HARP protein binds selectively to fork DNA. (A) HARP protein binds with higher affinity to fork DNA than to single-stranded DNA (ssDNA) or double-stranded DNA (dsDNA). Gel mobility shift experiments were performed with a 30 nt ssDNA, a 30 bp dsDNA, and a 30 nt fork DNA that is identical to the double-stranded DNA except for a 9 nt mismatch at one end. The relative concentrations of HARP are shown. The actual concentrations of HARP are 0, 0.05, 0.1, 0.2, and 0.4 nM. (B) HARP ATPase activity is stimulated to a greater extent by fork DNA than by single- or double-stranded DNA. The DNA substrates used in the ATPase assays are identical to those used in A, except that the DNA samples were not radiolabelled. Error bars represent SD (N = 3).