Chikungunya virus nonstructural protein 1 is a versatile RNA capping and decapping enzyme

Abstract

Chikungunya virus (CHIKV) non-structural protein 1 (nsP1) contains both the N7-guanine methyltransferase and guanylyltransferase activities and catalyzes the 5’ end cap formation of viral RNAs. To further understand its catalytic activity and role in virus-host interaction, we demonstrate that purified recombinant CHIKV nsP1 can reverse the guanylyl transfer reaction and remove the m7GMP from a variety of capped RNA substrates including host mRNAs. We then provide the structural basis of this function with a high-resolution cryogenic-electron microscopy structure of nsP1 in complex with the unconventional cap-1 substrate RNA m7GpppAmU. We show that the 5’ppRNA species generated by decapping can trigger RIG-I-mediated interferon (IFN) response. We further demonstrate that the decapping activity is conserved among the alphaviral nsP1s. To our knowledge, this is a new mechanism through which alphaviruses activate the antiviral immune response. This decapping activity could promote cellular mRNA degradation and facilitate viral gene expression which is functionally analogous to the cap-snatching mechanism by influenza virus.