Sulfur Mobilization for Fe-S Cluster Assembly by the Essential SUF Pathway in the Plasmodium falciparum Apicoplast and Its Inhibition

Abstract

ABSTRACTThe plastid of the malaria parasite, the apicoplast, is essential for parasite survival. It houses several pathways of bacterial origin that are considered attractive sites for drug intervention. Among these is the sulfur mobilization (SUF) pathway of Fe-S cluster biogenesis. Although the SUF pathway is essential for apicoplast maintenance and parasite survival, there has been limited biochemical investigation of its components and inhibitors ofPlasmodiumSUFs have not been identified. We report the characterization of two proteins,Plasmodium falciparumSufS (PfSufS) andPfSufE, that mobilize sulfur in the first step of Fe-S cluster assembly and confirm their exclusive localization to the apicoplast. The cysteine desulfurase activity ofPfSufS is greatly enhanced byPfSufE, and thePfSufS-PfSufE complex is detectedin vivo. Structural modeling of the complex reveals proximal positioning of conserved cysteine residues of the two proteins that would allow sulfide transfer from the PLP (pyridoxal phosphate) cofactor-bound active site ofPfSufS. Sulfide release from thel-cysteine substrate catalyzed byPfSufS is inhibited by the PLP inhibitord-cycloserine, which forms an adduct withPfSufS-bound PLP.d-Cycloserine is also inimical to parasite growth, with a 50% inhibitory concentration close to that reported forMycobacterium tuberculosis, against which the drug is in clinical use. Our results establish the function of two proteins that mediate sulfur mobilization, the first step in the apicoplast SUF pathway, and provide a rationale for drug design based on inactivation of the PLP cofactor ofPfSufS.