Nuclear type II [3H]estradiol binding sites: A histone H3-H4 complex

[3H]luteolin covalently labels two forms (11 kDa and 35 kDa proteins) of type II binding sites in rat uterine nuclear extracts [K. Shoulars, T. Brown, M. Alejandro, J. Crowley, B. Markaverich, Identification of rat uterine nuclear type II [3H]estradiol binding sites as histone H4, Biochem. Biophys. Res. Commun. 296 (2002) 1083-1090]. The 11 kDa protein was identified as histone H4. Levels of the 35 kDa protein were insufficient for sequencing; however, this protein was recognized by anti-histone H4 antibodies. Histones H3 and H4 exist as dimers in vivo (mw ≫ 35 kDa) and we suspected the 35 kDa [3H]luteolin-labeled protein in uterine nuclear extracts might be a complex of histones H3 and H4. This manuscript describes methods for the purification of commercially available calf thymus core histones that retain [3H]luteolin binding activity and are of sufficient purity for recombination studies. Mixing experiments with pure H3 and H4 from calf thymus demonstrate that a 35 kDa H3-H4 dimer capable of binding [3H]luteolin is generated and this protein appears equivalent to the 35 kDa [3H]luteolin binding protein in rat uterine nuclear extracts. If this is the case, type II site ligands including MeHPLA, luteolin, and other bioflavonoids and phytoestrogens may control histone-dependent gene transcription and cellular proliferation via binding to and modulating core histone/nucleosome function.