Structure of pig plasma retinol-binding protein at 1.65 Å resolution

The crystal structure of pig plasma retinol-binding protein (RBP) has been determined at 1.65 Å resolution. The space group is P2₁2₁2₁, with a = 45.81 (4), b = 53.14 (5), c = 72.97 (8) Å and one protein molecule in the asymmetric unit. The structure has been solved using the molecular replacement method and refined with restrained least squares to an R factor of 0.1844 and an R_free of 0.237 for 18 874 and 1001 independent reflections, respectively. The relatively high resolution structure of pig holoRBP has revealed some new structural details. Moreover, it has provided a description of the binding site for Cd²⁺, a metal ion which is required for protein crystallization. The hepta-coordination of the RBP-bound cadmium ion involves different residues of two symmetry-related RBP molecules, consistent with the participation of the cation in intermolecular interactions that in turn promote protein crystallization.