A point mutation in an unusual Sec7 domain is linked to brefeldin A resistance in a Plasmodium falciparum line generated by drug selection

The malaria parasite Plasmodium falciparum has an unusual organization of i ts secretory compartments. As an approach to a functional identification of auxiliary proteins involved in secretion, a parasite line was generated by drug selection that is resistant to brefeldin A, an inhibitor of the secre tory pathway. In the resistant line, neither protein secretion nor parasite viability were affected by the drug. The analysis of a sec7 domain, a cons erved structure of guanine nucleotide exchange factors (ARF-GEF) required f or the activation of ADP-ribosylation factors, revealed a single methionine -isoleucine substitution in the resistant parasite line. ARF-GEFs are key m olecules in the formation of transport vesicles and the main targets of bre feldin A. The methionine residue in this position of sec7 domains is highly conserved and confers brefeldin A sensitivity. Unlike other eukaryotes tha t have multiple ARF-GEFs, the plasmodial genome encodes a single sec7 domai n. This domain shows a distinct structural difference to all sec7 domains a nalysed so far; two conserved subdomains that are essential for protein fun ction are separated in the plasmodial protein by an insertion of 146 amino acids.