Suzuki S, Kasai K, Nishiyama N, Ishihara A, Yamauchi K. Characteristics of the brown hagfish Paramyxine atami transthyretin: Metal ion-dependent thyroid hormone binding.
Gen Comp Endocrinol 2017;
249:1-14. [PMID:
28242306 DOI:
10.1016/j.ygcen.2017.02.011]
[Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/30/2016] [Revised: 02/20/2017] [Accepted: 02/22/2017] [Indexed: 12/12/2022]
Abstract
Transthyretin (TTR) is a vertebrate-specific protein involved in thyroid hormone distribution in plasma, and its gene is thought to have emerged by gene duplication from the gene for the ancient TTR-related protein, 5-hydroxyisourate hydrolase, at some early stage of chordate evolution. We investigated the molecular and hormone-binding properties of the brown hagfish Paramyxine atami TTR. The amino acid sequence deduced from the cloned hagfish TTR cDNA shared 33-50% identities with those of other vertebrate TTRs but less than 24% identities with those of vertebrate and deuterostome invertebrate 5-hydroxyisourate hydrolases. Hagfish TTR, as well as lamprey and little skate TTRs, had an N-terminal histidine-rich segment, allowing purification by metal-affinity chromatography. The affinity of hagfish TTR for 3,3',5-triiodo-L-thyronine (T3) was 190 times higher than that for L-thyroxine, with a dissociation constant of 1.5-3.9nM at 4°C. The high-affinity binding sites were strongly sensitive to metal ions. Zn2+ and Cu2+ decreased the dissociation constant to one-order of magnitude, whereas a chelator, o-phenanthroline, increased it four times. The number of metal ions (mainly Zn2+ and Cu2+) was approximately 12/TTR (mol/mol). TTR was also a major T3-binding protein in adult hagfish sera and its serum concentration was approximately 8μM. These results suggest that metal ions and the acquisition of N-terminal histidine-rich segment may cooperatively contribute to the evolution toward an ancient TTR with high T3 binding activity from either 5-hydroxyisourate hydrolase after gene duplication.
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