Properties of solubilized nuclear triiodothyronine binding proteins

Rat liver nuclear proteins bind 3,5,3'-triiodo-L-thyronine (T3) to essentially one class of sites (Ka approximately 1 X 10(10) M-1). Gel filtration and sucrose gradient centrifugation studies show a main T3 binding component with a Stokes radius of 33 A and a sedimentation coefficient of 3.5S, and variable amounts of high molecular weight binding components, most of them being reversible aggregates of the main component formed during storage. But the uniqueness of the nuclean T3 binding proteins (NTBP) cannot be ascertained from polyacrylamide gel electrophoresis data. During storage in the absence of reducing agents, NTBP aggregate and rapidly lose their ability to bind T3; T3--NTBP complexes also aggregate and progressively dissociate. This can be reversed by dithiothreitol. Bound T3 could temporarily stabilize the binding site but cannot protect NTBP against general conformational changes which follow the oxidation of their essential --SH group(s). NTBP are DNA binding proteins with probably a relative independence of their DNA and T3 binding sites: they bind T3 to the same class of high affinity sites whether complexed or not with DNA; bound T3 is not a prerequisite for DNA binding.