Cysteine protease that cleaves viral polyprotein and specific host proteins. It is responsible for the autocatalytic cleavage between the P1 and P2 regions, which is the first cleavage occurring in the polyprotein. Cleaves also the host translation initiation factor EIF4G1, in order to shut down the capped cellular mRNA translation. Inhibits the host nucleus-cytoplasm protein and RNA trafficking by cleaving host members of the nuclear pores. Counteracts stress granule formation probably by antagonizing its assembly or promoting its dissassembly. Cleaves and inhibits host IFIH1/MDA5, thereby inhibiting the type-I IFN production and the establishment of the antiviral state. Cleaves and inhibits host MAVS, thereby inhibiting the type-I IFN production and the establishment of the antiviral state 