title: Insights into the role of the junctional region of Plasmodium falciparum dihydrofolate reductase-thymidylate synthase.
abstract: BACKGROUND
  Plasmodium falciparum dihydrofolate reductase-thymidylate synthase (pfDHFR-TS) is a well-defined target of anti-malarial drug, such as pyrimethamine and cycloguanil. Emergence of malaria parasites resistant to these drugs has been shown to be associated with point mutations of the gene coding for the target enzymes. Although the 3D-structure of P. falciparum bifunctional pfDHFR-TS has been reported previously, relatively little is known about the interactions between the pfDHFR and pfTS domains and the roles of the junctional region that links the two domains together. Therefore, a thorough understanding of the interaction of the two domains and the role of the junctional region of this target is important as the knowledge could assist the development of new effective anti-malarial drugs aimed at overcoming drug-resistant malaria.
  
  
  METHODS
  A system was developed to investigate the interaction between pfDHFR and pfTS domains and the role of the junctional region on the activity of the recombinant pfTS. Based on the ability of co-transformed plasmids coding for pfDHFR and pfTS with truncated junctional region to complement the growth of TS-deficient Escherichia coli strain χ2913recA(DE3) on minimum media without thymidine supplementation, active pfTS mutants with minimal length of junctional region were identified. Interactions between active pfDHFR and the pfTS domains were demonstrated by using a bacterial two-hybrid system.
  
  
  RESULTS
  Using TS-deficient E. coli strain χ2913recA(DE3), the authors have shown for the first time that in P. falciparum a junctional region of at least 44 amino acids or longer was necessary for the pfTS domain to be active for the synthesis of thymidylate for the cells. Truncation of the junctional region of the bifunctional pfDHFR-TS further confirmed the above results, and suggested that a critical length of the junctional peptide of pfDHFR-TS would be essential for the activity of TS to catalyze the synthesis of thymidylate.
  
  
  CONCLUSIONS
  The present study demonstrated the interactions between the pfDHFR and pfTS domains of the bifunctional pfDHFR-TS, and revealed that the junctional region linking the two protein domains is essential for the expression of catalytically active pfTS domain. The findings could be useful since inhibition of the pfDHFR-TS domain-domain interaction could form a basis for the development of new anti-malarial drugs based on targeting the non-active site region of this important enzyme.
authors.affiliation: Department of Biochemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand.
authors.forename: Natpasit
authors.initials: N
authors.lastname: Chaianantakul
authors.forename: Rachada
authors.initials: R
authors.lastname: Sirawaraporn
authors.forename: Worachart
authors.initials: W
authors.lastname: Sirawaraporn
mesh_terms: D004252:DNA Mutational Analysis; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development; D005816:Genetic Complementation Test; D010963:Plasmodium falciparum / Q000201:enzymology* / Q000235:genetics; D054730:Protein Interaction Domains and Motifs; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics* / Q000378:metabolism*; D013940:Thymidylate Synthase / Q000235:genetics* / Q000378:metabolism*; D020798:Two-Hybrid System Techniques
chemical_list: D013762:Tetrahydrofolate Dehydrogenase; D013940:Thymidylate Synthase