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.
Zanotti
Giuseppe
G
Department of Chemical Sciences and Institute of Biomolecular Chemistry-CNR, University of Padua, Italy. giuseppe.zanotti@unipd.it
Folli
Claudia
C
Cendron
Laura
L
Alfieri
Beatrice
B
Nishida
Sonia K
SK
Gliubich
Francesca
F
Pasquato
Nicola
N
Negro
Alessandro
A
Berni
Rodolfo
R
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D018360:Crystallography, X-Ray; D005454:Fluorescence Polarization; D006801:Humans; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry; D051379:Mice; D008958:Models, Molecular; D011228:Prealbumin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D054839:Retinol-Binding Proteins, Plasma / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
D007140:Immunoglobulin Fab Fragments; D011228:Prealbumin; D054839:Retinol-Binding Proteins, Plasma