CRYSTAL-STRUCTURE OF CELLULAR RETINOIC ACID-BINDING PROTEIN-I SHOWS INCREASED ACCESS TO THE BINDING CAVITY DUE TO FORMATION OF AN INTERMOLECULAR BETA-SHEET

A recombinant form of murine ape-cellular retinoic acid binding protei n I (apo-CRABPI) has been purified and crystallized at pH 5.0, and the crystal structure has been refined to an R-factor of 19.6% at a resol ution of 2.7 Angstrom. CRABPI binds all-trans retinoic acid and some r etinoic acid metabolites with nanomolar affinities. Coordinates of the hole form of CRABP were not available during the early stages of the study, and in spite of numerous homologs of known structure, phases we re not obtainable through molecular replacement. Instead, an interpret able electron density map was obtained by multiple isomorphous replace ment methods after improvement of the heavy-atom parameters with densi ty modified trial phases. Two molecules of apo-CRABPI occupy the P3(1) 21 asymmetric unit and are related by pseudo 2-fold rotational symmetr y Unique conformational differences are apparent between the two molec ules. In all of the family members studied to date, there is a lack of hydrogen bonds between two of the component beta-strands resulting in a gap in the interstand hydrogen bonding pattern. In the crystallogra phic dimer described here, a continuous intermolecular beta-sheet is f ormed by using this gap region. This is possible because of an 8 Angst rom outward maximum displacement of the tight turn between the third a nd fourth beta-strands on one of the molecules. The result is a double beta-barrel containing two apo-CRABPI molecules with a more open, lig and-accessible binding cavity, which has not been observed in other st ructures of a family of proteins that bind hydrophobic ligands. (C) 19 95 Academic Press Limited