Picornaviral 2C proteins: A unique ATPase family critical in virus replication

A cardioviral 2C-ATP complex structure reveals the essential role of a conserved arginine in regulation of cardioviral 2C activity

2C is a highly conserved picornaviral non-structural protein with ATPase activity and plays a multifunctional role in the viral life cycle as a promising target for anti-picornavirus drug development. While the structure-function of enteroviral 2Cs have been well studied, cardioviral 2Cs remain largely uncharacterized. Here, an endogenous ATP molecule was identified in the crystal structure of 2C from encephalomyocarditis virus (EMCV, Cardiovirus A). The ATP is bound into the ATPase active site with a unique compact conformation. Notably, the γ-phosphate of ATP directly interacts with Arg311 (conserved in cardioviral 2Cs), and its mutation significantly inhibits the ATPase activity. Unexpectedly, this mutation remarkably promotes 2C self-oligomerization and viral replication efficiency. Molecular dynamic simulations showed that the Arg311 side chain is highly dynamic, indicating it may function as a switch between the activation state and the inhibition state of ATPase activity. A hexameric ring model of EMCV 2C full length indicated that the C-terminal helix may get close to the N-terminal amphipathic helices to form a continuous positive region for RNA binding. The RNA-binding studies of EMCV 2C revealed that the RNA length is closely associated with the RNA-binding affinities and indicated that the substrate may wrap around the outer surface of the hexamer. Our studies provide a biochemical framework to guide the characterization of EMCV 2C and the essential role of arginine in cardioviral 2C functions.

IMPORTANCE

Encephalomyocarditis virus (Cardiovirus A) is the causative agent of the homonymous disease, which may induce myocarditis, encephalitis, and reproductive disorders in various mammals. 2C protein is functionally indispensable and a promising target for drug development involving broad-spectrum picornaviral inhibitors. Here, an endogenous ATP molecule with a unique conformation was discovered by a combination of protein crystallography and high-performance liquid chromatography in the encephalomyocarditis virus (EMCV) 2C structure. Biochemical and structural characterization analysis of EMCV 2C revealed the critical role of conserved Arg311 in ATPase activity and self-oligomerization of EMCV 2C. The viral replication kinetics and infectivity study suggested that the residue negatively regulated the infectivity titer and virus encapsulation efficiency of EMCV and is, therefore, crucial for 2C protein to promote viral replication. Our systemic structure-function analysis provides unique insights into the function and regulation mechanism of cardioviral 2C protein.

Leucoverdazyls as Novel Potent Inhibitors of Enterovirus Replication

Enteroviruses (EV) are important pathogens causing human disease with various clinical manifestations. To date, treatment of enteroviral infections is mainly supportive since no vaccination or antiviral drugs are approved for their prevention or treatment. Here, we describe the antiviral properties and mechanisms of action of leucoverdazyls-novel heterocyclic compounds with antioxidant potential. The lead compound, 1a, demonstrated low cytotoxicity along with high antioxidant and virus-inhibiting activity. A viral strain resistant to 1a was selected, and the development of resistance was shown to be accompanied by mutation of virus-specific non-structural protein 2C. This resistant virus had lower fitness when grown in cell culture. Taken together, our results demonstrate high antiviral potential of leucoverdazyls as novel inhibitors of enterovirus replication and support previous evidence of an important role of 2C proteins in EV replication.

Loxapine inhibits replication of hepatitis A virus in vitro and in vivo by targeting viral protein 2C

No antiviral drugs currently are available for treatment of infection by hepatitis A virus (HAV), a causative agent of acute hepatitis, a potentially life-threatening disease. Chemical screening of a small-compound library using nanoluciferase-expressing HAV identified loxapine succinate, a selective dopamine receptor D2 antagonist, as a potent inhibitor of HAV propagation in vitro. Loxapine succinate did not inhibit viral entry nor internal ribosome entry site (IRES)-dependent translation, but exhibited strong inhibition of viral RNA replication. Blind passage of HAV in the presence of loxapine succinate resulted in the accumulation of viruses containing mutations in the 2C-encoding region, which contributed to resistance to loxapine succinate. Analysis of molecular dynamics simulations of the interaction between 2C and loxapine suggested that loxapine binds to the N-terminal region of 2C, and that resistant mutations impede these interactions. We further demonstrated that administration of loxapine succinate to HAV-infected Ifnar1-/- mice (which lack the type I interferon receptor) results in decreases in the levels of fecal HAV RNA and of intrahepatic HAV RNA at an early stage of infection. These findings suggest that HAV protein 2C is a potential target for antivirals, and provide novel insights into the development of drugs for the treatment of hepatitis A.

An anti-picornaviral strategy based on the crystal structure of foot-and-mouth disease virus 2C protein

The foot-and-mouth disease virus (FMDV) 2C protein shares conserved motifs with enterovirus 2Cs despite low sequence identity. Here, we determine the crystal structure of an FMDV 2C fragment to 1.83 Å resolution, which comprises an ATPase domain, a region equivalent to the enterovirus 2C zinc-finger (ZFER), and a C-terminal domain harboring a loop (PBL) that occupies a hydrophobic cleft (Pocket) in an adjacent 2C molecule. Mutations at ZFER, PBL, and Pocket affect FMDV 2C ATPase activity and are lethal to FMDV infectious clones. Because the PBL-Pocket interaction between FMDV 2C molecules is essential for its functions, we design an anti-FMDV peptide derived from PBL (PBL-peptide). PBL-peptide inhibits FMDV 2C ATPase activity, binds FMDV 2C with nanomolar affinity, and disrupts FMDV 2C oligomerization. FMDV 2C targets lipid droplets (LDs) and induces LD clustering in cells, and PBL-peptide disrupts FMDV 2C-induced LD clustering. Finally, we demonstrate that PBL-peptide exhibits anti-FMDV activity in cells.