Prothymosin α (ProTα) is an abundant acidic nuclear protein that may be involved in cell proliferation. In our search for its cellular partners, we haver recently found that ProTα binds to linker histone H1. We now provide further evidence for the physiological relevance of this interaction by immunoisolation of a histone H1-ProTα complex from NIH 3T3 cell extracts. A detailed analysis of the interaction between the two proteins suggests contacts between the acidic region of ProTα and histone H1. In the context of a physiological chromatin reconstitution reaction, the presence of ProTα does not affect incorporation of an amount of histone H1 sufficient to increase the nucleosome repeat length by 20 bp, but prevents association of all further H1. Consistent with this finding, a fraction of histone H1 is released when H1-containing chromatin is challenged with ProTα. These results imply at least two different interaction modes of H1 with chromatin, which can be distinguished by their sensitivity to ProTα. The properties of ProTα suggest a role in fine tuning the stoichiometry and/or mode of interaction of H1 with chromatin.
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