. These findings unravel a novel antiviral mechanism via the interaction of viral PB2 and host protein BAG6 during avian or human influenza virus infection and highlight a potential application of BAG6 for antiviral drug development. Author summary: Influenza A virus (IAV) is a major public health threat worldwide. The viral polymerase subunit PB2 of IAV is not only responsible for transcription and replication of viral RNA but also acts as a key determinant for host adaptation, making it a key target of host defense system. In this study, we report BAG6 as a novel negative regulator of viral PB2 protein. It interacts with the N-terminus of PB2 and promotes PB2 ubiquitination at the K189 residue, which is highly conserved in all IAV subtypes, resulting in the competitive inhibition of RdRp assembly and the ubiquitination degradation of PB2. These findings emphasize the potent antiviral activity of BAG6 against both avian and human influenza viruses and provide a detailed insight to facilitate antivirals targeting PB2 protein. [ABSTRACT FROM AUTHOR]

Salmonella enterica serovar Typhimurium (S. Typhimurium) is one of the leading causes of food-borne illnesses worldwide. To colonize the gastrointestinal tract, S. Typhimurium produces multiple virulence factors that facilitate cellular invasion. Chitinases have been recently emerging as virulence factors for various pathogenic bacterial species, and the S. Typhimurium genome contains two annotated chitinases: STM0018 (chiA) and STM0233. However, the role of these chitinases during S. Typhimurium pathogenesis is unknown. The putative chitinase STM0233 has not been studied previously, and only limited data exists on ChiA. Chitinases typically hydrolyze chitin polymers, which are absent in vertebrates. However, chiA expression was detected in infection models and purified ChiA cleaved carbohydrate subunits present on mammalian surface glycoproteins, indicating a role during pathogenesis. Here, we demonstrate that expression of chiA and STM0233 is upregulated in the mouse gut and that both chitinases facilitate epithelial cell adhesion and invasion. S. Typhimurium lacking both chitinases showed a 70% reduction in invasion of small intestinal epithelial cells in vitro. In a gastroenteritis mouse model, chitinase-deficient S. Typhimurium strains were also significantly attenuated in the invasion of small intestinal tissue. This reduced invasion resulted in significantly delayed S. Typhimurium dissemination to the spleen and the liver, but chitinases were not required for systemic survival. The invasion defect of the chitinase-deficient strain was rescued by the presence of wild-type S. Typhimurium, suggesting that chitinases are secreted. By analyzing N-linked glycans of small intestinal cells, we identified specific N-acetylglucosamine-containing glycans as potential extracellular targets of S. Typhimurium chitinases. This analysis also revealed a differential abundance of Lewis X/A-containing glycans that is likely a result of host cell modulation due to the detection of S. Typhimurium chitinases. Similar glycomic changes elicited by chitinase deficient strains indicate functional redundancy of the chitinases. Overall, our results demonstrate that S. Typhimurium chitinases contribute to intestinal adhesion and invasion through modulation of the host glycome. Author summary: Salmonella Typhimurium infection is one of the leading causes of food-borne illnesses worldwide. In order for S. Typhimurium to effectively cause disease, it has to invade the epithelial cells lining the intestinal tract. This invasion step allows S. Typhimurium to replicate efficiently, causing further tissue damage and inflammation. In susceptible patients, S. Typhimurium can spread past the intestines and infect peripheral organs. It is essential to fully understand the invasion mechanism used by S. Typhimurium to design better treatments for infection. Here, we demonstrate that the two chitinases produced by S. Typhimurium are involved in this invasion process. We show that S. Typhimurium chitinases interact with surface glycans of intestinal epithelial cells and promote adhesion and invasion. Using a mouse infection model, we show that S. Typhimurium chitinases are required for the invasion of the small intestine and enhance the dissemination of S. Typhimurium to other organs. This study reveals an additional mechanism by which S. Typhimurium invades and causes infection. [ABSTRACT FROM AUTHOR]