Molecular and thyroid hormone binding properties of lamprey transthyretins: The role of an N-terminal histidine-rich segment in hormone binding with high affinity

Transthyretin (TTR) is a plasma thyroid hormone (TH) binder that emerged from an ancient hydroxyisourate hydrolase by gene duplication. To know how an ancient TTR had high affinity for THs, molecular and TH binding properties of lamprey TTRs were investigated. In adult serum, the lipoprotein LAL was a major T3 binder with low affinity. Lamprey TTRs had an N-terminal histidine-rich segment, and had two classes of binding sites for 3,3′,5-triiodo-L-thyronine (T3): a high-affinity and a low-affinity site. Mutant TTRΔ3-11, lacking the N-terminal histidine-rich segment, lost the high-affinity T3 binding site. [125I]T3 binding to wild type TTR and mutant TTRΔ3-11, was differentially modulated by Zn2+. Zn2+ contents of wild type TTR were 7-10/TTR (mol/mol). Our results demonstrate that lamprey TTR is a Zn2+-dependent T3 binder. The N-terminal histidine-rich segment may be essential for neo-functionalization (i.e., high-affinity T3 binding activity) of an ancient TTR after gene duplication.