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9.B.35 The Putative Thyroxine-Transporting Transthyretin (Transthyretin) Family

Transthyretin is a thyroid hormone (thyroxine) binding protein that may function in the transport of thyroxine from the blood stream to the brain across the mammalian blood brain barrier of the choroid plexus. It is most abundant in this tissue but is also found in liver and other tissues. The transthyretin precursor is also known as amyloidosis type I prealbumin. There are amylogenic transthyretin variants that may be important in Alzheimer's disease. These mammalian proteins may also bind vitamin A/retinol binding proteins. Homologues are found in birds, amphibians, worms, yeast and bacteria. An example of a bacterial homologue is the periplasmic YedX protein of E. coli (137aas; spP76341). Based on 3-D structures, two dimer pairs of the mammalian proteins are each believed to form internal channels that are the binding sites for thyroxine. Less than 1% of plasma prealbumin is believed to be involved in thyroxine transport. Numerous references are provided under Swiss Prot acc #P02767. Distant bacterial homologues have been identified (see 9.B.35.2.1).  These bacterial homologues may belong to the Peptidase M14NE-CP-C_like superfamily. 

The proposed transport reaction catalyzed by the transthyretin channel is:

Thyroxine (blood) (and other compounds) → Thyroxine (brain) (and other compounds)

References associated with 9.B.35 family:

Blake, C.C., M.J. Geisow, S.J. Oatley, B. Rerat, and C. Rerat. (1978). Structure of prealbumin: secondary, tertiary and quaternary interactions determined by Fourier refinement at 1.8 Å. J. Mol. Biol. 121: 339-356. 671542
Doniselli, N., E. Monzeglio, A. Dal Palù, A. Merli, and R. Percudani. (2015). The identification of an integral membrane, cytochrome c urate oxidase completes the catalytic repertoire of a therapeutic enzyme. Sci Rep 5: 13798. 26349049
Jung, D.K., Y. Lee, S.G. Park, B.C. Park, G.H. Kim, and S. Rhee. (2006). Structural and functional analysis of PucM, a hydrolase in the ureide pathway and a member of the transthyretin-related protein family. Proc. Natl. Acad. Sci. USA 103: 9790-9795. 16782815
Wojtczak, A. (1997). Crystal structure of rat transthyretin at 2.5 Å resolution: first report on a unique tetrameric structure. Acta Biochim. Pol. 44: 505-517. 9511961