The emerging roles of glutamine amidotransferases in metabolism and immune defense.

Glutamine amidotransferases (GATs) catalyze the synthesis of nucleotides, amino acids, glycoproteins and an enzyme cofactor, thus serving as key metabolic enzymes for cell proliferation. Carbamoyl-phosphate synthetase, Aspartate transcarbamoylase, and Dihydroorotase (CAD) is a multifunctional enzyme of the GAT family and catalyzes the first three steps of the de novo pyrimidine synthesis. Following our findings that cellular GATs are involved in immune evasion during herpesvirus infection, we discovered that CAD reprograms cellular metabolism to fuel aerobic glycolysis and nucleotide synthesis via deamidating RelA. Deamidated RelA activates the expression of key glycolytic enzymes, rather than that of the inflammatory NF-κB-responsive genes. As such, cancer cells prime RelA for deamidation via up-regulating CAD activity or accumulating RelA mutations. Interestingly, the recently emerged SARS-CoV-2 also activates CAD to couple evasion of inflammatory response to activated nucleotide synthesis. A small molecule inhibitor of CAD depletes nucleotide supply and boosts antiviral inflammatory response, thus greatly reducing SARS-CoV-2 replication. Additionally, we also found that CTP synthase 1 (CTPS1) deamidates interferon (IFN) regulatory factor 3 (IRF3) to mute IFN induction. Our previous studies have implicated phosphoribosyl formylglycinamidine synthase (PFAS) and phosphoribosyl pyrophosphate amidotransferase (PPAT) in deamidating retinoic acid-inducible gene I (RIG-I) and evading dsRNA-induced innate immune defense in herpesvirus infection. Overall, these studies have uncovered an unconventional enzymatic activity of cellular GATs in metabolism and immune defense, offering a molecular link intimately coupling these fundamental biological processes.

Metabolic Enzymes in Viral Infection and Host Innate Immunity.

Metabolic enzymes are central players for cell metabolism and cell proliferation. These enzymes perform distinct functions in various cellular processes, such as cell metabolism and immune defense. Because viral infections inevitably trigger host immune activation, viruses have evolved diverse strategies to blunt or exploit the host immune response to enable viral replication. Meanwhile, viruses hijack key cellular metabolic enzymes to reprogram metabolism, which generates the necessary biomolecules for viral replication. An emerging theme arising from the metabolic studies of viral infection is that metabolic enzymes are key players of immune response and, conversely, immune components regulate cellular metabolism, revealing unexpected communication between these two fundamental processes that are otherwise disjointed. This review aims to summarize our present comprehension of the involvement of metabolic enzymes in viral infections and host immunity and to provide insights for potential antiviral therapy targeting metabolic enzymes.

Molecular cloning of Y-Box binding protein-1 from mandarin fish and its roles in stress-response and antiviral immunity.

Mandarin fish (Siniperca chuatsi) is a universally farmed fish species in China and has a large farming scale and economic value. With the high-density cultural mode in mandarin fish, viral diseases, such as infectious spleen and kidney necrosis virus (ISKNV) and Siniperca chuatsi rhabdovirus (SCRV), have increased loss, which has seriously restricted the development of aquaculture. Y-Box binding protein 1 (YB-1) is a member of cold shock protein family that regulates multiple cellular processes. The roles of mammalian YB-1 protein in environmental stress and innate immunity have been studied well, but its roles in teleost fishes remain unknown. In the present study, the characteristic of S. chuatsi YB-1 (scYB-1) and its roles in cold stress and virus infection were investigated. The scYB-1 obtained an 1541 bp cDNA that contains a 903 bp open reading frame encoding a protein of 300 amino acids. Tissue distribution results showed that the scYB-1 is a ubiquitously expressed gene found among tissues from mandarin fish. Overexpression of scYB-1 can increase the expression levels of cold shock-responsive genes, such as scHsc70a, scHsc70b, and scp53. Furthermore, the role of scYB-1 in innate immunity was also investigated in mandarin fish fry (MFF-1) cells. The expression level of scYB-1 was significant change in response to poly (I:C), poly (dG:dC), PMA, ISKNV, or SCRV stimulation. The overexpression of scYB-1 can significantly increase the expression levels of NF-κB-responsive genes, including scIL-8, scTNF-α, and scIFN-h. The NF-κB-luciferase report assay results showed that the relative expression of luciferin was significantly increased in the cells overexpressed with scYB-1 compared with those in cells overexpressed with control plasmid. These results indicate that scYB-1 can induce the NF-κB signaling pathway in MFF-1 cells. Overexpressed scYB-1 can downregulate the expression of ISKNV viral major capsid protein (mcp) gene but upregulates the expression of SCRV mcp gene. Moreover, knockdown of scYB-1 using siRNA can upregulate the expression of ISKNV mcp gene but downregulates the expression of SCRV mcp gene. These results indicate that scYB-1 suppresses ISKNV infection while enhancing SCRV infection. The above observations suggest that scYB-1 is involved in cold stress and virus infection. Our study will provide an insight into the roles of teleost fish YB-1 protein in stress response and innate immunity.