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[摘要]：The H2A-H2B histone heterodimer folds via monomeric and dimeric kinetic intermediates. Within similar to 5 ms, the H2A and H2B polypeptides associate in a nearly diffusion limited reaction to form a dimeric ensemble, denoted I-2 and I-2*, the latter being a subpopulation characterized by a higher content of nonnative structure (NNS). The I-2 ensemble folds to the native heterodimer, N-2, through an observable, first-order kinetic phase. To determine the regions of structure in the I-2 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-2, the transition state and N-2; however, only residues in the hydrophobic core of the dimer interface perturbed the I-2* population. Destabilization of I-2* correlated with slower folding rates, implying that NNS is not a kinetic trap but rather accelerates folding. The pattern of Phi values indicated that residues forming intramolecular interactions in the peripheral helices contributed similar stability to I-2 and N-2, but residues involved in intermolecular interactions in the hydrophobic core are only partially folded in I-2. These findings suggest a dimerize-then-rearrange model. Residues throughout the histone fold contribute to the stability of I-2, 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-2, more native-like interactions are developed and nonnative interactions are rearranged. (C) 2011 Elsevier Ltd. All rights reserved. |
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