Putative domain-domain interactions in the vesicular stomatitis virus L polymerase protein appendage region

The multidomain polymerase protein (L) of nonsegmented negative-strand (NNS) RNA viruses catalyzes transcription and replication of the virus genome. The N-terminal half of the protein forms a ring-like polymerase structure, while the C-terminal half encoding viral mRNA transcript modifications consists of a flexible appendage with three distinct globular domains. To gain insight into putative transient interactions between L domains during viral RNA synthesis, we exchanged each of the four distinct regions encompassing the appendage region of vesicular stomatitis virus (VSV) Indiana serotype L protein with their counterparts from VSV New Jersey and analyzed effects on virus polymerase activity in a minigenome system. The methyltransferase domain exchange yielded a fully active polymerase protein, which functioned as well as wild-type L in the context of a recombinant virus. Exchange of the downstream C-terminal nonconserved region abolished activity, but coexchanging it with the methyltransferase domain generated a polymerase favoring replicase over transcriptase activity, providing strong evidence of interaction between these two regions. Exchange of the capping enzyme domain or the adjacent nonconserved region thought to function as an "unstructured" linker also abrogated polymerase activity even when either domain was coexchanged with other appendage domains. Further probing of the putative linker segment using in-frame enhanced green fluorescent protein (EGFP) insertions similarly abrogated activity. We discuss the implications of these findings with regard to L protein appendage domain structure and putative domain-domain interactions required for polymerase function.

NNS viruses include many well-known human pathogens (e.g., rabies, measles, and Ebola viruses), as well as emerging viral threats (e.g., Nipah and Hendra viruses). These viruses all encode a large L polymerase protein similarly organized into multiple domains that work in concert to enable virus genome transcription and replication. But how the unique L protein carries out the multiplicity of individual steps in these two distinct processes is poorly understood. Using two different approaches, i.e., exchanging individual domains in the C-terminal appendage region of the protein between two closely related VSV serotypes and inserting unrelated protein domains, we shed light on requirements for domain-domain interactions and domain contiguity in polymerase function. These findings further our understanding of the conformational dynamics of NNS L polymerase proteins, which play an essential role in the pathogenic properties of these viruses and represent attractive targets for the development of antiviral measures.