A non-canonical sensing pathway mediates Plasmodium adaptation to AA deficiency

Eukaryotes have canonical pathways for responding to amino acid (AA) availability. Under AA-limiting conditions, the TOR complex is repressed, whereas the sensor kinase GCN2 is activated. While these pathways have been highly conserved throughout evolution, malaria parasites are a rare exception. Despite auxotrophic for most AA, Plasmodium does not have either a TOR complex nor the GCN2-downstream transcription factors. While Ile starvation has been shown to trigger eIF2 phosphorylation and a hibernatory-like response, the overall mechanisms mediating detection and response to AA fluctuation in the absence of such pathways has remained elusive. Here we show that Plasmodium parasites rely on an efficient sensing pathway to respond to AA fluctuations. A phenotypic screen of kinase knockout mutant parasites identified nek4, eIK1 and eIK2 - the last two clustering with the eukaryotic eIF2 kinases - as critical for Plasmodium to sense and respond to distinct AA-limiting conditions. Such AA-sensing pathway is temporally regulated by these kinases at distinct life cycle stages and allows parasites to actively fine-tune replication and development in response to AA availability. Collectively, our data identify a previously unknown set of heterogeneous responses to AA depletion, mediated by a complex mechanism that is critical for modulating parasite cell cycle and survival.