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