Crystallization and preliminary X-ray data for the human transthyretin-retinol-binding protein (RBP) complex bound to an anti-RBP Fab

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.

Review: TTR amyloidosis-structural features leading to protein aggregation and their implications on therapeutic strategies

Transthyretin amyloidosis represents a spectrum of clinical syndromes that, in all cases except senile systemic amyloidosis, are dependent on the mutation present in the transthyretin (TTR) protein. Although the role of amyloid deposits in the pathogenesis of the disease is not clear, preventing their formation or promoting their disaggregation is necessary to control the development of clinical symptoms. The design of therapies aiming at preventing amyloid formation or promoting its dissociation requires detailed knowledge of the fibrils' molecular structure and a complete view about the factors responsible for protein aggregation. This review is focused on the structural studies, performed on amyloid fibrils and amyloidogenic TTR variants, aiming at understanding the aggregation mechanism as well as the atomic structure of the fibril assembly. Based on the available information possible therapies are also surveyed.