Increasing the length and hydrophobicity of the C‐terminal sequence of transthyretin strengthens its binding affinity to retinol binding protein

Transthyretin ( TTR ) is a transporter for thyroid hormone ( TH ) and retinol, the latter via binding with retinol binding protein ( RBP ). Both the N‐terminal and C‐terminal regions of the TTR subunit are located in close proximity to the central binding channel for ligands. During the evolution of vertebrates, these regions changed in length and hydropathy. The changes in the N‐terminal sequence were demonstrated to affect the binding affinities for TH s and RBP . Here, the effects of changes in the C‐terminal sequence were determined. Three chimeric TTR s, namely pigC/hu TTR (human TTR with the C‐terminal sequence changed to that of Sus scrofa TTR ), xenoN/pigC/hu TTR (human TTR with the N‐terminal and C‐terminal sequences changed to those of Xenopus laevis and S. scrofa , respectively), and pigC/croc TTR ( Crocodylus porosus TTR with the C‐terminal sequence changed to that of S. scrofa TTR ), were constructed and their binding affinities for human RBP were determined at low TTR / RBP molar ratio using chemiluminescence immunoblotting. The binding dissociation constant ( K d ) values of pigC/hu TTR , xenoN/pigC/hu TTR and pigC/croc TTR were 3.20 ± 0.35, 1.53 ± 0.38 and 0.31 ± 0.04 μ m , respectively, and the K d values of human and C. porosus TTR were 4.92 ± 0.68 and 1.42 ± 0.45 μ m , respectively. These results demonstrate chimeric TTR s bound RBP with a higher strength than wild‐type TTR s, and the changes in the C‐terminal sequence of TTR had a positive effect on its binding affinity for RBP . In addition, changes to the N‐terminal and C‐terminal sequences showed comparable effects on the binding affinity.