The structure of human retinol-binding protein (RBP) with its carrier protein transthyretin reveals an interaction with the carboxy terminus of RBP

Whether ultimately utilized as retinoic acid, retinal, or retinol, vitamin A is transported to the target cells as all-trans-retinol bound to retinol-binding protein (RBP). Circulating in the plasma, REP itself is bound to transthyretin (TTR, previously referred to as thyroxine-binding prealbumin). In vitro one tetramer of TTR can bind two molecules of retinol-binding protein. However, the concentration of REP in the plasma is limiting, and the complex isolated from serum is composed of TTR and REP in a I to 1 stoichiometry. We report here the crystallographic structure at 3.2 Angstrom of the protein-protein complex of human REP and TTR. REP binds at a 2-fold axis of symmetry in the TTR tetramer, and consequently the recognition site itself has 2-fold symmetry: Four TTR amino acids (Arg-21, Val-20, Leu-82, and Ile-84) are contributed by two monomers. Amino acids Trp-67, Phe-96, and Leu-63 and -97 from REP are flanked by the symmetry-related side chains from TTR. In addition, the structure reveals an interaction of the carboxy terminus of REP at the protein-protein recognition interface. This interaction, which involves Leu-182 and Leu-183 of REP, is consistent with the observation that naturally occurring truncated forms of the protein are more readily cleared from plasma than full-length REP. Complex formation prevents extensive loss of REP through glomerular filtration, and the loss of Leu-182 and Leu-183 would result in a decreased affinity of REP for TTR.