The paradigm for viral transcription initiation involving cap snatching is based on orthomyxovirus, influenza, and posits that heterotrimeric viral RNA dependent RNA polymerase (RdRp) acquires 5'caps from cellular mRNAs through the endonuclease activity of influenza RdRp. This general cap snatching mechanism has been assumed for all minus stranded segmented RNA viruses including the bunyaviruses and arenaviruses, although their RdRps are structurally different and they replicate at different locations inside the host cell. Since viral RdRp uses capped oligoribonucleotides as primers during transcription initiation, viruses have to effectively compete with the cellular mRNA decapping machinery to preserve these mRNA caps for the efficient synthesis of viral mRNAs. Our preliminary studies show that hantavirus nucleocapsid protein (N) binds with high affinity to the 5'caps of cellular mRNAs and protects their degradation from 5'termini. The protected 5'capped oligos of at least 180 nucleotides in length are stored in cellular P bodies by N. These capped oligos sequestered in P bodies are efficiently used as primers by replicating Sin Nombre hantavirus for the synthesis of viral mRNA. Since the length of RNA primers used by viral RdRp during transcription initiation varies from 6-18 nucleotides, further trimming of sequestered capped oligos in P bodies must take place for the generation of capped RNA primer of appropriate length and specificity. We hypothesize that endonuclease activity required for such trimming resides in viral RdRp. Since N binds the mRNA caps with high affinity, it is possible that N works in conjunction with viral RdRp in generating the RNA primer. We also hypothesize that sequestered capped oligos in P bodies are selected on the basis of nucleotide sequence in the vicinity of 5'cap to serve as primers for the initiation of viral mRNA synthesis. It is highly likely that hantavirus replication also takes place in P bodies. Multifaceted experimental approaches have been designed to check these hypotheses. We have strong preliminary data showing that N has distinct cap binding and RNA binding domains. Hence identification and characterization of cap binding domain of N is warranted. These studies will help in understanding the molecular mechanism of bunya and arenavirus pathogenesis with a focus on cap snatching mechanism of transcription initiation. In future these studies will also help in the identification of targets for the antiviral therapeutics and design of future potential antiviral agents for diseases caused by negative stranded RNA viruses which use cap snatching mechanism for the initiation of viral mRNA synthesis.
Hantavirus cardio pulmonary syndrome (HPS) caused by rodent borne category A virus has a mortality of fifty percent. The studies on """"""""cap snatching"""""""" will help in understanding the mechanism of hantavirus pathogenesis, identification of targets for the antiviral therapy and design of future potential antiviral agents for the treatments of HPS.
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