A divergent nonsense-mediated decay machinery in Plasmodium falciparum is inefficient and non-essential

Abstract

Nonsense-mediated decay (NMD) is a conserved mRNA quality control process that eliminates transcripts bearing a premature termination codon. In addition to its role in removing erroneous transcripts, NMD is involved in post-transcriptional regulation of gene expression via programmed intron retention in metazoans. The apicomplexan parasite Plasmodium falciparum shows relatively high levels of intron retention, but it is unclear whether these variant transcripts are functional targets of NMD. In this study, we use CRISPR-Cas9 to disrupt and epitope-tag two core NMD components: PfUPF1 (PF3D7_1005500) and PfUPF2 (PF3D7_0925800). Using RNA-seq, we find that NMD in P. falciparum is highly derived and requires UPF2, but not UPF1 for transcript degradation. Furthermore, our work suggests that the majority of intron retention in P. falciparum has no functional role and that NMD is not required for parasite growth ex vivo. We localise both PfUPF1 and PfUPF2 to puncta within the parasite cytoplasm, which may represent processing bodies - ribonucleoparticles that are sites of cytoplasmic mRNA decay. Finally, we identify a number of mRNA-binding proteins that co-immunoprecipitate with the NMD core complex and propose a model for a divergent NMD that does not require PfUPF1 and incorporates novel accessory proteins to elicit mRNA decay.