Chaperonin-like repeats in a 34-kDa Plasmodium berghei phosphoprotein

A Plasmodium homologue of cochaperone p23 and its differential expression during the replicative cycle of the malaria parasite

The complete gene sequence of a major phosphoprotein from the malaria parasite reveals that it is a homologue to cochaperone p23. This p23 homologue is highly conserved between Plasmodium falciparum and other malaria parasites and exhibits 44% sequence identity with the Schizosaccharomyces pombe p23 homologue. The Plasmodium p23 is a relatively abundant cytoplasmic protein with a molecular mass of 34-36 kDa depending on species. Expression of this 34 kDa protein and its mRNA commences in the early ring stage and continues throughout the trophozoite stage. At the beginning of schizogony there is a decrease in the transcription and translation rates and a decline in the amount of the 34 kDa protein. The exact role of the 34 kDa phosphoprotein in parasite replication and differentiation is not known, but the Plasmodium p23 homologue may play a role in parasite proliferation and differentiation through its interactions with protein kinases and other chaperones.

Evidence for epitope-specific thymus-independent response against a repeat sequence in a protein antigen

We have studied the immunogenicity of a recombinant heat-shock protein-related stress protein of 70,000 MW (Pfhsp) of the human malaria parasite Plasmodium falciparum in H-2 congenic and non-congenic strains of inbred and outbred mice. Most mice of different genetic backgrounds produced antibodies to Pfhsp, indicating a lack of any apparent genetic restriction of immune response. A peptide corresponding to the GGMP repeat sequence in the C-terminal region of Pfhsp was recognized by more than 75% of sera from immunized mice. The GGMP repeat epitope-specific antibodies were largely of the IgM isotype, especially in all seven inbred strains of mice tested. The lack of significant boosting of the immune response, a predominantly IgM isotype of antibodies and generation of antibody responses in athymic nude mice suggest a thymus-independent response against the GGMP repeat epitope in the Pfhsp molecule.

Further characterization of a 58 kDa Plasmodium berghei phosphoprotein as a cochaperone

Molecular chaperones are important for proper protein folding during protein biogenesis. This report describes a protein from Plasmodium berghei which is 30% identical and 40% similar to a recently described mammalian cochaperone, or heat shock protein 70 interacting protein. The P. berghei cochaperone accumulates throughout the trophozoite stage and decreases during the schizont stage. The stage specific expression is consistent with its presumed role in protein folding or protein-protein interactions. The largest difference between the Plasmodium and mammalian sequences is a more extensive domain of imperfect glycine-glycine-methionine-proline (GGMP) tandem repeats in the parasite's cochaperone sequence. Immunofluorescence studies show that the protein is an abundant cytosolic protein of the parasite. However, antibodies raised against the GGMP repeat domain, which is also found in other parasite chaperones, react with both the parasite and host erythrocyte membrane. The reactivity with the host membrane suggests that the parasite exports molecular chaperones into the infected erythrocyte.

Proteolysis of a 34 kDa phosphoprotein coincident with a decrease in protein kinase activity during the erythrocytic schizont stage of the malaria parasite

Protein phosphorylation events may play important roles in the replication and differentiation of the malarial parasite. Investigations into the lability of a Plasmodium protein kinase revealed that a 34 kDa parasite phosphoprotein is rapidly converted into a 19 kDa fragment. Coincident with this conversion is a nearly total loss of a protein kinase activity, as determined from the phosphorylation of endogenous protein substrates. Both the conversion of the 34 kDa protein to the 19 kDa protein and the loss of protein kinase activity are inhibited by thio-protease inhibitors. The presence of low levels of the intact 34 kDa protein restores the protein kinase activity to almost maximum levels. However, it was not possible to demonstrate protein kinase activity associated with the 34 kDa protein, thus suggesting that the 34 kDa protein is probably an activator or regulator of the protein kinase activity and not a protein kinase. The conversion to the 19 kDa fragment also occurs in vivo and only during the schizont stage prior to the appearance of ring forms. During this same period the protein kinase activity decreases suggesting that the proteolytic processing of the 34 kDa protein may be a physiological regulator of the protein kinase.