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The N-terminal region of the hepatitis-C virus (HCV) core protein is rich in basic residues, while the C-terminal end of the protein comprises of a stretch of hydrophobic amino acids. Between these two extremes is an amphipathic region with two predicted α-helical segments. This region embodies Leu or hydrophobic residues in positions of heptad repeats and is possibly capable of self-association. To investigate this possibility, the core sequence was divided into two fragments and expressed separately as recombinant proteins. Recombinant proteins with the N-terminal fragment remained as monomers even at high concentrations in SDS/PAGE. Recombinant protein with the C-terminal fragment appeared largely monomeric on denaturing gels but some oligomers were also detected. Furthermore, proline mutations in either one of the predicted α helices adversely affected the observed oligomerization. The self-association capacity of the core protein C-terminal region was further supported by results from a yeast two-hybrid system. To affirm our conclusion, a peptide covering the heptad repeats and the predicted α helices was synthesized. Data from mass spectrometry and gel-filtration chromatography concluded that this peptide readily self-associated into the homodimer. Therefore, our results suggest that the oligomerization motifs of the HCV core protein may not be limited to the previously suggested N-terminal region. [ABSTRACT FROM AUTHOR]