Ligand interactions in the ArsA protein, the catalytic component of an anion-translocating adenosinetriphosphatase

The ars operon of the conjugative R-factor R773 produces resistance to arsenicals in cells of Escherichia coli. The operon encodes an oxyanion pump which is composed of a membrane subunit, the 45.5-kDa ArsB protein, and a catalytic subunit, the 63-kDa ArsA protein. Purified ArsA protein is an arsenite(antimonite)-stimulated ATPase. From its amino acid sequence, as deduced from the nucleotide sequence, the ArsA protein has four tryptophanyl residues which could serve as intrinsic fluorescent probes for the study of substrate-induced conformational changes. Both static and dynamic measurements of tryptophan fluorescence were performed with the ArsA protein. Results from static anisotropy measurements indicated differences in molecular motion with addition of ATP, SbO2-, or Mg2+. These results were supported by time decay measurements of fluorescence anisotropy. The results of time decay measurements indicated a shorter correlation time, reflecting localized motion in the vicinity of the probe, and a longer correlation time, which could have arisen from rotation of the major portion of the molecule. The longer correlation time changed with addition of the various effectors, especially MgCl2, suggesting that binding of Mg2+ decreases probe mobility.