The N-terminal domain of NS3 of hepatitis C virus (HCV) possesses serine protease activity, which is essential for virus replication. This portion is also implicated in malignant transformation of hepatocytes. We previously demonstrated that an N-terminal portion of NS3 formed a complex with the tumor suppressor p53 and suppressed actinomycin D-induced apoptosis. We report here that single-point mutations of NS3 at position 106 from Leu to Ala (L106A), and position 43 from Phe to Ala (F43A) to a lesser extent, significantly impaired complex formation with p53. Moreover, the L106A Mutation impaired an otherwise more distinct anti-apoptotic activity of NS3. F43A and L106A mutations also inhibited serine protease activity of NS3. These results collectively suggest the possibility that Leu(106) and Phe(43) are involved in p53 interaction and serine protease activity, and therefore, can be a good target for certain low-molecular-weight compound(s) to inhibit both oncogenic and replicative abilities of HCV. (c) 2005 Elsevier Inc. All rights reserved.