The expression and processing of the Hepatitis C virus core protein (HCcAg) were analyzed in the methylotrophic yeast Pichia pastoris. Two proteins with 21 (p21) and 23 kDa (p23) were detected in immunoblot with a serum from a chronic carrier patient, as the major products for HCcAg. Both proteins, p21 and p23, produced by proteolytic processing in P. partoris, share the same N-terminal region and reacted with a monoclonal antibody towards the first 35 amino acids of HCcAg. The proteolytic processing of the recombinant polypeptide, having the HCcAg and the first 148 as of El protein, was also confirmed by immunoblot analysis using mAbs with HCcAg and E1 specificities, respectively. The 32 kDa glycosilated El protein was then immuno-identified, as well as the processed HCcAg. These data demonstrated the usefulness of P. pastoris, as expression system, to study the processing of HCV structural proteins. (C) 2002 Elsevier Science (USA). All rights reserved.