Identification of PfSR1-MT as melatonin-responsive GPCR-like candidate from the human malaria parasite Plasmodium falciparum.

The synchronizing effect of melatonin on the malaria parasite Plasmodium falciparum is well documented, yet its receptor remains elusive. In mammals, melatonin signals through seven-transmembrane (7TM) G protein-coupled receptors. Here, we characterized two putative Plasmodium melatonin receptor candidates, PfSR1-MT and PfSR10, expressed during the asexual parasite stage. We show that PfSR1-MT, but not PfSR10, binds 2-[¹²⁵I]iodomelatonin with high affinity (pM) when heterologously expressed in human cells. Molecular modeling and mutagenesis studies identified the melatonin-binding site within the transmembrane core of PfSR1-MT. Melatonin binding promoted the extracellular domain (ECD) movement of PfSR1-MT and cytosolic Ca²⁺ mobilization. We identified ARL34, an indole compound specifically binding and activating PfSR1-MT, but not human melatonin receptors, and impairing both asexual parasite proliferation and gametocyte development, supporting its potential as a novel antimalarial targeting the PfSR1-MT signaling pathway. Together, these findings identify PfSR1-MT as a melatonin-responsive GPCR-like candidate in the human malaria parasite Plasmodium falciparum.