Development of heterologous systems to produce useful HCV vaccine candidates is an important part of HCV research. In this study different HCV structural region variants were designed to express the first 120 aa, 176 aa, 339 aa, and 650 aa of HCV polyprotein, and aa 384 to 521, or aa 384-605 or aa 384-746 of HCV E2 protein fused to the leader sequence of sucrose invertase 2 allowing the secretion of recombinant E2 proteins. Low expression levels were observed for HCV core protein (HCcAg) variants expressing the first 120 aa and 176 aa (HCcAg. 120 and HCcAg. 176, respectively). Higher expression levels were observed when HCcAg was expressed as a polypeptide with either El or E1 and E2 proteins. In addition, HCcAg was processed to produce two antigenic bands with 21 and 23 kDa (P21 and P23, respectively) when expressed as a polypeptide with HCV El and E2 proteins. Results also suggest E I processing in the context of HCcAg.E1.E2 polyprotein. On the other hand, E2.521, E2.605, and E2.680 were efficiently excreted to the culture medium. However, the entire E2.746 variant predominantly localized in the insoluble fraction of ruptured cells. Results suggest that the hydrophobic C-terminal E2 region from aa, 681 to 746 is critical for intracellular retention of recombinant E2.746 protein in Pichia pastoris cells. Endo H or PNGase F treatment suggests that E2.746 was modified with high-mannose type oligosaccharides in P. pastoris. These data justify the usefulness of P. pastoris expression system to express HCV structural viral proteins which may be useful targets for HCV vaccine candidates. (c) 2006 Elsevier Inc. All rights reserved.