Nucleosome core protein: asymmetric dissociation of the octamer

Abstract

The octameric nucleosomal core-histone complex, (H2A)₂-(H2B)₂-(H3)₂-(H4)₂, isolated from rat liver, undergoes dissociation during gel exclusion chromatography as a result of dilution occurring in the columns. The elution pattern at pH 7.0 and 4° C showed a sharp leading peak containing all four histones but predominantly H3 and H4, and a trailing peak containing equal amounts of histones H2A and H2B. As column length was increased the area under the leading peak decreased and that under the trailing peak increased. In addition the relative positions of the two peaks varied with column length. From an analysis of the data on increase in elution volume of the leading peak in relation to column length an apparent molecular weight of 86 000 was calculated for the undissociated molecule. Its apparent molecular weight, histone composition and pattern of further dissociation in relation to column length suggest that this species is the hexamer, (H2A-H2B)-(H3)₂-(H4)₂. At pH 7.0 and 4° C the dissociation of the core complex appears to be as follows: (H2A)₂-(H2B)₂-(H3)₂-(H4)₂ -> (H2A-H2B) + (H2A-H2B)-(H3)₂-(H4)₂ -> 2(H2A-H2B) + (H3)₂-(H4)₂. This dissociation was accelerated by an increase in temperature or decrease in pH and was accompanied by marked conformational changes as judged by circular dichroism measurements.