Cloning, purification, and nucleotide-binding traits of the catalytic subunit A of the V₁V_O ATPase from Aedes albopictus

The Asian tiger mosquito, Aedes albopictus, is commonly infected by the gregarine parasite Ascogregarina taiwanensis, which develops extracellularly in the midgut of infected larvae. The intracellular trophozoites are usually confined within a parasitophorous vacuole, whose acidification is generated and controlled by the V₁V_O ATPase. This proton pump is driven by ATP hydrolysis, catalyzed inside the major subunit A. The subunit A encoding gene of the Aedes albopictus V₁V_O ATPase was cloned in pET9d1-His₃ and the recombinant protein, expressed in the Escherichia coli Rosetta™ 2 (DE3) strain, purified by immobilized metal affinity- and ion-exchange chromatography. The purified protein was soluble and properly folded. Analysis of secondary structure by circular dichroism spectroscopy showed that subunit A comprises 43% α-helix, 25% β-sheet and 40% random coil content. The ability of subunit A of eukaryotic V-ATPases to bind ATP and/or ADP is demonstrated by photoaffinity labeling and fluorescence correlation spectroscopy (FCS). Quantitation of the FCS data indicates that the ADP-analogues bind slightly weaker to subunit A than the ATP-analogues. Tryptophan fluorescence quenching of subunit A after binding of different nucleotides provides evidence for secondary structural alterations in this subunit caused by nucleotide-binding.