Enhancement of DNA flexibility in vitro and in vivo by HMGB box A proteins carrying box B residues

摘要

HMGB proteins are abundant non-histone components of eukaryotic chromatin. The biological function of DNA sequence nonspecific HMGB proteins is obscure. These proteins are composed of one or two conserved HMG box domains, each forming three alpha helices that fold into a sequence nonspecific DNA-binding module recognizing the DNA minor groove. Box A and box B homology domains have subtle sequence differences such that box B domains bend DNA strongly while DNA bending by isolated box A domains is weaker. Both box A and box B domains preferentially bind to distorted DNA structures. Here we show using DNA cyclization kinetics assays in vitro and E. coli DNA looping assays in vivo that an isolated HMG box A domain derived from human HMGB2 folds poorly and does not enhance apparent DNA flexibility. Surprisingly, substitution of a small number of cationic residues from the N-terminal leader of a functional yeast box B protein, Nhp6Ap, confers the ability to enhance DNA flexibility. These results demonstrate important roles for cationic leader amino acids in HMGB folding, DNA interaction, and DNA bending.