Triiodothyronine nuclear receptor and the role of non-histone protein factors in in vitro triiodothyronine binding

Abstract The rat liver triiodothyronine (T 3 ) nuclear receptor rapidly looses, after a partial purification from the nuclear extract, its ability to bind T 3 . We previously reported that histones, in the presence of DNA, could protect against inactivation enhancing the T 3 binding site concentration and maintaining the high affinity for T 3 . A nuclear fraction discarded during the receptor purification (fraction A) was also found able to restore T 3 binding and was analyzed. As histones + DNA, fraction A stabilized the T 3 binding site from irreversible inactivation during incubation with T 3 , increasing its concentration while keeping the same high affinity for T 3 . It was active even at relatively high receptor concentration, appeared lightly more active than histones (+ DNA) in the same protein concentration range (up to 50-fold increment of T 3 binding at the optimal concentration of 25 μg/ml) and was unaffected or slightly inhibited by DNA. Other proteins (ovalbumin, soybean trypsin inhibitor, RNAase) and rat liver cytosol were several times less effective, suggesting a major role of some nuclear constituents. The active factors in fraction A essentially belong to non-histone nuclear proteins. Fraction A was found heterogenous regarding the molecular size and pH i of the active factors, the existence of subfractions more active on a protein concentration basis being suggested but not yet clearly evidenced. Efficient in vitro T 3 binding to the isolated T 3 nuclear receptor thus depends on the presence of several different nuclear constituents, histones + DNA or some non-histone proteins. Whether interactions with these constituents could modulate T 3 binding within the nucleus remains to be elucidated.