Multifunctional protein that contains an N-terminal protease and a C-terminal helicase, playing essential roles in viral polyprotein processing and viral genome replication. The chymotrypsin-like serine protease activity utilizes NS4A as an essential cofactor and catalyzes the cleavage of the polyprotein leading to the release of NS4A, NS4B, NS5A, and NS5B. Interacts with NS5B to enhance RNA-dependent RNA polymerase activity