Triiodothyronine nuclear receptor and the role of non-histone protein factors in in vitro triiodothyronine binding.
The rat liver triiodothyronine (T3) nuclear receptor rapidly looses, after a partial purification from the nuclear extract, its ability to bind T3. We previously reported that histones, in the presence of DNA, could protect against inactivation enhancing the T3 binding site concentration and maintaining the high affinity for T3. A nuclear fraction discarded during the receptor purification (fraction A) was also found able to restore T3 binding and was analyzed. As histones + DNA, fraction A stabilized the T3 binding site from irreversible inactivation during incubation with T3, increasing its concentration while keeping the same high affinity for T3. It was active even at relatively high receptor concentration, appeared slightly more active than histones (+ DNA) in the same protein concentration range (up to 50-fold increment of T3 binding at the optimal concentration of 25 micrograms/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 heterogeneous regarding the molecular size and pHi 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 T3 binding to the isolated T3 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 T3 binding within the nucleus remains to be elucidated.
Bismuth
J
J
Anselmet
A
A
Torresani
J
J
D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism*; D002852:Chromatography, Ion Exchange; D002868:Chromosomal Proteins, Non-Histone / Q000378:metabolism*; D004247:DNA / Q000302:isolation & purification / Q000378:metabolism; D006657:Histones / Q000378:metabolism; D066298:In Vitro Techniques; D007525:Isoelectric Focusing; D008099:Liver / Q000378:metabolism*; D008297:Male; D011487:Protein Conformation; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011988:Receptors, Thyroid Hormone; D014284:Triiodothyronine / Q000378:metabolism*
D002868:Chromosomal Proteins, Non-Histone; D006657:Histones; D011956:Receptors, Cell Surface; D011988:Receptors, Thyroid Hormone; D014284:Triiodothyronine; D004247:DNA