Site-directed mutations in motif VI of Escherichia coli DNA helicase II result in multiple biochemical defects: evidence for the involvement of motif VI in the coupling of ATPase and DNA binding activities via conformational changes

Abstract

Two site-directed mutants of Escherichia coli DNA helicase II (UvrD) were constructed to examine the functional significance of motif VI in a superfamily I helicase. Threonine 604 and arginine 605, representing two of the most highly conserved residues in motif VI, were replaced with alanine, generating the mutant alleles uvrD-T604A and uvrD-R605A. Genetic complementation studies indicated that UvrD-T604A, but not UvrD-R605A, functioned in methyl-directed mismatch repair and UvrABC-mediated nucleotide excision repair. Both mutant enzymes were purified and single-stranded DNA (ssDNA)-stimulated ATP hydrolysis, duplex DNA unwinding, and ssDNA binding were studied in the steady-state and compared to wild-type UvrD. UvrD-T604A exhibited a serious defect in ssDNA binding in the absence of nucleotide. However, in the presence of a non-hydrolyzable ATP analog, DNA binding was only slightly compromised. Limited proteolysis experiments suggested that UvrD-T604A had a "looser" conformation and could not undergo conformational changes normally associated with ATP binding/hydrolysis and DNA binding. UvrD-R605A, on the other hand, exhibited nearly normal DNA binding but had a severe defect in ATP hydrolysis (kcat=0.063 s-1 compared to 162 s-1 for UvrD). UvrD-T604A exhibited a much less severe decrease in ATPase activity (kcat=8.8 s-1). The Km for ATP for both mutants was not significantly changed. The results suggest that residues within motif VI of helicase II are essential for multiple biochemical properties associated with the enzyme and that motif VI is potentially involved in conformational changes related to the coupling of ATPase and DNA binding activities.