Plasmodium falciparum: variations in the C-terminal cysteine-rich region of the merozoite surface protein-1 in field samples among Indian isolates.

The cysteine-rich C-terminal region of the merozoite surface protein-1, MSP-119, of Plasmodium falciparum has been the most promising vaccine target antigen to date, based on protective immunization studies with recombinant proteins in mice and monkey models. To be further developed as a vaccine candidate, it is essential to study its sequence heterogeneity in field isolates from diverse geographical areas. We have analyzed the DNA sequences encoding the C-terminal region of P. falciparum MSP-1 (1526-1744 aa, corresponding to part of the 16th and all of the 17th blocks) of 16 isolates from different regions in India. The PNG-MAD20 type of MSP-1 sequence predominated in this subcontinent. The MSP-119 region as usual was found to be highly conserved, with amino acid variations at four positions. Based on these variations, only three MSP-119 forms (Q-KNG, E-KNG, and E-TSG, a novel variant) were detected among these isolates. The two MSP-119 variant forms (Q-KNG and E-TSG) were expressed in Escherichia coli as histidine-tagged polypeptides and were characterized immunologically to corroborate the sequence data.