Plasmodium falciparum Raf kinase inhibitor is a lipid binding protein that interacts with CDPK1 and regulates its activity in asexual blood stage

Raf Kinase Inhibitor Protein (RKIP) is an important regulator of MAPK signaling pathway in multicellular eukaryotes. Plasmodium falciparum RKIP (PfRKIP) is a putative phosphatidylethanolamine binding protein (PEBP) that shares limited similarity with Homo sapiens RKIP (HsRKIP). Interestingly, critical components of MAPK pathway are not expressed in malaria parasite and the physiological function of PfRKIP remains unknown. PfRKIP is expressed throughout the asexual schizogony with maximum expression in late schizonts. Interestingly, PfRKIP and HsRKIP show pH dependent differential interaction profiles with various lipids. At physiological pH, PfRKIP show interaction with PE and lipids containing phosphorylated phosphatidylinositol group; however, HsRKIP show no interaction under the same conditions. Mutation of conserved residues in the PEBP domain of PfRKIP decreases its interaction with PI(3)P. Furthermore, our results suggest that PfRKIP leads to increase in the autophosphorylation of PfCDPK1 that leads to transphosphorylation of substrates by PfCDPK1. Using various in vitro and in vivo experiments we have demonstrated the interaction of PfRKIP with PfCDPK1 and have also identified key residues in PfRKIP that play important role in this interaction. Interestingly, locostatin, a specific inhibitor of mammalian RKIP increased the interaction of PfRKIP with PfCDPK1 that perhaps leads to the sequestration of PfCDPK1 in a heterodimeric complex. Importantly, treatment of malaria parasite with locostatin shows dose dependent inhibition of parasite growth. This study suggests that specific inhibitors that modify PfRKIP leading to increase in its interaction with PfCDPK1 may be designed and explored as novel anti-malarial compounds to inhibit malaria parasite growth.