Nucleosides for the treatment of respiratory RNA virus infections

Jordan, Paul C; Stevens, Sarah K; Deval, Jerome

The core RSV polymerase consists of a 250 kDa large (L) protein of approximately 2000 amino acids and a 27 kDa phosphoprotein (P) that synthesizes an RNA product upon the addition of an RNA template. [59] [60] [61] [62] The P protein is thought to act as a chaperone to aid in the stability and expression of the polymerase. During RNA synthesis, the P protein anchors the L protein to the N protein and also binds to the M2-1 transcription antitermination factor. [63] [64] [65] [66] This matrix protein, M2-1, serves as an elongation factor and is necessary for the polymerase to be fully processive in producing long mRNA products. 67 No structure is available for any L protein from the paramyxoviruses and pneumoviruses, largely due to the size, solubility, and complexities with yielding enough highly pure protein. However, the cryo-electron microscopic (EM) structure of the L-P complex from a highly similar virus, vesicular stomatitis virus (VSV), has recently been solved. 68 VSV is a non-segmented, negative strand virus from the Rhabdoviridae family and given the high sequence conservation of the L protein, the structure of VSV L has provided important structural insights. The L proteins of RSV, VSV, and other related negative sense RNA polymerases can be divided into six CR. 69 The first three regions (CRI-III) of VSV generate a doughnut-like structure in negative stain EM and cryo-EM analysis. 68, 70 The remaining CRs appear as globular appendages on this doughnut. The doughnut structure adopts a classic right-handed configuration in the cryo-EM reconstruction composed of fingers, palm, and thumb domains, like other polymerases (see Figure 3 ). The CRIV and CRV contain conserved residues and catalytic motifs necessary for enzymatic function.