Coupled ATP and DNA Binding of Adeno-Associated Virus Rep40 Helicase^†

Adeno-associated virus 2 Rep40 helicase is involved in packaging single-stranded genomic DNA into virions. ATPase activity was stimulated 5−10-fold by DNA, depending upon assay conditions. The concentration dependence of Rep40 ATPase activity in the absence and presence of DNA indicates that the monomer is inactive and that the active enzyme is at least a dimer. Binding to oligonucleotides, examined by fluorescence anisotropy, was positively cooperative and required ATP or ATPγS; ADP and AMPPCP did not promote binding. The cooperativity and the nucleotide requirement were also demonstrated by surface plasmon resonance. Although the Rep40 behaves as a monomer in solution, it binds to DNA as an oligomer. The requirement of a nucleotide for DNA binding and the stimulation of ATPase activity by DNA indicate that the two processes are linked. Glutaraldehyde cross-linking generated a species that migrates as a trimer on sodium dodecyl sulfate (SDS) gel electrophoresis; ATPγS promoted the formation of this species and higher order oligomers. The predominant cross-linked species was a trimer in the absence of ATPγS, regardless of whether duplex or single-stranded DNA was present. In the presence of duplex or single-stranded DNA and ATPγS, glutaraldehyde cross-linking generated a species that behaved as a dimer on SDS gel elctrophoresis. Sucrose-gradient velocity sedimentation of Rep40 gave an S_20,w of 3 in the absence of ligands or in the presence of a 26 bp duplex DNA. The S_20,w was 3.5 in the presence of ATPγS and 7 and 7.6 in the presence of DNA and ATPγS.