Identification of the retinol-binding protein (RBP) interaction site and functional state of RBPs for the membrane receptor

Inhalant Mammal-Derived Lipocalin Allergens and the Innate Immunity

A major part of important mammalian respiratory allergens belongs to the lipocalin family of proteins. By this time, 19 respiratory mammalian lipocalin allergens have been registered in the WHO/IUIS Allergen Nomenclature Database. Originally, lipocalins, small extracellular proteins (molecular mass ca. 20 kDa), were characterized as transport proteins but they are currently known to exert a variety of biological functions. The three-dimensional structure of lipocalins is well-preserved, and lipocalin allergens can exhibit high amino acid identities, in several cases more than 50%. Lipocalins contain an internal ligand-binding site where they can harbor small principally hydrophobic molecules. Another characteristic feature is their capacity to bind to specific cell-surface receptors. In all, the physicochemical properties of lipocalin allergens do not offer any straightforward explanations for their allergenicity. Allergic sensitization begins at epithelial barriers where diverse insults through pattern recognition receptors awaken innate immunity. This front-line response is manifested by epithelial barrier-associated cytokines which together with other components of immunity can initiate the sensitization process. In the following, the crucial factor in allergic sensitization is interleukin (IL)-4 which is needed for stabilizing and promoting the type 2 immune response. The source for IL-4 has been searched widely. Candidates for it may be non-professional antigen-presenting cells, such as basophils or mast cells, as well as CD4+ T cells. The synthesis of IL-4 by CD4+ T cells requires T cell receptor engagement, i.e., the recognition of allergen peptides, which also provides the specificity for sensitization. Lipocalin and innate immunity-associated cell-surface receptors are implicated in facilitating the access of lipocalin allergens into the immune system. However, the significance of this for allergic sensitization is unclear, as the recognition by these receptors has been found to produce conflicting results. As to potential adjuvants associated with mammalian lipocalin allergens, the hydrophobic ligands transported by lipocalins have not been reported to enhance sensitization while it is justified to suppose that lipopolysaccharide plays a role in it. Taken together, type 2 immunity to lipocalin allergens appears to be a harmful immune response resulting from a combination of signals involving both the innate and adaptive immunities.

β-Lactoglobulin and Glycodelin: Two Sides of the Same Coin?

The two lipocalins, β-lactoglobulin (βLg) and glycodelin (Gd), are possibly the most closely related members of the large and widely distributed lipocalin family, yet their functions appear to be substantially different. Indeed, the function of β-lactoglobulin, a major component of ruminant milk, is still unclear although neonatal nutrition is clearly important. On the other hand, glycodelin has several specific functions in reproduction conferred through distinct, tissue specific glycosylation of the polypeptide backbone. It is also associated with some cancer outcomes. The glycodelin gene, PAEP, reflecting one of its names, progestagen-associated endometrial protein, is expressed in many though not all primates, but the name has now also been adopted for the β-lactoglobulin gene (HGNC, www.genenames.org). After a general overview of the two proteins in the context of the lipocalin family, this review considers the properties of each in the light of their physiological functional significance, supplementing earlier reviews to include studies from the past decade. While the biological function of glycodelin is reasonably well defined, that of β-lactoglobulin remains elusive.

A role of heparan sulphate proteoglycan in the cellular uptake of lipocalins ß-lactoglobulin and allergen Fel d 4

Lipocalins, small extracellular hydrophobic molecule carriers, can be internalized by a variety of different cells. However, to date receptors have only been identified for human lipocalins. Here, we specifically investigated uptake mechanisms for lipocalins ß-lactoglobulin and Fel d 4 in HeLa and Chinese hamster ovary (CHO) cells. We provide evidence that cell surface heparan sulphate proteoglycan is essential for internalization of these lipocalins. In HeLa cells, lipocalin uptake was inhibited by competition with soluble heparin, enzymatic digestion of cellular heparan sulphate by heparinase and inhibition of its biosynthesis by sodium chlorate. Biochemical studies by heparin affinity chromatography and colocalization studies further supported a role of heparan sulphate proteoglycan in lipocalin uptake. Finally, lipocalin uptake was blocked in CHO mutant cells defective in glycosaminoglycan biosynthesis whereas in wild-type cells it was clearly detectable. Thus, cell surface heparan sulphate proteoglycan represents a novel component absolutely participating in the cellular uptake of some lipocalins.

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

Structure of the STRA6 receptor for retinol uptake

Vitamin A homeostasis is critical to normal cellular function. Retinol-binding protein (RBP) is the sole specific carrier in the bloodstream for hydrophobic retinol, the main form in which vitamin A is transported. The integral membrane receptor STRA6 mediates cellular uptake of vitamin A by recognizing RBP-retinol to trigger release and internalization of retinol. We present the structure of zebrafish STRA6 determined to 3.9-angstrom resolution by single-particle cryo-electron microscopy. STRA6 has one intramembrane and nine transmembrane helices in an intricate dimeric assembly. Unexpectedly, calmodulin is bound tightly to STRA6 in a noncanonical arrangement. Residues involved with RBP binding map to an archlike structure that covers a deep lipophilic cleft. This cleft is open to the membrane, suggesting a possible mode for internalization of retinol through direct diffusion into the lipid bilayer.

New insights and changing paradigms in the regulation of vitamin A metabolism in development

Vitamin A and its active metabolite retinoic acid are essential for embryonic development and adult homeostasis. Surprisingly, excess or deficiency of vitamin A and retinoic acid can cause similar developmental defects. Therefore, strict feedback and other mechanisms exist to regulate the levels of retinoic acid within a narrow physiological range. The oxidation of vitamin A to retinal has recently been established as a critical nodal point in the synthesis of retinoic acid, and over the past decade, RDH10 and DHRS3 have emerged as the predominant enzymes that regulate this reversible reaction. Together they form a codependent complex that facilitates negative feedback maintenance of retinoic acid levels and thus guard against the effects of dysregulated vitamin A metabolism and retinoic acid synthesis. This review focuses on advances in our understanding of the roles of Rdh10 and Dhrs3 and their impact on development and disease. WIREs Dev Biol 2017, 6:e264. doi: 10.1002/wdev.264 For further resources related to this article, please visit the WIREs website.

Estradiol's interesting life at the cell's plasma membrane

Clearly, we have presented here evidence of a very complex set of mechanisms and proteins involved with various and intricate actions of steroids at the plasma membrane. Steroids do MUCH more at the plasma membrane than simply passing passively through it. They may sit in the membrane; they are bound by numerous proteins in the membrane, including ERs, SHBG, steroid-binding globulin receptors, and perhaps elements of cellular architecture such as tubulin. It also seems likely that the membrane itself responds graphically to the presence of steroids by actually changing its shape as well, perhaps, as accumulating steroids. Clara Szego suggested in the 1980s that actions of E2 at one level would act synergistically with its actions at another level (e.g. membrane actions would complement nuclear actions). Given the sheer number of proteins involved in steroid actions, just at the membrane level, it seems unlikely that every action of a steroid on every potential protein effector will act to the same end. It seems more likely that these multiple effects and sites of effect of steroids contribute to the confusion that exists as to what actions steroids always have. For example, there is confusion with regard to synthetic agents (SERMs etc.) that have different and often opposite actions depending on which organ they act upon. A better understanding of the basic actions of steroids should aid in understanding the variability of their clinical effects.

The effect of retinol binding protein on the proteome of C2C12 muscle cells

BACKGROUND

Retinol binding protein (RBP) and its membrane receptor, STRA6, are vital for the management of vitamin A in the body. Recently, elevated serum RBP levels have been implicated as a contributing factor to the development of insulin resistance and type 2 diabetes. However, conflicting opinions exist as to how these increased levels can cause insulin resistance.

METHODS

In order to better understand the influences of RBP, a proteomic study was devised to determine the direct effect of RBP on a mouse muscle cell line, because the muscle is the principal site of insulin induced glucose uptake. C2C12 cells were treated with RBP for 16 h and the proteome analysed for alterations in protein abundance and phosphorylation by 2-DE.

RESULTS

A number of changes were observed in response to retinol binding protein treatment, of which the most interesting were decreased levels of the phosphatase, protein phosphatase 1 β. This phosphatase is responsible for regulating glycogen synthase and glycogen phosphorylase, the rate-limiting enzymes involved in glycogen storage and utilization. Retinol binding protein treatment resulted in increased phosphorylation and inhibition of glycogen synthase, with detrimental effects on insulin stimulated glycogen production in these cells.

CONCLUSION

The results indicate that RBP may have a negative effect on energy storage in the cell and could contribute to the development of insulin resistance in muscle tissue. Understanding how retinol binding protein influences insulin resistance may reveal novel strategies to target this disease.

STRA6: role in cellular retinol uptake and efflux

Distribution of vitamin A throughout the body is important to maintain retinoid function in peripheral tissues and to ensure optimal vision. A critical step of this process is the transport of vitamin A across cell membranes. Increasing evidence indicates that this process is mediated by a multidomian membrane protein that is encoded by the stimulated by retinoic acid 6 (STRA6) gene. Biochemical studies revealed that STRA6 is a transmembrane pore which transports vitamin A bidirectionally between extra- and intracellular retinoid binding proteins. Vitamin A accumulation in cells is driven by coupling of transport with vitamin A esterification. Loss-of-function studies in zebrafish and mouse models have unraveled the critical importance of STRA6 for vitamin A homeostasis of peripheral tissues. Impairment in vitamin A transport and uptake homeostasis are associated with diseases including type 2 diabetes and a microphthalmic syndrome known as Matthew Wood Syndrome. This review will discuss the advanced state of knowledge about STRA6's biochemistry, biology and association with disease.

STRA6 is critical for cellular vitamin A uptake and homeostasis

Vitamin A must be adequately distributed within the body to maintain the functions of retinoids in the periphery and chromophore production in the eyes. Blood transport of the lipophilic vitamin is mediated by the retinol-binding protein, RBP4. Biochemical evidence suggests that cellular uptake of vitamin A from RBP4 is facilitated by a membrane receptor. This receptor, identified as the Stimulated by retinoic acid gene 6 (Stra6) gene product, is highly expressed in epithelia that constitute blood-tissue barriers. Here we established a Stra6 knockout mouse model to analyze the metabolic basis of vitamin A homeostasis in peripheral tissues. These mice were viable when bred on diets replete in vitamin A, but evidenced markedly reduced levels of ocular retinoids. Ophthalmic imaging and histology revealed malformations in the choroid and retinal pigmented epithelium, early cone photoreceptor cell death, and reduced lengths of rod outer segments. Similar to the blood-retina barrier in the RPE, vitamin A transport through the blood-cerebrospinal fluid barrier in the brain's choroid plexus was impaired. Notably, treatment with pharmacological doses of vitamin A restored vitamin A transport across these barriers and rescued the vision of Stra6(-/-) mice. Furthermore, under conditions mimicking vitamin A excess and deficiency, our analyses revealed that STRA6-mediated vitamin A uptake is a regulated process mandatory for ocular vitamin A uptake when RBP4 constitutes the only transport mode in vitamin A deficiency. These findings identifying STRA6 as a bona fide vitamin A transporter have important implications for disease states associated with impaired blood vitamin A homeostasis.

Enthalpy/entropy compensation effects from cavity desolvation underpin broad ligand binding selectivity for rat odorant binding protein 3

Evolution has produced proteins with exquisite ligand binding specificity, and manipulating this effect has been the basis for much of modern rational drug design. However, there are general classes of proteins with broader ligand selectivity linked to function, the origin of which is poorly understood. The odorant binding proteins (OBPs) sequester volatile molecules for transportation to the olfactory receptors. Rat OBP3, which we characterize by X-ray crystallography and NMR, binds a homologous series of aliphatic γ-lactones within its aromatic-rich hydrophobic pocket with remarkably little variation in affinity but extensive enthalpy/entropy compensation effects. We show that the binding energetics are modulated by two desolvation processes with quite different thermodynamic signatures. Ligand desolvation follows the classical hydrophobic effect; however, cavity desolvation is consistent with the liberation of "high energy" water molecules back into bulk solvent with a strong, but compensated, enthalpic contribution, which together underpin the origins of broad ligand binding selectivity.

Expression, characterization and ligand specificity of lipocalin-1 interacting membrane receptor (LIMR)

Human lipocalin-1 interacting membrane receptor (LIMR) was the first lipocalin receptor to be identified, as a specific receptor for lipocalin-1 (Lcn1). Subsequently LIMR has been reported to interact with other ligands as well, notably with the bovine lipocalin β-lactoglobulin (BLG) and with the unrelated secretoglobin uteroglobin (UG). To study the ligand-binding behaviour of this prototypic lipocalin receptor in more detail, a system was developed for the recombinant expression of LIMR in Drosophila Schneider 2 (S2) cells, and for the subsequent solubilization and purification of the protein. The receptor forms dimers or larger oligomers when solubilized in n-dodecyl β-D-maltoside (DDM). The full-length, functional receptor was captured onto a surface plasmon resonance (SPR) chip via an α-V5 antibody, and the binding of various potential ligands was followed in time. In this way, LIMR was shown to be highly specific for Lcn1, binding the lipocalin with low micromolar to high nanomolar affinity. No interactions with any of the other putative ligands could be detected, raising doubts about the physiological relevance of the reported binding of BLG and UG to the receptor.

Protein HP1028 from the human pathogen Helicobacter pylori belongs to the lipocalin family

Helicobacter pylori is a bacterial pathogen that causes severe diseases, including gastritis, ulcers and gastric cancer. Although this bacterium has been extensively studied, the physiological functions of a large number of the proteins encoded by its genome are unknown. HP1028 is a protein that is relevant to colonization and to the survival of the bacterium in the stomach, but its function is not clearly understood. Bioinformatics studies suggest that HP1028 is a monomeric protein that is secreted in the H. pylori periplasm. The crystal structure of HP1028 has been determined at 2.6 Å resolution using the SAD method. The three-dimensional structure of the protein reveals that it belongs to the lipocalin family, a group of proteins that bind and transport (often hydrophobic) small molecules. The structure of HP1028, together with the possible localization of the mature protein in the bacterial periplasm and the position of the hp1028 gene in the bacterial genome, point to a role in H. pylori chemotaxis.

Effects of estradiol on the endocytic transport of vitamin D carrier protein in hepatocytes

BACKGROUND

The possible modulation of receptor-mediated endocytosis (RME) by sex steroids is not well understood, especially in terms of the different receptor-ligand systems and cell types that may exhibit such regulation. The main objective of the current study was to examine the short-term effects of 17β-estradiol (E2) on RME of an extracellular carrier protein for calciferols, vitamin D-binding protein (DBP).

METHODS

Murine male and female primary hepatocytes were treated for 30min in the absence (controls) or presence of Ε2 (1μM). Labeled DBP was then added, and its endocytosis was measured after an incubation of 10min at 37°C using standard ELISA techniques. To obtain further insight into potential molecular mechanisms, fulvestrant and 17α-ethinyl estradiol (EE) were also analyzed. And as part of comparative analyses, a second nutrient carrier protein, vitamin A-binding protein (RBP), was also analyzed.

RESULTS

The results provide the first evidence for an estradiol-dependent stimulation of DBP endocytosis (p<0.05 relative to controls without Ε2). This stimulation, however, was only observed in female hepatocytes. Uptake of RBP was enhanced to a similar extent as DBP by estradiol. In normal (non-estradiol treated) male and female hepatocytes such changes in DBP or RBP endocytosis were not observed. Both fulvestrant and EE exhibited a significant (p<0.05), but incomplete, inhibition of Ε2-dependent stimulation of endocytosis.

CONCLUSIONS

The results provide novel evidence for Ε2 effects on endocytic transport; and for gender-related differences in E2-enhanced transport. These Ε2 effects may be partly dependent on estrogen receptors; but possible, additional or alternative mechanisms are also proposed.

GENERAL SIGNIFICANCE

Endocytic transport is a fundamental function whose regulation has implications for cell signaling, growth, survival, differentiation, and death. This study helps delineate a possible endocrine regulatory pathway involving modulation of endocytosis by a steroid hormone. It also provides a potential, new relation between different hormonal regulators, e.g., estradiol effects on cellular assimilation of calciferols.

Retinoid-binding proteins: similar protein architectures bind similar ligands via completely different ways

BACKGROUND

Retinoids are a class of compounds that are chemically related to vitamin A, which is an essential nutrient that plays a key role in vision, cell growth and differentiation. In vivo, retinoids must bind with specific proteins to perform their necessary functions. Plasma retinol-binding protein (RBP) and epididymal retinoic acid binding protein (ERABP) carry retinoids in bodily fluids, while cellular retinol-binding proteins (CRBPs) and cellular retinoic acid-binding proteins (CRABPs) carry retinoids within cells. Interestingly, although all of these transport proteins possess similar structures, the modes of binding for the different retinoid ligands with their carrier proteins are different.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, we analyzed the various retinoid transport mechanisms using structure and sequence comparisons, binding site analyses and molecular dynamics simulations. Our results show that in the same family of proteins and subcellular location, the orientation of a retinoid molecule within a binding protein is same, whereas when different families of proteins are considered, the orientation of the bound retinoid is completely different. In addition, none of the amino acid residues involved in ligand binding is conserved between the transport proteins. However, for each specific binding protein, the amino acids involved in the ligand binding are conserved. The results of this study allow us to propose a possible transport model for retinoids.

CONCLUSIONS/SIGNIFICANCE

Our results reveal the differences in the binding modes between the different retinoid-binding proteins.

Fenretinide derivatives act as disrupters of interactions of serum retinol binding protein (sRBP) with transthyretin and the sRBP receptor

Serum retinol binding protein (sRBP) is released from the liver as a complex with transthyretin (TTR), a process under the control of dietary retinol. Elevated levels of sRBP may be involved in inhibiting cellular responses to insulin and in generating first insulin resistance and then type 2 diabetes, offering a new target for therapeutic attack for these conditions. A series of retinoid analogues were synthesized and examined for their binding to sRBP and their ability to disrupt the sRBP-TTR and sRBP-sRBP receptor interactions. A number inhibit the sRBP-TTR and sRBP-sRBP receptor interactions as well as or better than Fenretinide (FEN), presenting a potential novel dual mechanism of action and perhaps offering a new therapeutic intervention against type 2 diabetes and its development. Shortening the chain length of the FEN derivative substantially abolished binding to sRBP, indicating that the strength of the interaction lies in the polyene chain region. Differences in potency against the sRBP-TTR and sRBP-sRBP receptor interactions suggest variant effects of the compounds on the two loops of sRBP guarding the entrance of the binding pocket that are responsible for these two protein-protein interactions.

High resolution X-ray structures of mouse major urinary protein nasal isoform in complex with pheromones

In mice, the major urinary proteins (MUP) play a key role in pheromonal communication by binding and transporting semiochemicals. MUP-IV is the only isoform known to be expressed in the vomeronasal mucosa. In comparison with the MUP isoforms that are abundantly excreted in the urine, MUP-IV is highly specific for the male mouse pheromone 2-sec-butyl-4,5-dihydrothiazole (SBT). To examine the structural basis of this ligand preference, we determined the X-ray crystal structure of MUP-IV bound to three mouse pheromones: SBT, 2,5-dimethylpyrazine, and 2-heptanone. We also obtained the structure of MUP-IV with 2-ethylhexanol bound in the cavity. These four structures show that relative to the major excreted MUP isoforms, three amino acid substitutions within the binding calyx impact ligand coordination. The F103 for A along with F54 for L result in a smaller cavity, potentially creating a more closely packed environment for the ligand. The E118 for G substitution introduces a charged group into a hydrophobic environment. The sidechain of E118 is observed to hydrogen bond to polar groups on all four ligands with nearly the same geometry as seen for the water-mediated hydrogen bond network in the MUP-I and MUP-II crystal structures. These differences in cavity size and interactions between the protein and ligand are likely to contribute to the observed specificity of MUP-IV.

Tissue-specific increases in endogenous all-trans retinoic acid: possible contributing factor in ethanol toxicity

All-trans retinoic acid (atRA) is required for neurogenesis and dendritic growth in the hippocampus. The toxic effects of ethanol include developmental defects, cognitive dysfunction, and increased risk of cancer and have some similarities to the detrimental effects of excess atRA, the active form of vitamin A. It is therefore possible that disruption of atRA homeostasis would contribute to the deleterious effects of ethanol. However, previous work, using very high exogenous doses of retinol, found that ethanol toxicity led to decreased formation of atRA, apparently due to competitive inhibition of alcohol dehydrogenase, which is purportedly involved in the conversion of retinol to retinal. A new study, using assays that are highly sensitive for endogenous atRA, has reported that ethanol toxicity in mice actually increased atRA concentration in certain tissues, including brain hippocampus, apparently due to a mobilization of hepatic retinyl esters that led to increased retinol and atRA in specific tissues.

Ethanol elevates physiological all-trans-retinoic acid levels in select loci through altering retinoid metabolism in multiple loci: a potential mechanism of ethanol toxicity

All-trans-retinoic acid (atRA) supports embryonic development, central nervous system function, and the immune response. atRA initiates neurogenesis and dendritic growth in the hippocampus and is required for spatial memory; superphysiological atRA inhibits neurogenesis, causes teratology and/or embryo toxicity, and alters cognitive function and behavior. Because abnormal atRA shares pathological conditions with alcoholism, inhibition of retinol (vitamin A) activation into atRA has been credited widely as a mechanism of ethanol toxicity. Here, we analyze the effects of ethanol on retinoid concentrations in vivo during normal vitamin A nutriture, using sensitive and analytically robust assays. Ethanol either increased or had no effect on atRA, regardless of changes in retinol and retinyl esters. Acute ethanol (3.5 g/kg) increased atRA in adult hippocampus (1.6-fold), liver (2.4-fold), and testis (1.5-fold). Feeding dams a liquid diet with 6.5% ethanol from embryonic day 13 (e13) to e19 increased atRA in fetal hippocampus (up to 20-fold) and cortex (up to 50-fold), depending on blood alcohol content. One-month feeding of the 6.5% ethanol diet increased atRA in adult hippocampus (20-fold), cortex (2-fold), testis (2-fold), and serum (10-fold). Tissue-specific increases in retinoid dehydrogenase mRNAs and activities, extrahepatic retinol concentrations, and atRA catabolism combined to produce site-specific effects. Because a sustained increase in atRA has deleterious effects on the central nervous system and embryo development, these data suggest that superphysiological atRA contributes to ethanol pathological conditions, including cognitive dysfunction and fetal alcohol syndrome.-Kane, M. A., Folias, A. E., Wang, C., Napoli, J. L. Ethanol elevates physiological all-trans-retinoic acid levels in select loci through altering retinoid metabolism in multiple loci: a potential mechanism of ethanol toxicity.

Structural and mutational analyses of protein-protein interactions between transthyretin and retinol-binding protein

Transthyretin is a tetrameric binding protein involved in the transport of thyroid hormones and in the cotransport of retinol by forming a complex in plasma with retinol-binding protein. In the present study, we report the crystal structure of a macromolecular complex, in which human transthyretin, human holo-retinol-binding protein and a murine anti-retinol-binding protein Fab are assembled according to a 1 : 2 : 2 stoichiometry. The main interactions, both polar and apolar, between retinol-binding protein and transthyretin involve the retinol hydroxyl group and a limited number of solvent exposed residues. The relevance of transthyretin residues in complex formation with retinol-binding protein has been examined by mutational analysis, and the structural consequences of some transthyretin point mutations affecting protein-protein recognition have been investigated. Despite a few exceptions, in general, the substitution of a hydrophilic for a hydrophobic side chain in contact regions results in a decrease or even a loss of binding affinity, thus revealing the importance of interfacial hydrophobic interactions and a high degree of complementarity between retinol-binding protein and transthyretin. The effect is particularly evident when the mutation affects an interacting residue present in two distinct subunits of transthyretin participating simultaneously in two interactions with a retinol-binding protein molecule. This is the case of the amyloidogenic I84S replacement, which abolishes the interaction with retinol-binding protein and is associated with an altered retinol-binding protein plasma transport in carriers of this mutation. Remarkably, some of the residues in mutated human transthyretin that weaken or abolish the interaction with retinol-binding protein are present in piscine transthyretin, consistent with the lack of interaction between retinol-binding protein and transthyretin in fish.