Molecular characterization of Plasmodium falciparum Bruno/CELF RNA binding proteins

The human malaria parasite Plasmodium falciparum employs intricate post-transcriptional regulatory mechanisms in different stages of its life cycle. Despite the importance of post-transcriptional regulation, key elements of these processes, namely RNA binding proteins (RBPs), are poorly characterized. In this study, the RNA binding properties of P. falciparum proteins were characterized including two putative members of the Bruno/CELF family of RBPs (PfCELF1 and PfCELF2), dihydrofolate reductase-thymidylate synthase (PfDHFR-TS), and adenosine deaminase (PfAda). RNA binding activity was tested using UV-crosslinking and electrophoretic mobility shift assays. PfCELF1 and PfDHFR-TS demonstrated RNA binding activity, whereas PfAda and PfCELF2 were RBP-negative. Intracellular protein localization of RBPs was studied using GFP-tagged transgenic parasite lines. PfCELF1 protein may shuttle between nucleus and cytoplasm, as shown by a predominantly nuclear PfCELF1 cell population and another predominantly cytoplasmic. In contrast, PfDHFR-TS protein is predominantly cytoplasmic. PfCELF1 may thus have several roles, including pre-mRNA processing. The mRNA targets of these P. falciparum proteins were investigated by ribonomics using DNA microarrays. A sequence motif similar to that recognized by CELF proteins in other species is common in the introns of target mRNAs identified for PfCELF1, suggesting that nuclear-localized PfCELF1 may regulate pre-mRNA splicing in P. falciparum, as has been found for CELF proteins in other species. In contrast, none or very few mRNA targets were found for the other proteins, suggesting that they do not have biologically relevant roles as RBPs in the asexual stages of P. falciparum.