HMGB binding to DNA: single and double box motifs.

High mobility group (HMG) proteins are nuclear proteins believed to significantly affect DNA interactions by altering nucleic acid flexibility. Group B (HMGB) proteins contain HMG box domains known to bind to the DNA minor groove without sequence specificity, slightly intercalating base pairs and inducing a strong bend in the DNA helical axis. A dual-beam optical tweezers system is used to extend double-stranded DNA (dsDNA) in the absence as well as presence of a single box derivative of human HMGB2 [HMGB2(box A)] and a double box derivative of rat HMGB1 [HMGB1(box A+box B)]. The single box domain is observed to reduce the persistence length of the double helix, generating sharp DNA bends with an average bending angle of 99+/-9 degrees and, at very high concentrations, stabilizing dsDNA against denaturation. The double box protein contains two consecutive HMG box domains joined by a flexible tether. This protein also reduces the DNA persistence length, induces an average bending angle of 77+/-7 degrees , and stabilizes dsDNA at significantly lower concentrations. These results suggest that single and double box proteins increase DNA flexibility and stability, albeit both effects are achieved at much lower protein concentrations for the double box. In addition, at low concentrations, the single box protein can alter DNA flexibility without stabilizing dsDNA, whereas stabilization at higher concentrations is likely achieved through a cooperative binding mode.