Is an RNA-dependent RNA polymerase active in viral RNA replication. Is a methyltransferase active in RNA capping and an RNA helicase. Methyltransferase displays a cytoplasmic capping enzyme activity. This function is necessary since all viral RNAs are synthesized in the cytoplasm, and host capping enzymes are restricted to the nucleus. Helicase region probably exhibits NTPase and RNA unwinding activities. Contains a cysteine protease activity responsible for autocatalytic processing of the 237 kDa protein (Potential). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) ATP + H2O = ADP + H(+) + phosphate Autocatalytically cleaved by the cysteine protease. Belongs to the ssRNA positive-strand viruses RNA-directed RNA polymerase family.