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Transthyretin (TTR) amyloid fibril formation is observed systemically in familial amyloid polyneuropathy and senile systemic amyloidosis and appears to be the causative agent in these diseases. Herein, we demonstrate conclusively that thyroxine (10.8 M) inhibits TTR fibril formation efficiently in vitro and does so by stabilizing the tetramer against dissociation and the subsequent conformational changes required for amyloid fibril formation. In addition, the nonnative ligand 2,4,6-triiodophenol, which binds to TTR with slightly increased affinity also inhibits TTR fibril formation by this mechanism. Sedimentation velocity experiments were employed to show that TTR undergoes dissociation (linked to a conformational change) to form the monomeric amyloidogenic intermediate, which self-assembles into amyloid in the absence, but not in the presence of thyroxine. These results demonstrate the feasibility of using small molecules to stabilize the native fold of a potentially amyloidogenic human protein, thus preventing the conformational changes, which appear to be the common link in several human amyloid diseases. This strategy and the compounds resulting from further development should prove useful for critically evaluating the amyloid hypothesis-i.e., the putative cause-and-effect relationship between TTR amyloid deposition and the onset of familial amyloid polyneuropathy and senile systemic amyloidosis.Transthyretin (TTR) is present in human plasma (0.2 mg͞ml; 3.63 M, tetramer) and is composed of four identical ␤-sheetrich subunits that bind and transport thyroxine (T4) and the retinol binding protein (1). In unfortunate individuals, TTR is converted into an insoluble fibrillar structure called amyloid. These fibrils putatively cause senile systemic amyloidosis (wild-type TTR composes the fibrils-late onset) and familial amyloid polyneuropathy (FAP; predominantly variant TTR composing the fibrils-earlier onset) by virtue of the amyloid's neurotoxicity and͞or by physically interfering with normal organ function (2-8). A TTR amyloid fibril is Ϸ130Å in diameter and made up of four protofilaments, each having a twisted cross-␤-helix structure (9, 10). TTR amyloid fibril formation is observed during partial acid denaturation from a conformational intermediate formed under conditions simulating a lysosome (pH 5.5 Ϯ 0.5), which has been implicated in fibril formation in vivo (11,12). TTR amyloid fibril formation can be avoided under acidic conditions by working at low TTR concentrations and low temperature (25ЊC) allowing identification of the quaternary, tertiary, and secondary structure of the intermediate(s) that can form amyloid (12). These studies reveal that tetrameric TTR is nonamyloidogenic; however, the dissociation of the tetramer into a monomeric intermediate having an altered, but defined, tertiary structure is capable of amyloid fibril formation and is therefore called the amyloidogenic intermediate (Fig. 1). Several of the 50 FAP-associated TTR single-site mutations still adopt a normal tetrameric s...

Insoluble protein fibrils resulting from the self-assembly of a conformational intermediate are implicated as the causative agent in several severe human amyloid diseases, including Alzheimer's disease, familial amyloid polyneuropathy, and senile systemic amyloidosis. The latter two diseases are associated with transthyretin (TTR) amyloid fibrils, which appear to form in the acidic partial denaturing environment of the lysosome. Here we demonstrate that f lufenamic acid (Flu) inhibits the conformational changes of TTR associated with amyloid fibril formation. The crystal structure of TTR complexed with Flu demonstrates that Flu mediates intersubunit hydrophobic interactions and intersubunit hydrogen bonds that stabilize the normal tetrameric fold of TTR. A small-molecule inhibitor that stabilizes the normal conformation of a protein is desirable as a possible approach to treat amyloid diseases. Molecules such as Flu also provide the means to rigorously test the amyloid hypothesis, i.e., the apparent causative role of amyloid fibrils in amyloid disease.

We had previously identified the WW domain as a novel globular domain that is composed of 38 -40 semiconserved amino acids and is involved in mediating protein-protein interaction. The WW domain is shared by proteins of diverse functions including structural, regulatory, and signaling proteins in yeast, nematode, and mammals. Functionally it is similar to the Src homology 3 domain in that it binds polyproline ligands. By screening a 16-day mouse embryo expression library, we identified two putative ligands of the WW domain of Yes kinase-associated protein which we named WW domainbinding proteins 1 and 2. These proteins interacted with the WW domain via a short proline-rich motif with the consensus sequence of four consecutive prolines followed by a tyrosine. Herein, we report the cDNA cloning and characterization of the human orthologs of WW domain-binding proteins 1 and 2. The products encoded by these cDNA clones represent novel proteins with no known function. Furthermore, these proteins show no homology to each other except for a proline-rich motif. By fluorescence in situ hybridization on human metaphase chromosomes, we mapped the human genes for WW domain-binding proteins 1 and 2 to chromosomes 2p12 and 17q25, respectively. In addition, using sitedirected mutagenesis, we determined which residues in the WW domain of Yes kinase-associated protein are critical for binding. Finally, by synthesizing peptides in which the various positions of the four consecutive proline-tyrosine motif and the five surrounding residues were replaced by all possible amino acid residues, we further elucidated the binding requirements of this motif.The Src homology (SH) 1 2 and SH3 domains have assumed essential roles in furthering the understanding of how an extracellular signal is transmitted from the cellular membrane, through the cytoplasm, and finally into the nucleus where the signal is interpreted through the process of gene-specific transcription. The SH2 domain has been shown to interact specifically with sequences containing a phosphotyrosine residue, whereas the SH3 domain mediates binding to proline-rich sequences with the minimal consensus of PXXP (P represents proline, and X designates any amino acid) (1, 2). The SH2 and SH3 domains thus consist of a common binding core that recognizes phosphotyrosine-or proline-rich motifs, respectively, and which achieve binding specificity through unique flanking sequences (3-5). As a result, these domains determine which proteins can interact, and equally important, in what order the interaction occurs in the closely regulated pathways of signal transduction. Recently, two other important signaling modules were characterized: the pleckstrin homology domain and the protein interaction domain/phosphotyrosine binding domain (6 -10). These modular repeats represent true protein domains in that they constitute structurally distinct three-dimensional units that can properly fold and function in the context of other proteins or in isolation (11,12).We have previously identified a Yes ki...

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