PROTHYMOSIN-ALPHA MODULATES THE INTERACTION OF HISTONE H1 WITH CHROMATIN

Prothymosin alpha (ProT alpha) is an abundant acidic nuclear protein t hat may be involved in cell proliferation. In our search for its cellu lar partners, we have recently found that ProT alpha binds to linker h istone H1;We now provide further evidence for the physiological releva nce of this interaction by immunoisolation of a histone H1-ProT alpha complex from NIH 3T3 cell extracts. A detailed analysis of the interac tion between the two proteins suggests contacts between the acidic reg ion of ProT alpha and histone H1. In the context of a physiological ch romatin reconstitution reaction, the presence of ProT alpha does not a ffect 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 ProT alpha. These results imply at least two different interaction modes of H1 wit h chromatin, which can be distinguished by their sensitivity to ProT a lpha. The properties of ProT alpha suggest a role in fine tuning the s toichiometry and/or mode of interaction of H1 with chromatin.