The Adaptive Proline Response in P. falciparum Is Independent of PfeIK1 and eIF2α Signaling.

We have previously identified the cytoplasmic prolyl tRNA synthetase in Plasmodium falciparum as the functional target of the natural product febrifugine and its synthetic analogue halofuginone (HFG), one of the most potent antimalarials discovered to date. However, our studies also discovered that short-term treatment of asexual blood stage P. falciparum with HFG analogues causes a 20-fold increase in intracellular proline, termed the adaptive proline response (APR), which renders parasites tolerant to HFG. This novel resistance phenotype lacks an apparent genetic basis but remains stable after drug withdrawal. On the basis of our findings that HFG treatment induces eIF2α phosphorylation, a sensitive marker and mediator of cellular stress, we here investigate if eIF2α-signaling is functionally linked to the APR. In our comparative studies using a parasite line lacking PfeIK1, the Plasmodium orthologue of the eIF2α-kinase GCN2 that mediates amino acid deprivation sensing, we show that HFG activity and the APR are independent from PfeIK1 and eIF2α signaling.