The RNA polymerase of influenza virus consists of three subunits: PA, PB1 and PB2. It uses a unique `cap-snatching' mechanism for the transcription of viral mRNAs. The cap-binding domain of the PB2 subunit (PB2cap) in the viral polymerase binds the cap of a host pre-mRNA molecule, while the endonuclease of the PA subunit cleaves the RNA 10-13 nucleotides downstream from the cap. The capped RNA fragment is then used as the primer for viral mRNA transcription. The structure of PB2cap from influenza virus H1N1 A/California/07/2009 and of its complex with the cap analog m(7)GTP were solved at high resolution. Structural changes are observed in the cap-binding site of this new pandemic influenza virus strain, especially the hydrophobic interactions between the ligand and the target protein. m(7)GTP binds deeper in the pocket than some other virus strains, much deeper than the host cap-binding proteins. Analysis of the new H1N1 structures and comparisons with other structures provide new insights into the design of small-molecule inhibitors that will be effective against multiple strains of both type A and type B influenza viruses.

In a negative-strand RNA virus, the genomic RNA is sequestered inside the nucleocapsid when the viral RNA-dependent RNA polymerase uses it as the template for viral RNA synthesis. It must require a conformational change in the nucleocapsid protein (N) to make the RNA accessible to the viral polymerase during this process. The structure of an empty mumps virus (MuV) nucleocapsid-like particle was determined to 10.4-Å resolution by cryo-electron microscopy (cryo-EM) image reconstruction. By modeling the crystal structure of parainfluenza virus 5 into the density, it was shown that the ␣-helix close to the RNA became flexible when RNA was removed. Point mutations in this helix resulted in loss of polymerase activities. Since the core of N is rigid in the nucleocapsid, we suggest that interactions between this region of the mumps virus N and its polymerase, instead of large N domain rotations, lead to exposure of the sequestered genomic RNA. Some pathogens appear to reemerge in spite of available vaccines, such as mumps virus and measles virus (3-5). Effective controls are needed to combat these pathogens. In order to develop more effective countermeasures, the mechanism of NSV replication should be better understood. One of the unique features in NSVs is that the genomic RNA is sequestered in the nucleocapsid (6). During transcription and replication, the viral RNA-dependent RNA polymerase (vRdRp) must be able to gain access to the sequestered genomic RNA in order to use it as the template. For Rhabdoviridae and Paramyxoviridae, the virus encodes a single nucleocapsid protein (N) that polymerizes as a linear capsid to encapsidate the genomic RNA (7). The viral polymerase complex consists of the large protein (L) and the phosphoprotein (P). IMPORTANCE Mumps virus (MuVThe structure of the nucleocapsid or a nucleocapsid-like particle has been solved for several members of Rhabdoviridae and Paramyxoviridae by X-ray crystallography or cryo-electron microscopy (cryo-EM) three-dimensional (3D) reconstruction (8-12). The common features among various structures are that the N protein has an N-terminal domain and C-terminal domain in its core, composed mostly of ␣-helices. When the N subunits assemble into a polymeric capsid, they are aligned in parallel in a linear fashion (13). There are extensive side-by-side interactions between the neighboring domains and domain swaps of extended loops and long termini. The genomic RNA is encapsidated in a cavity formed between the two core domains. Most of the RNA bases are stacked, some of which face the exterior and some the interior of the N protein core. The tight assembly of the nucleocapsid clearly suggests that vRdRp must open the N protein core in order to unveil the genomic RNA. How this action is carried out remains to be discovered. The interaction of the polymerase cofactor P with the nucleocapsid may provide some insights on this subject. The C-terminal domain of vesicular stomatitis virus (VSV) P protein binds between the extended loops in the C-terminal domains ...

In the face of increasing drug resistance and the rapidly increasing necessity for practicality in clinical settings, drugs targeting different viral proteins are needed in order to control influenza A and B. A small molecule that tenaciously adheres to the PB2cap binding domain, part of the heterotrimeric RNA polymerase machinery of influenza A virus and influenza B virus, is a promising drug candidate. Understanding the anatomic behavior of PB2cap upon ligand binding will aid in the development of a more robust inhibitor. In this report, the anatomic behavior of the influenza A virus PB2cap domain is established by solving the crystal structure of native influenza A virus PB2cap at 1.52 Å resolution. By comparing it with the ligand-bound structure, the dissociation and rotation of the ligand-binding domain in PB2cap from the C-terminal domain is identified. This domain movement is present in many PB2cap structures, suggesting its functional relevance for polymerase activity.

X-ray crystallographic structural determinations of the PB2 cap binding domain (PB2cap) have improved the conformational characterization of the RNA-dependent RNA polymerase machinery (PA, PB2, and PB1) of the influenza virus. Geometrically, the catalytic PB1 subunit resembles the palm of a human hand. PA lies near the thumb region, and PB2 lies near the finger region. PB2 binds the cap moiety in the pre-mRNA of the host cell, while the endonuclease of PA cleaves the pre-mRNA 10-13 nucleotides downstream. The truncated RNA piece performs as a primer for PB1 to synthesize the viral mRNA. Precisely targeting PB2cap with a small molecule inhibitor will halt viral proliferation via interference of the cap-snatching activity. Wild-type and mutant PB2cap from A/California/07/2009 H1N1 were expressed in Escherichia coli, purified by nickel affinity and size exclusion chromatography, crystallized, and subjected to X-ray diffraction experiments. The crystal of mutant PB2cap liganded with m 7 GTP was prepared by co-crystallization. Structures were solved by the molecular replacement method, refined, and deposited in the Protein Data Bank (PDB). Structural determination and comparative analyses of these structures revealed the functions of Glu361, Lys376, His357, Phe404, Phe323, Lys339, His432, Asn429, Gln406, and Met401 in PB2cap, and the dissociation of the influenza A PB2cap C-terminal subdomain (residues 446-479) upon ligand binding. Understanding the role of these residues will aid in the ultimate development of a small-molecule inhibitor that binds both Influenza A and B virus PB2cap.