Membrane orientation and antigenic peptides of an immunoprotective 74 kDa Plasmodium knowlesi glycoprotein

Vaccination trials have shown that a purified, 74 kDa glycoprotein, GP74, isolated from the host cell membrane of Plasmodium knowlesi-infected rhesus erythrocytes, can provide protective immunity against P. knowlesi malaria. We have extended this work by a tryptic peptide analysis of the disposition of GP74 in the host cell membrane. Of the 18 peptides characterized by high-performance liquid chromatography only four were accessible to lactoperoxidase-catalyzed radioiodination of non-leaky, schizont-infected host cells from the extracellular space. Metabolic labeling with radioactive glucosamine indicates that two of the surface exposed peptides are glycopeptides, and one of these, peptide 12 appears to carry a dominant antigenic site, according to its reactivity with immunoglobulin from sera of monkeys protected against P. knowlesi malaria.