Reexamination of hormone-binding properties of protein disulfide-isomerase.
Protein disulfide-isomerase (PDI), an abundant multifunctional protein, has been described as a 3,3',5-triiodo-L-thyronine (T3)-binding protein. As pointed out by several authors, the physiological significance of this hormone-binding property has not been fully addressed. To clarify this point, we have analyzed the T3-binding properties of purified PDI. At equilibrium, T3 binds PDI at two binding sites: first, at a high-affinity site with a Kd of 21 nM and a Bmax of 1.8 x 10(-3) mol T3/mol PDI monomer, and second at a very low affinity site that is unsaturated up to 100 microM T3. Thus, T3 binding is mainly non-specific and the specific part represents only about 0.2% of the protein monomer. Cross-linking experiments at a concentration where mainly specific binding occurs indicate that PDI does not bind L-T3 exclusively; a wide variety of analogs are also bound. Refolding of reduced denatured ribonuclease A by PDI is inhibited by T3 and analogs, and the inhibition profile reflects the binding properties very closely. Since purified PDI displays neither the specificity expected for a physiological receptor, nor significant T3-binding activity, results are discussed in terms of a necessary PDI association with another component to form a T3 receptor.
Guthapfel
R
R
Commissariat à l'Energie Atomique, Département d'Ingénierie et d'Etudes des Protéines, Gif-sur-Yvette, France.
Gueguen
P
P
Quemeneur
E
E
D001665:Binding Sites; D003001:Cloning, Molecular; D004926:Escherichia coli; D006801:Humans; D007457:Iodine Radioisotopes; D007535:Isomerases / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D011489:Protein Denaturation; D019704:Protein Disulfide-Isomerases; D017510:Protein Folding; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D012259:Ribonuclease, Pancreatic / Q000737:chemistry; D014284:Triiodothyronine / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology
D007457:Iodine Radioisotopes; D011994:Recombinant Proteins; D014284:Triiodothyronine; D012259:Ribonuclease, Pancreatic; D007535:Isomerases; D019704:Protein Disulfide-Isomerases