Binding of 3,5,3′-Triiodothyronine (T3) and its Analogs to thein VitroTranslational Products of c-erbA Protooncogenes: Differences in the Affinity of the α- and β-Forms for the Acetic Acid Analog and Failure of the Human Testis and Kidney α-2 Products to Bind T3

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Abstract
We have compared the affinities for T3 and the T3 analog binding characteristics of the in vitro translational products of seven c-erbA cDNAs (chicken c-erbA .alpha.; human placental c-erbA .beta.; rat c-erbA .beta.-1; rat c-erbA .alpha.-1; rat c-erbA .alpha.-2; human testis c-erbA .alpha.-2; and human kidney c-erbA .alpha.-2). Four of these (chicken c-erbA .alpha., human placental c-erbA'' .beta., rat c-erbA .beta.-1, rat c-erbA .alpha.-1) bound T3 with high affinity as previously described. When compared under identical conditions of synthesis and [125I]T3 binding, there was no significant differences between the affinity of the chicken c-erbA A.alpha.-1 and the human c-erbA .beta. but in a more limited series the affinity of rat c-erbA .beta.-1 for T3 was 4.6-fold higher than that of the rat c-erbA .alpha.-1. In vitro translational products of the .beta.-probes showed a characteristic 2.2-fold higher triiodothyroacetic acid/T3 ratio than did the products of the .alpha.-probes, regardless of the species of origin of the probe. As previously established, the rat c-erbA .alpha.-2 product did not bind T3. However, in contrast to two published reports, the human testis and kidney .alpha.-2 probe products also failed to bind T3. These findings indicate that highly conserved C-terminal 37-40 residues are important for high affinity T3 binding by proteins encoded by the c-erb A family of genes.

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