The role of the labile amino-termini (histone """"""""tails"""""""") on the stability of the H2A-H2B dimer and the (H3-H4)/(H3-H4)2 system was studied by circular dichroism spectroscopy and differential scanning calorimetry. The ionic strength and pH dependence of the thermodynamic stability of the truncated subunits were compared to the same properties of the corresponding full-length complexes. The unfolding transition temperature of the truncated H2A-H2B dimer increases as a function of ionic strength, in a way very similar to that of the full length protein. At extremely low ionic strength, the truncated complex is only slightly more stable than the full-length protein, whereas from above 50 mM NaCl salt concentrations the truncated complex is marginally less stable than its full-length counterpart. At neutral pH and physiological ionic strength, the thermal denaturation of the truncated H2A-H2B dimer is centered at about 47 C with a corresponding enthalpy change of about 38 kcal/mol of 11.5 kDa monomer, compared to 50 C and 40 kcal/mol of 14 Kda monomer, respectively, for the full-length H2A-H2B dimer under similar conditions. The thermal stability of the truncated (H3-H4)/(H3-H4)2 system was studied at low ionic strength and pH 4.5, where the predominant molecular species present in solution is the truncated H3-H4 dimer. Its unfolding transition temperature increases with ionic strength, in a way that parallels the behavior of the full-length H3-H4 dimer. At 25 mM NaCl, the thermal denaturation of the truncated H3-H4 dimer is centered at about 50 C with a corresponding enthalpy change of 29 kcal/mol of 10 Kda monomer, compared to 49 C and 29 kcal/mol of 13.5 Kda monomer, respectively, for the full-length protein. The pH dependence of the stability of the truncated (H3-H4)/(H3-H4)2 system is very similar to that exhibited by the full-length system. Both the truncated H2A-H2B dimer and the (H3-H4)/(H3-H4)2 system at low ionic strength and pH 7.5 or 4.5, respectively, behave as highly cooperative systems, each melting as single unit without any detectable intermediates of folded monomers, exactly like the corresponding full-length complexes. In summary, the limited trypsinization of the H2A-H2B dimer and the (H3-H4)/(H3-H4)2 system, i.e. the removal of the labile histone """"""""tails"""""""", does not affect the ionic strength and pH dependence of the stability of these proteins in any significant way. Therefore, it is the centrally located folded region of the core histone complexes (i.e., the histone fold) that determine the stabilities and regulate their associative behavior.
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