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We recently identified three novel thioredoxin-like genes in the genome of the protozoan parasite  Plasmodium  that belong to the Phosducin-like family of proteins (PhLP). PhLPs are small cytosolic proteins hypothesized to function in G-protein signaling and protein folding. Although PhLPs are highly conserved in eukaryotes from yeast to mammals, only a few representatives have been experimentally characterized to date. In addition, while PhLPs contain a thioredoxin domain, they lack a CXXC motif, a strong indicator for redox activity, and it is unclear whether members of the PhLP family are enzymatically active. Here, we describe PbPhLP-3 as the first phosducin-like protein of a protozoan organism,  Plasmodium berghei . Initial transcription analysis revealed continuous low-level expression of  pbphlp-3  throughout the complex  Plasmodium  life cycle. Attempts to knockout  pbphlp-3  in  P .  berghe i did not yield live parasites, suggesting an essential role for the gene in  Plasmodium . We cloned, expressed and purified PbPhLP-3 and determined that the recombinant protein is redox active  in vitro  in a thioredoxin-coupled redox assay. It also has the capacity to reduce the organic compound  tert-Butyl hydroperoxide  (TBHP)  in vitro , albeit at low efficiency. Sequence analysis, structural modeling, and site-directed mutagenesis revealed a conserved cysteine in the thioredoxin domain to be the redox active residue. Lastly, we provide evidence that recombinant human PhLP-3 exhibits redox activity similar to that of PbPhLP-3 and suggest that redox activity may be conserved in PhLP-3 homologs of other species. Our data provide new insight into the function of PhLP-3, which is hypothesized to act as co-chaperones in the folding and regulation of cytoskeletal proteins. We discuss the potential implications of PhLP-3 as a thioredoxin-target protein and possible links between the cellular redox network and the eukaryotic protein folding machinery.

Characterization of a protozoan Phosducin-like protein-3 (PhLP-3) reveals conserved redox activity