Acts as a cysteine protease, methyltransferase and deubiquitinase (PubMed:22117220, PubMed:23966860, PubMed:29117247, PubMed:32732284) (Probable). The cysteine protease activity cleaves the polyprotein giving rise to mature proteins (PubMed:23966860). The methyltransferase domain is probably involved in viral RNA capping (Probable). The deubiquitylating activity counteracts the degradation of the viral polymerase mediated by the host ubiquitin-proteasome system (PubMed:22117220). The polymerase is thus stabilized and infectivity is increased (PubMed:22117220). Favors K63 poly-Ub linkage (PubMed:23345508). RNA-directed RNA polymerase is responsible for the replication and transcription of the genome. Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Interacts with host ubiquitin. The viral OTU domain (vOTU) is responsible for the deubiquitination activity (PubMed:23345508). Both protease (PRO) and deubiquitination (DUB) activities rely on the single catalytic site of the cysteine proteinase (PubMed:29117247). The switch in the PRO/DUB activities is due to the mobility of a GPP flap (PubMed:29117247). The deubiquitinase activity is low compared to that of Bunyaviruses or coronaviruses. Belongs to the tymovirus NS35 RNA replicase polyprotein family.