The H2A-H2B dimeric kinetic intermediate is stabilized by widespread hydrophobic burial with few fully native interactions.

ABSTRACT: The H2A-H2B histone heterodimer folds via monomeric and dimeric kinetic intermediates. Within ?5 ms, the H2A and H2B polypeptides associate in a nearly diffusion limited reaction to form a dimeric ensemble, denoted I? and I?*, the latter being a subpopulation characterized by a higher content of nonnative structure (NNS). The I? ensemble folds to the native heterodimer, N?, through an observable, first-order kinetic phase. To determine the regions of structure in the I? ensemble, we characterized 26 Ala mutants of buried hydrophobic residues, spanning the three helices of the canonical histone folds of H2A and H2B and the H2B C-terminal helix. All but one targeted residue contributed significantly to the stability of I?, the transition state and N?; however, only residues in the hydrophobic core of the dimer interface perturbed the I?* population. Destabilization of I?* correlated with slower folding rates, implying that NNS is not a kinetic trap but rather accelerates folding. The pattern of ? values indicated that residues forming intramolecular interactions in the peripheral helices contributed similar stability to I? and N?, but residues involved in intermolecular interactions in the hydrophobic core are only partially folded in I?. These findings suggest a dimerize-then-rearrange model. Residues throughout the histone fold contribute to the stability of I?, but after the rapid dimerization reaction, the hydrophobic core of the dimer interface has few fully native interactions. In the transition state leading to N?, more native-like interactions are developed and nonnative interactions are rearranged.

SUBMITTER: Guyett PJ 

PROVIDER: S-EPMC3259194 | biostudies-literature | 2012 Jan

REPOSITORIES: biostudies-literature