Binding mode of nucleosome-assembly protein (AP-I) and histones

Studies were made on the binding mode of the nucleosome-assembly protein AP-I with histones H2A + H2B and/or H3 + H4. Histones H2A + H2B bound with AP-I to form a 7-S complex which contained twice as much AP-I (by weight) as histones. Histone H3 + H4 formed an 8-S complex with AP-I. The 7-S and 8-S complexes did not form a new complex when mixed, but significant amounts of two histone pairs were assembled into a 12-S complex on mixing the (H2A + H2B)--AP-I complex (7-S) with free H3 + H4. In contrast, when the (H3 + H4)--AP-I complex (8-S) was incubated with free H2A + H2B, almost no assembly occurred, but the 7-S complex of H2A + H2B was newly formed. Binding studies by enzyme-linked immunosorbent assay showed that AP-I bound with H2A + H2B faster than with H3 + H4. From these results, it is suggested that AP-I has a higher binding affinity for histone H2A + H2B than for H3 + H4, and that the 7-S complex is an intermediate in formation of the 12-S octamer complex (H2A + H2B + H3 + H4)2.