Binding of thyroxine to pig transthyretin, its cDNA structure, and other properties

The catalytic kinetics of chicken transthyretin towards human Aβ1-42

The novel property of transthyretin (TTR) as a protease has been proposed to be significant. However, the study of TTR proteolysis properties has not been completely elucidated. Herein, we first report the catalytic activity of chicken TTR from plasma determined by using fluorescently labeled amyloid beta 1-42 peptide (Aβ_1-42), and compared it with human TTR (human TTR) from plasma and recombinant Crocodylus porosus TTR. The enzyme kinetic study revealed that the affinity for Aβ_1-42 of chicken TTR and C. porosus TTR (K_M values were 12.72 ± 0.27 μM and 16.21 ± 0.02 μM, respectively) were significantly lower than human TTR (K_M was 43.05 ± 0.39 μM). In addition, the catalytic efficiency of chicken TTR (K_cat/K_M was 310,386.87 ± 13,627.12 M^-1 s^-1) was 4.3 and 5.5 folds higher than those of C. porosus TTR and human TTR (K_cat/K_M were 72,893.80 ± 355.74 M^-1 s^-1 and 56,519.12 ± 5009.50 M^-1 s^-1, respectively), respectively. These results does not only indicated the relationship between structure and the proteolytic activity of TTR, but also suggested a potential development of TTR as a therapeutic anti-Aβ agent.

The hydrophobic C-terminal sequence of transthyretin affects its catalytic kinetics towards amidated neuropeptide Y

Transthyretin (TTR) is a transporter for thyroid hormone and retinol binding protein that has recently been reported to have proteolytic activity against certain substrates, including amidated neuropeptide Y (NPY). However, the proteolytic activity of TTR towards NPY is not fully understood. Here, we used fluorescence-based assays to determine the catalytic kinetics of human TTR towards human amidated NPY. The Michaelis constant (KM) and catalytic efficiency (kcat/KM) of TTR proteolysis were 15.88 ± 0.44 μm and 687 081 ± 35 692 m -1·s-1, respectively. In addition, we demonstrated an effect of the C-terminal sequence of TTR. When the C-terminal sequence of TTR was made more hydrophobic, the KM and kcat/KM changed to 12.87 ± 0.22 μm and 983 755 ± 18 704 m -1·s-1, respectively. Our results may be useful for the development of TTR as a therapeutic agent with low risk of the undesirable symptoms that develop from amidated NPY, and for further improvement of the kcat/KM of TTR.

Evolution of thyroid hormone distributor proteins

Thyroid hormones (THs) are evolutionarily old hormones, having effects on metabolism in bacteria, invertebrates and vertebrates. THs bind specific distributor proteins (THDPs) to ensure their efficient distribution through the blood and cerebrospinal fluid in vertebrates. Albumin is a THDP in the blood of all studied species of vertebrates, so may be the original vertebrate THDP. However, albumin has weak affinity for THs. Transthyretin (TTR) has been identified in the blood across different lineages in adults vs juveniles. TTR has intermediate affinity for THs. Thyroxine-binding globulin has only been identified in mammals and has high affinity for THs. Of these THDPs, TTR is the only one known to be synthesised in the brain and is involved in moving THs from the blood into the cerebrospinal fluid. We analysed the rates of evolution of these three THDPs: TTR has been most highly conserved and albumin has had the highest rate of divergence.

Species-dependent effects of the phenolic herbicide ioxynil with potential thyroid hormone disrupting activity: modulation of its cellular uptake and activity by interaction with serum thyroid hormone-binding proteins

Ioxynil, a phenolic herbicide, is known to exert thyroid hormone (TH) disrupting activity by interfering with TH-binding to plasma proteins and a step of the cellular TH-signaling pathway in restricted animal species. However, comparative studies are still lacking on the TH disruption. We investigated the interaction of [125I]ioxynil with serum proteins from rainbow trout, bullfrog, chicken, pig, rat, and mouse, using native polyacrylamide gel electrophoresis. Candidate ioxynil-binding proteins, which included lipoproteins, albumin and transthyretin (TTR), differed among the vertebrates tested. Rainbow trout and bullfrog tadpole serum had the lowest binding activity for ioxynil, whereas the eutherian serum had the highest binding activity. The cellular uptake of, and response to, ioxynil were suppressed by rat serum greater than by tadpole serum. The cellular uptake of [125I]ioxynil competed strongly with phenols with a single ring, but not with THs. Our results suggested that ioxynil interferes with TH homeostasis in plasma and with a step of cellular TH-signaling pathway other than TH-uptake system, in a species-specific manner.

Characterization of plasma triiodophenol binding proteins in vertebrates and tissue distribution of triiodophenol in Rana catesbeiana tadpoles

We investigated the interaction of 2,4,6-triiodophenol (TIP), a potent thyroid hormone disrupting chemical, with serum proteins from rainbow trout (Onchorhynchus mykiss), bullfrog (Rana catesbeiana), chicken (Gallus gallus), pig (Sus scrofa domesticus), and rat (Rattus norvegicus) using a [(125)I]TIP binding assay, gel filtration chromatography, and native polyacrylamide gel electrophoresis. [(125)I]TIP bound non-specifically to proteins in trout serum, specifically but weakly to proteins in bullfrog serum, and specifically and strongly to proteins in chicken, pig, and rat serum samples. Candidate TIP-binding proteins included lipoproteins (220-320kDa) in trout, albumin in bullfrog, albumin and transthyretin (TTR) in chicken and pig, and TTR in rat. TTR in the chicken, pig, and rat serum samples was responsible for the high-affinity, low-capacity binding sites for TIP (dissociation constant 2.2-3.5×10(-10)M). In contrast, a weak interaction of [(125)I]TIP with tadpole serum proteins accelerated [(125)I]TIP cellular uptake in vitro. Intraperitoneal injection of [(125)I]TIP in tadpoles revealed that the radioactivity was predominantly accumulated in the gallbladder and the kidney. The differences in the molecular and binding properties of TIP binding proteins among vertebrates would affect in part the cellular availability, tissue distribution and clearance of TIP.

Evolutionary changes to transthyretin: structure-function relationships

Transthyretin is one of the three major thyroid hormone-binding proteins in plasma and/or cerebrospinal fluid of vertebrates. It transports retinol via binding to retinol-binding protein, and exists mainly as a homotetrameric protein of approximately 55 kDa in plasma. The first 3D structure of transthyretin was an X-ray crystal structure from human transthyretin. Elucidation of the structure-function relationship of transthyretin has been of significant interest since its highly conserved structure was shown to be associated with several aspects of metabolism and with human diseases such as amyloidosis. Transthyretin null mice do not have an overt phenotype, probably because transthyretin is part of a network with other thyroid hormone distributor proteins. Systematic study of the evolutionary changes of transthyretin structure is an effective way to elucidate its function. This review summarizes current knowledge about the evolution of structural and functional characteristics of vertebrate transthyretins. The molecular mechanism of evolutionary change and the resultant effects on the function of transthyretin are discussed.

Cell and Molecular Biology of Transthyretin and Thyroid Hormones

Advances in four areas of transthyretin (TTR) research result in this being a timely review. Developmental studies have revealed that TTR is synthesized in all classes of vertebrates during development. This leads to a new hypothesis on selection pressure for hepatic TTR synthesis during development only, changing the previous hypotheses from "onset" of hepatic TTR synthesis in adulthood to "maintaining" hepatic TTR synthesis into adulthood. Evolutionary studies have revealed the existence of TTR-like proteins (TLPs) in nonvertebrate species and elucidated some of their functions. Consequently, TTR is an excellent model for the study of the evolution of protein structure, function, and localization. Studies of human diseases have demonstrated that TTR in the cerebrospinal fluid can form amyloid, but more recently there has been recognition of the roles of TTR in depression and Alzheimer's disease. Furthermore, amyloid mutations in human TTR that are the normal residues in other species result in cardiac deposition of TTR amyloid in humans. Finally, a revised model for TTR-thyroxine entry into the cerebrospinal fluid via the choroid plexus, based on data from studies in TTR null mice, is presented. This review concentrates on TTR and its thyroid hormone binding, in development and during evolution, and summarizes what is currently known about TLPs and the role of TTR in diseases affecting the brain.

Structural and functional evolution of transthyretin and transthyretin-like proteins

Transthyretin (TTR) is a tetrameric protein involved in the distribution of thyroid hormones in vertebrates. The amino acid sequence of TTR is highly conserved across vertebrates. Hypothetical TTR-like proteins (TLPs) were inferred from the identification of genes in nonvertebrate species. Here, we identified five motifs defining TLPs and three motifs defining both TTRs and TLPs. These motifs were mapped onto structurally conserved and functionally important regions of TTRs. These motifs were used to build hidden Markov models for accurate identification of TLPs in other organisms. TLPs were divided into three main groups based on their N-terminal regions. Most TLPs are cytosolic, but in plants and slime mold, we predict they are peroxisomal. We verified that the TLPs from enterobacteria were periplasmic. We demonstrated that TLP genes are expressed in a bacterium (E. coli), an invertebrate animal (C. elegans), and a plant (A. thaliana). These TLPs have similar subunit molecular weights to TTRs, are tetramers, and are predicted to have similar three-dimensional (3D) structures to TTRs, but do not bind thyroid hormones or similar ligands. We suggest that like TTRs, the N-terminal and C-terminal regions of TLPs are integral in defining the function of TLPs in nonvertebrate species and that the TLP gene duplicated in primitive vertebrates to produce the TTR gene. TLP/TTR has retained its overall structure, but changed function and localization during evolution in bacteria, invertebrates, plants, and vertebrates.

The evolution of transthyretin synthesis in vertebrate liver, in primitive eukaryotes and in bacteria

Thyroid hormones are evolutionarily old signal molecules, which can partition between compartments by partitioning into lipid membranes. The role of thyroid hormone distributor proteins is to ensure that sufficient thyroid hormone remains in the bloodstream. Of particular interest is the role of transthyretin, synthesised by the liver and secreted into the blood. In this review, three hypotheses are presented, suggesting the selection pressures leading to the onset of transthyretin synthesis in the liver during evolution. A thyroid hormone distribution network would be a selection advantage over a single protein performing this function. Similarly to the situation in eutherians, hepatic transthyretin synthesis in marsupials is under negative acute phase regulation. The overall three-dimensional structure of transthyretin did not change appreciably during vertebrate evolution. The region of the primary sequence which evolved most was the N-terminal region of the subunit. The N-termini of transthyretin changed from longer and more hydrophobic in reptiles/birds, to shorter and more hydrophilic in eutherians. These changes are correlated with a change in preference from binding of triiodothyronine, to binding thyroxine. As the rest of the molecule had not changed significantly during vertebrate evolution, the gene coding for transthyretin must have evolved prior to the divergence of the vertebrates from the non-vertebrates. Five open reading frames in the genomes of C. elegans (2), S. dublin, S. pombe and E. coli were identified. The protein products are predicted to form tetramers similar to transthyretins. Two possible functions of these proteins are suggested.

Structural distribution of mutations associated with familial amyloidotic polyneuropathy in human transthyretin

The human plasma protein transthyretin (TTR) is a highly stable soluble homotetrameric protein. Still, conformational changes in the wild type protein can lead to self-assembly into insoluble amyloid fibrils. In addition, 74 point mutations are known to enhance amyloid formation causing familial amyloidotic polyneuropathy (PAP). Alignment of TTR sequences from twenty different species shows that only six of these mutations occur as natural amino acids in other organisms. In this paper we analyse the distribution of FAP mutations within the three-dimensional structure of TTR. Contradictory to what might be expected from protein stability studies, the mutations are not restricted to structurally rigid parts of the molecule, nor are they concentrated at the monomer interaction sites.

Structure and expression of the transthyretin gene in the choroid plexus: a model for the study of the mechanism of evolution

Thyroid hormones are key regulators of brain differentiation and function. They permeate strongly into lipid membranes. However, a substantial portion of thyroid hormone is retained in the intravascular/extracellular compartments by binding to plasma proteins. In the brain, transthyretin is the most important of these proteins. This transthyretin is synthesized in the epithelial cells of the choroid plexus and exclusively secreted towards the brain. A net movement of thyroid hormones from the blood to the brain ensues. During evolution, transthyretin synthesis in the choroid plexus and the beginnings of a neocortex first appeared at the stage of the stem reptiles. The affinity of transthyretin for thyroxine increased and that for triiodothyronine decreased during evolution. This could augment the importance of deiodination for regulation of metabolism and gene expression by thyroid hormones in the brain. Successive shifts of the splice site at the 5' end of exon 2 of transthyretin precursor mRNA in the 3' direction led to a shortening of the N-terminal sections and to an increase in hydrophilicity of the N-terminal regions of transthyretin. This shift can be explained by a sequence of single base mutations. It could be an example for a molecular mechanism of positive Darwinian evolution. The selection pressure, which led to the expression of the transthyretin gene in the choroid plexus during evolution, might have been the maintenance of thyroid hormone homeostasis in the extracellular compartment of the brain in the presence of the greatly increasing volume of the lipid phase.

Inhibition of transthyretin-met30 expression using Inosine(15.1)-Hammerhead ribozymes in cell culture

Hereditary amyloidosis is primarily caused by mutations within the transthyretin gene. More than 75 mutations within transthyretin have been reported in causing amyloidosis. The most common mutation is the val30met mutation in the transthyretin protein (TTR-met30) caused by a mononucleic substitution from G to A (GUC to AUC) in the transthyretin gene resulting in the exchange for the amino acids valine to methionine in the corresponding protein sequence. The aim of this work is the development of a specific cleavage of TTR-met30 mRNA in the cell culture system using hammerhead ribozymes. We showed previously that chemically modified nuclease stable Inosine(15.1)-Hammerhead ribozymes are able to target the TTR-met30 mRNA with high specificity on the RNA level (Biochem. Biophys. Res. Commun. 260, 313-317, 1999). Now we present data confirming our observations on the cellular level. We used the wild-type human normal (hn) TTR expressing cell line HepG2 and the stable transfected cell line 293-TTR-met30 for TTR-met30 experiments. We cleaved the TTR-met30 and hnTTR mRNA with specific nuclease stable chemically modified Inosine(15.1)-Hammerhead ribozymes and analyzed the protein after immunoprecipitation and subsequent Western blotting. We were able to downregulate the TTR concentration by 54.5% (100% = 1.5 mg/l TTR) and also specifically to target the TTR-met30 expression in the cell culture system. The therapeutic effect was improved using cationic liposomes resulting in a total downregulation by 92.1 and 62.7% targeting hnTTR mRNA and TTR-met30 mRNA, respectively. The successful employment of Inosine(15.1)-Hammerhead ribozymes in cell culture is therefore a promising tool for the development of a gene therapeutic strategy for hereditary amyloidosis.

The transthyretin-retinol-binding protein complex

Transthyretin (TTR, formerly called prealbumin), one of the transporters of the hormone thyroxine and the lipocalin retinol-binding protein (RBP), the specific carrier of the vitamin, are known to form, under physiological conditions, a macromolecular complex that is believed to play an important physiological role: prevention of glomerular filtration of the low molecular weight RBP in the kidneys. The physiological significance of complex formation is discussed first, followed by a brief description of the three-dimensional structure of the two participating proteins. The two X-ray models of the complex available are subsequently discussed and compared and finally the non-crystallographic evidence that supports these models is reviewed.

Evolution of the thyroid hormone-binding protein, transthyretin

Transthyretin (TTR) belongs to a group of proteins, which includes thyroxine-binding globulin and albumin, that bind to and transport thyroid hormones in the blood. TTR is also indirectly implicated in the carriage of vitamin A through the mediation of retinol-binding protein (RBP). It was first identified in 1942 in human serum and cerebrospinal fluid and was formerly called prealbumin for its ability to migrate faster than serum albumin on electrophoresis of whole plasma. It is a single polypeptide chain of 127 amino acids (14,000 Da) and is present in the plasma as a tetramer of noncovalently bound monomers. The major sites of synthesis of TTR in eutherian mammals, marsupials, and birds are the liver and choroid plexus but in reptiles it is synthesised only in the choroid plexus. The observation that TTR is strongly expressed in the choroid plexus but not in the liver of the stumpy-tailed lizard and the strong conservation of expression in the choroid plexus from reptiles to mammals have been taken as evidence to suggest that extrahepatic synthesis of TTR evolved first. The identification and cloning of TTR from the liver of an amphibian, Rana catesbeiana, and a teleost fish, Sparus aurata, and its absence from the choroid plexus of both species suggest an alternative model for its evolution. Protein modelling studies are presented that demonstrate differences in the electrostatic characteristics of the molecule in human, rat, chicken, and fish, which may explain why, in contrast to TTR from human and rat, TTR from fish and birds preferentially binds triiodo-l-thyronine.

The evolution of the thyroid hormone distributor protein transthyretin in the order insectivora, class mammalia

Thyroid hormones are involved in the regulation of growth and metabolism in all vertebrates. Transthyretin is one of the extracellular proteins with high affinity for thyroid hormones which determine the partitioning of these hormones between extracellular compartments and intracellular lipids. During vertebrate evolution, both the tissue pattern of expression and the structure of the gene for transthyretin underwent characteristic changes. The purpose of this study was to characterize the position of Insectivora in the evolution of transthyretin in eutherians, a subclass of Mammalia. Transthyretin was identified by thyroxine binding and Western analysis in the blood of adult shrews, hedgehogs, and moles. Transthyretin is synthesized in the liver and secreted into the bloodstream, similar to the situation for other adult eutherians, birds, and diprotodont marsupials, but different from that for adult fish, amphibians, reptiles, monotremes, and Australian polyprotodont marsupials. For the characterization of the structure of the gene and the processing of mRNA for transthyretin, cDNA libraries were prepared from RNA from hedgehog and shrew livers, and full-length cDNA clones were isolated and sequenced. Sections of genomic DNA in the regions coding for the splice sites between exons 1 and 2 were synthesized by polymerase chain reaction and sequenced. The location of splicing was deduced from comparison of genomic with cDNA nucleotide sequences. Changes in the nucleotide sequence of the transthyretin gene during evolution are most pronounced in the region coding for the N-terminal region of the protein. Both the derived overall amino sequences and the N-terminal regions of the transthyretins in Insectivora were found to be very similar to those in other eutherians but differed from those found in marsupials, birds, reptiles, amphibians, and fish. Also, the pattern of transthyretin precursor mRNA splicing in Insectivora was more similar to that in other eutherians than to that in marsupials, reptiles, and birds. Thus, in contrast to the marsupials, with a different pattern of transthyretin gene expression in the evolutionarily "older" polyprotodonts compared with the evolutionarily "younger" diprotodonts, no separate lineages of transthyretin evolution could be identified in eutherians. We conclude that transthyretin gene expression in the liver of adult eutherians probably appeared before the branching of the lineages leading to modern eutherian species.

Evolution of thyroid hormone binding by transthyretins in birds and mammals

Transthyretin, a protein synthesized and secreted by the choroid plexus and liver, binds thyroid hormones in extracellular compartments. This binding prevents accumulation of thyroid hormones in the lipids of membranes, establishing extracellular thyroid hormone pools for the distribution of the hormones throughout the body and brain. The N-termini of the transthyretin subunits are longer and more hydrophobic in chicken than in eutherian transthyretins. Here, we show that this is a general structural feature of avian transthyretins. Systematic changes of protein structure during evolution result from selection pressure leading to changes in function. The evolution of transthyretin function, namely, the binding of thyroid hormones, was studied in nine vertebrate species. The affinity of thyroxine binding to transthyretin is lowest in avians (mean Kd of about 30 nm), intermediate in metatherians (mean Kd of about 17 nm) and highest in eutherians (mean Kd of about 11 nm). The affinity for 3,5,3'-triiodothyronine shows an opposite trend, being four times higher for avian transthyretins than for mammalian transthyretins.

Synthetic flavonoids cross the placenta in the rat and are found in fetal brain

The synthetic flavonoid EMD-49209 is a potent inhibitor of the in vivo and in vitro binding of thyroxine (T4) to transthyretin (TTR). We studied the distribution of 125I-labeled EMD-49209 in maternal tissues, intestinal contents, and fetal tissues in rats that were 20 days pregnant (from 1 to 24 h after intravenous injection). The percent dose of EMD decreased quickly with time. In maternal brain no radioactive flavonoid could be detected. EMD was excreted very rapidly from the intestines. In the fetal compartment the percent dose of EMD increased with time; after 24 h it contained 17% of the EMD. The flavonoid was found in all fetal tissues investigated and also in the fetal brain. Because TTR concentrations are high in the fetal rat, especially in the brain, the transfer of flavonoid to the fetal brain might be linked to TTR expression. The presence of flavonoid in the fetal brain raises the possibility of an essential interference of flavonoids with the availability of T4 in the fetal compartment.

Abundant synthesis of transthyretin in the brain, but not in the liver, of turtles

The binding of thyroxine to proteins in the blood plasma of the turtle, Trachemys scripta, was analyzed by incubation with radioactive thyroxine, electrophoresis and autoradiography. Albumin and an alpha-globulin were found to bind thyroxine; no thyroxine-binding transthyretin was detected in the prealbumin region. In contrast to blood plasma, a thyroxine-binding prealbumin was observed in medium from T. scripta choroid plexus incubated in vitro. RNA was extracted from brain tissue containing choroid plexus and from liver of T. scripta and Chelydra serpentina and analyzed by hybridization with transthyretin cDNA from the lizard Tiliqua rugosa. The brain RNAs contained substantial amounts of transthyretin mRNA, whereas only trace amounts of transthyretin mRNA were detected in RNA from liver. No transthyretin mRNA was observed in RNA from kidney. The results support the hypothesis that the expression of the transthyretin gene first evolved in the choroid plexus of the brain at the stage of the stem reptiles, whereas abundant transthyretin synthesis in liver evolved much later, and independently, in mammals and birds.

Evolution of shorter and more hydrophilic transthyretin N-termini by stepwise conversion of exon 2 into intron 1 sequences (shifting the 3' splice site of intron 1)

Transthyretin cDNA was cloned from Eastern Grey Kangaroo liver and its nucleotide sequence determined. Analysis of the derived amino acid sequence of kangaroo transthyretin, together with data obtained previously for transthyretins from other vertebrate species [Duan, W., Richardson, S. J., Babon, J. J., Heyes, R. J., Southwell, B. R., Harms, P. J., Wettenhall, R. E. H., Dziegielewska, K. M., Selwood, L., Bradley, A. J., Brack, C. M. & Schreiber, G. (1995) Eur. J. Biochem. 227, 396-406], showed that the N-terminus is the region which changes most distinctly during evolution. It has been shown for human, mouse and rat transthyretins, that this region is encoded by DNA at the border of exon 1 and exon 2. Therefore, this section of transthyretin genomic DNA was amplified by PCR and directly sequenced for the Buffalo Rat, Tammar Wallaby, Eastern Grey Kangaroo, Stripe-faced Dunnart, Short-tailed Grey Opossum and White Leghorn Chicken. The splice sites at both ends of intron 1 were identified by comparison with the cDNA sequences. The obtained data suggest that the N-termini of transthyretin evolved by successive shifts of the 3' splice site of intron 1 in the 3' direction, resulting in successive shortening of the 5' end of exon 2. At the protein level, this resulted in a shorter and more hydrophilic N-terminal region of transthyretin. Successive shifts in splice sites may be an evolutionary mechanism of general importance, since they can lead to stepwise changes in the properties of proteins. This could be a molecular mechanism for positive Darwinian selection.

The evolution of gene expression, structure and function of transthyretin

Thyroxine, the most abundant thyroid hormone in blood, partitions into lipid membranes. In a network-like system, thyroxine-binding plasma proteins counteract this partitioning and establish intravascular, protein-bound thyroxine pools. These are far larger than the free thyroxine pools. In larger eutherians, proteins specifically binding thyroxine are albumin, transthyretin, and thyroxine-binding globulin. Some binding of thyroxine can also occur to lipoproteins. During evolution, transthyretin synthesis first appeared in the choroid plexus of the stem reptiles, about 300 million years ago. Transthretin synthesis in the liver evolved much later, independently, in birds, eutherians and some marsupial species. Analysis of 57 human transthyretin variants suggests that most mutations in transthyretin are not compatible with its normal metabolism and lead to its deposition as amyloid. Analysis of transthyretin or its gene in 20 different species shows that evolutionary changes of transthyretin predominantly occurred near the N-termini. A change in RNA splicing between exon 1 and exon 2 led to a decrease in hydrophobicity and length of the N-termini. It is proposed that the selection pressure producing these changes was the need for a more effective prevention of thyroxine partitioning into lipids. Lipid pools increased during evolution with the increases in relative sizes of brains and internal organs and changes in lipid composition of membranes in ectothermic and endothermic species.

The crystal structure of transthyretin from chicken

The crystal structure of chicken transthyretin has been solved at 290-pm resolution by molecular-replacement techniques. Transthyretin is the protein component of the amyloid fibrils found in patients suffering from either familial amyloidotic polyneuropathy or senile systemic amyloidosis. Familial amyloidotic polyneuropathy is an autosomal dominant hereditary type of amyloidosis which involves transthyretin with either one or two amino acid substitutions. The three-dimensional structure of chicken transthyretin was determined in order to compare a non-amyloidogenic, species-variant transthyretin with wild-type and mutant transthyretin molecules. Of the 31 chicken-to-human residue differences, 9 occur at positions which in human transthyretin give rise to amyloidogenic variants although none corresponds to the appropriate side-chain substitutions. The model of chicken transthyretin has been refined to an R-factor of 19.9%. The overall fold of the protein is that of an all-beta protein. Compared with wild-type human transthyretin the avian transthyretin shows quite large differences in the region known to be involved in binding to retinol-binding protein, it has a much shorter helical component than the human protein and some of the monomer-monomer interactions are different.