The circumsporozoite antigen of the simian malarial parasite,Plqsmodium knowlesi, consists of tandemly repeated immunodominant peptide units which may play a role in evading the immune system. To study the immunogenicity of this antigen in the absence of the immunodominant repeats, the whole of the non-repetitive region of this antigen has been expressed inEscherichia coli. The entire amino-terminal region up to the start of the repeats, and the full non-repetitive carboxyl region starting from the end of the repeats up to the termination codon, have been expressed separately, as fusion proteins with a 26 kD glutathione-S-transferase protein ofSchistosomq japonicum. A repeat-less truncated antigen has also been expressed as the same fusion protein. The amino-terminal fusion protein (GST-CSN), is a soluble protein of a molecular weight of 38 kD, which could be purified by affinity chromatography on immobilized glutathione. The carboxylterminal fusion protein (GST-CSC), is insoluble, migrates with an anomalous molecular weight of 32 kD, and binds to the affinity matrix weakly. The truncated repeat-less fusion protein (GST-CSNC) is also, an insoluble protein of molecular weight of 48 kD. Unlike the two separate domains, GST-CSNC is an extremely unstable protein inEscherichia coli.

The ribosomal phosphoprotein P0 of the human malarial parasitePlasmodium falciparum (PfP0) has been identified as a protective surface protein. InDrosophila, P0 protein functions in the nucleus. The ribosomal function of P0 is mediated at the stalk of the large ribosomal subunit at the GTPase centre, where the elongation factor eEF2 binds. The multiple roles of the P0 protein presumably occur through interactions with other proteins. To identify such interacting protein domains, a yeast two-hybrid screen was carried out. Out of a set of sixty clones isolated, twelve clones that interacted strongly with both PfP0 and theSaccharomyces cerevisiae P0 (ScP0) protein were analysed. These belonged to three broad classes: namely (i) ribosomal proteins; (ii) proteins involved in nucleotide binding; and (iii) hypothetical integral membrane proteins. One of the strongest interactors (clone 67B) mapped to the gene YFL034W which codes for a hypothetical integral membrane protein, and is conserved amongst several eukaryotic organisms. The insert of clone 67B was expressed as a recombinant protein, and immunoprecipitaion (IP) reaction with anti-P0 antibodies pulled down this protein along with PfP0 as well as ScP0 protein. Using deletion constructions, the domain of ScP0, which interacted with clone 67B, was mapped to 60–148 amino acids. It is envisaged that the surface localization of P0 protein may be mediated through interactions with putative YFL034W-like proteins inP. falciparum