The Mechanism Of Semliki Forest Virus Evading Antiviral RNAi Immunity

RNAi is an evolutionarily conserved post-transcriptional gene silencing mechanism,which has been well considered as an antiviral immunity in fungi,plants,invertebrates and mammal.When virusus infect cells,the dsRNA replicated intermediates that are generated during their replication can be recognized and processed by Dicer into virus-derived siRNAs(vsiRNAs).Then these vsiRNAs are loaded into RNA-induced silencing complex(RISC),directing the degradation of the homologous viral RNAs and leading to the clearance of virus.However,in the course of long-term evolution,multiple viruses have evolved immune evasion mechanism to antagonize antiviral RNAi by encoding viral suppressors of RNA silencing(VSRs).Semliki forest virus(SFV)belongs to the genus Alphavirus in the family Togaviridae which is transmitted by mosquitoes.Because of its weak pathogenicity,it is a model virus of this genus.Other Alphavirus include Sindbis virus(SINV),Chikungunya virus(CHIKV)and so on.In our study,we used SFV to infect Aedes mosquito Aag2 cells and human 293 T cells,and found that Core protein of SFV could inhibit RNAi in vitro.In addition,the in vitro experiments showed that Core protein could inhibit RNAi pathway by binding dsRNA and siRNA,and at the same time protect dsRNA from Dicer cleavage.When the key residues K124/K128 and K139/K142 were mutated,the Core protein almost lost the ability to bind dsRNA and siRNA,and the replication ability of the mutant virus was also attenuated.Together,our studies uncovered a novel alphaviral VSR that inhibits RNAi mainly by binding dsRNA and siRNA.