Functional Characterization of a Novel Thioredoxin Domain-Containing Protein of the Malaria Parasite Plasmodium

A novel thioredoxin domain-containing protein of the malaria parasite Plasmodium was identified and found to be conserved among eukaryotes. This protein belongs to the phosducin-like family of proteins (PhLPs), and was therefore assigned the name PhLP1, since it is the first phosducin-like protein to be identified in Plasmodium. PhLPs have been found to have various roles in G-protein signaling, cell cycle progression, and protein folding. However, the biochemical mechanism by which PhLPs perform their function is unknown. Here is described the cloning and biochemical characterization of both PhLP1 and its human homolog TXNDC9. Both purified PhLP1 and TXNDC9 showed enzymatic activity in the insulin reduction assay and were also active in the thioredoxin-couple reduction assay. Sequence alignment and homologous modeling of PhLP1 and TXNDC9 indicated a conserved, putative atypical active site in place of the typical catalytic CXXC motif found in classical redox active thioredoxins. Site-directed mutagenesis of the putative redox active cysteine (C106) in PhLP1 abolished the redox functions of the protein, confirming the role of C106 in the catalytic mechanisms of the protein. These results show for the first time that PhLPs are redox active enzymes that can be efficiently reduced by the thioredoxin system. These findings shed new light on the biochemical mechanism and biological function of these highly conserved proteins