Plasmodium falciparum: molecular analysis of a putative protective antigen, the thermostable 96-kDa protein.
A group of three Plasmodium falciparum antigens of distinct pI, migrating with an apparent MW of 96 kDa has been previously identified as a target of protective immunity both in humans and in monkeys (Jouin et al. 1987, Dubois et al. 1987). These antigens are produced during the late stages of asexual intraerythrocytic development. One of these 96-kDa proteins, the 96 tR, has a pI of 5.25, is thermostable, and is released in the culture supernatant (Jouin et al. 1987). We report here the cloning and expression in Escherichia coli of the gene coding for this antigen. Antibodies raised to the recombinant 96 tR immunoprecipitated exclusively the 96 tR, indicating that the other two antigens of 96 kDa are the product(s) of distinct gene(s). Northern and Southern blots as well as DNA sequencing of the gene showed that the 96 tR antigen is identical to proteins identified in other laboratories as the glycophorin binding protein GBP 130 (Perkins 1984, Ravetch et al. 1985) and Ag 78 (Bianco et al. 1987). The 96-kDa antigen is produced at the trophozoite stage and more actively in the schizonts. It is released in the culture supernatant at the time of schizont rupture, together with two minor products, forming a characteristic triplet. This triplet was also detected in immunoblots of merozoites. An approximate quantification on immunoblots indicated that the largest proportion of the protein is found in the culture supernatant, a minor fraction being loosely associated with merozoites. By immunofluorescence and immunoelectron microscopy, intense signals were observed in the erythrocyte cytoplasm. The 50-amino acid repeats were found in all strains examined, the protein showing some size polymorphism. The antigen was detected in the serum of infected monkeys as well as in that of infected humans.