Background: Plasmodium falciparum P2 (PfP2) protein plays nonribosomal roles through SDS-and DTT-resistant oligomerization. Results: For SDS-and DTT-sensitive oligomerization, the 53rd cysteine of PfP2 plays an important role. Conclusion: DTT-and SDS-resistant oligomerization of PfP2 was propagated by differentially expressed parasite proteins. Significance: Analysis of regulation of PfP2 oligomerization in parasite-infected erythrocytes may help in understanding the export of P2 to erythrocyte surface.

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The P2 protein (equivalent of L7/L12 in prokaryotes), a member of the ribosomal stalk in eukaryotes, is highly conserved, particularly its C-terminal domain. In order to understand the sequence-structure-function relationships in eukaryotic C-terminal stretches, about which nothing is known at the moment, we have investigated here, the structural characteristics of these domains of P2 proteins from three different species, namely, human, Plasmodium falciparum, and Toxoplasma gondii; the sequence homology among these is 70% although sequence identity is only 36%. About 50 amino acids of the C-terminal domains of P2 from the three species were expressed and purified. Gel filtration studies indicated peaks for both monomer and oligomer at milimolar concentrations and also suggested monomer-multimer equilibrium. Circular Dichroism showed that this domain does not have stable secondary structures. (1)H-(15)N HSQC spectra in every case showed one set of requisite number of peaks as per the sequence. This indicated that there is rapid multimer-monomer equilibrium in solution and the observed peaks which originate from the monomer reflect average chemical shifts. The spectral dispersion in all the cases is narrow, although there are noticeable differences in the three proteins. Detailed NMR investigations revealed that this protein domain is intrinsically disordered although there are short segments with preferred secondary structural propensities at similar places along the sequence. This may suggest that the sequence is selected in evolution to impart disorder, and thereby accord conformational adaptability.

Passive immunization with antibodies to recombinant Plasmodium falciparum P0 riboprotein (rPfP0, 61–316 amino acids) provides protection against malaria. Carboxy-terminal 16 amino acids of the protein (PfP0C0) are conserved and show 69% identity to human and mouse P0. Antibodies to this domain are found in 10–15% of systemic lupus erythematosus patients. We probed the nature of humoral response to PfP0C0 by repeatedly immunizing mice with rPfP0. We failed to raise stable anti-PfP0C0 hybridomas from any of the 21 mice. The average serum anti-PfP0C0 titer remained low (5.1 ± 1.3 × 104). Pathological changes were observed in the mice after seven boosts. Adsorption with dinitrophenyl hapten revealed that the anti-PfP0C0 response was largely polyreactive. This polyreactivity was distributed across all isotypes. Similar polyreactive responses to PfP0 and PfP0C0 were observed in sera from malaria patients. Our data suggests that PfP0 induces a deviant humoral response, and this may contribute to immune evasion mechanisms of the parasite.