Malaria Proteins Implicated in Host-Parasite Interactions
Anderson, Laura Fay
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The invasive and transmission stages of the malaria parasite Plasmodium falciparum express several proteins with domains implicated in host-parasite interactions, that are potential vaccine candidates or drug targets. The expression patterns of two proteins PfTRAMP (Plasmodium Related Apical Merozoite Protein) and PCRAGS (Plasmodium cysteine related antigen of gametocytes and schizonts), containing such putative domains, are examined and their potential roles in merozoite invasion and egress are discussed. PfTRAMP has a possible role in merozoite invasion. Transcription occurs in mature schizonts as shown by Northern blot. Recombinant protein was successfully expressed in insect cells indicating that this eukaryotic system can be utilised for the expression of Plasmodium proteins. PCRAGS is a member of the Plasmodium Cysteine Repeat Modular Proteins (PCRMPs), a family of high molecular weight proteins with highly conserved, cysteine-rich regions and multipass transmembrane domains. The gene encodes a signal sequence, a truncated extracellular domain, an EGF-like domain and a multipass-transmembrane domain. PCRAGS is highly conserved in Plasmodium spp and other Apicomplexa. The extracellular domain has been under purifying selection, suggesting that the sequence or the structure of this domain is important for function. The gene is transcribed throughout the asexual erythrocytic cycle and is expressed in both gametocytes and schizonts in P. falciparum and in the rodent malaria P.berghei. Antibodies raised against a short peptide in the C-terminus detect PfCRAGS during schizont rupture and in mature segmenting schizonts but not in merozoites. Western blotting showed that PfCRAGS is present in the membrane fraction. Co-localisation studies showed that PfCRAGS is associated with the infected erythrocyte membrane, suggesting a role in merozoite egress. PfCRAGS is also expressed in stage II-IV male gametocytes in association with a membrane and is the earliest known male specific protein expressed. Gene knock-out of pbcrags in P. berghei showed that PbCRAGS is not essential for asexual development. In vivo evaluation of phenotype showed that pbcrags knock-out parasites are less virulent than wildtype parasites and have an increased gametocyte production in a non-susceptible host. The unique expression and localisation pattern of PfCRAGS in combination with putative host-parasite binding domains implicate this novel protein as a potential vaccine candidate or drug target.