Characterization of a high-molecular-weight phosphoprotein synthesized by the human malarial parasite Plasmodium falciparum

During its intra-erythrocytic cycle. Plasmodium falciparum synthesizes a protein of apparent M r 250000—300000. Its precise size is dependent on the P. falciparum isolate examined. This protein contains phosphate covalently bound to one or more serine residues and hence is termed PP300. Monoclonal antibody, McAb4-1F, binds to PP300 on immunoblots of protein extracts from all parasite isolates tested, both those exhibiting and those lacking the knob phenotype. Using McAb4-lF, the polypeptide was shown to be physically associated with the plasma membrane in a membrane-isolation procedure. However, in an indirect immuno-fluorescence assay the McAb appeared to bind to antigen associated with the erythrocyte plasma membrane in parasitized cells. However, it reacted only to fixed, not unfixed, parasitized erythrocytes indicating that the epitope is not normally exposed to extracellular antibodies. Clone 29-2 was isolated by a McAb4-lF immunoscreen of a P. falciparum complementary DNA (cDNA) expression library created in pUC8. Rat anti-clone serum which was raised to the purified protein encoded by the lacZ-29-2 fusion in pUC8 reacted with PP300 in immunoblots of parasite antigen. In Southern-blot analyses of parasite DNA digested with EcoRI, HinddIII, or EcoRV, the 29-2 DNA insert hybridized to more than one fragment even though the insert lacked internal sites for these enzymes. In addition, hybridization studies were conducted using two oligodeoxy-nucleotides which were constructed based on the sequence of a cDNA clone which encoded part of a similar high-molecular-weight P. falciparum protein [Coppel et al., Mol. Biochem. Parasitol. 20 (1986) 265–277]. Analysis of these results indicates that the two cDNA sequences are parts of the same gene or a family of related genes.