Two Plasmodium vivax hypnozoite-expressed RNA-binding proteins inhibit liver stage replication

Plasmodium vivax (Pv) forms non-replicating liver stages called hypnozoites, which activate after primary infection, and cause relapses of symptomatic blood-stage malaria. We hypothesized that hypnozoites must actively suppress schizogony to maintain a quiescent state. Differential transcriptome prospecting idenfied two hypnozoite-expressed transcripts encoding putative RNA-binding proteins. We assessed the functional role of the two encoded proteins in Plasmodium yoelii (Py), a rodent malaria parasite that naturally does not form hypnozoites. Strikingly, individual expression of each protein in Py liver stages blocked liver stage schizogony, with parasites remaining small and uninucleate. A screen of RNA sequences that interact with the putative RNA-binding domains of these proteins showed enrichment of distinct, highly specific motifs, indicating that these protein might block schizogony by binding RNAs containing these motifs. Our findings provide the unprecedented functional evidence for one potential molecular mechanism of hypnozoite formation. Based on their function we named these proteins IESI-1 and IESI-2 (Initiation of Exo-erythrocytic Schizogony inhibited).