Crystal Structure of Cellular Retinoic Acid Binding Protein I Shows Increased Access to the Binding Cavity due to Formation of an Intermolecular Β-Sheet

James R. Thompson, Judy M. Bratt, Leonard J. Banaszak

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A recombinant form of murine apo-cellular retinoic acid binding protein I (apo-CRABPI) has been purified and crystallized at pH 5.0, and the crystal structure has been refined to anR-factor of 19.6% at a resolution of 2.7 Å. CRABPI binds all-transretinoic acid and some retinoic acid metabolites with nanomolar affinities. Coordinates of the holo form of CRABP were not available during the early stages of the study, and in spite of numerous homologs of known structure, phases were not obtainable through molecular replacement. Instead, an interpretable electron density map was obtained by multple isomorphous replacement methods after improvement of the heavy-atom parameters with density modified trial phases. Two molecules of apo-CRABPI occupy theP3121 asymmetric unit and are related by pseudo 2-fold rotational symmetry. Unique conformational differences are apparent between the two molecules. IN all of the family members studied to date, there is a lack of hydrogen bonds between two of the component β-strands resulting in a gap in the interstand hydrogen bonding pattern. In the crystallographic dimer described here, a continuous intermolecular β-sheet is formed by using this gap region. This is possible because of an 8 Å outward maximum displacement of the tight turn between the third and fourth β-strands on one of the molecules. The result is a double β-barrel containing two apo-CRABPI molecules with a more open, ligand-accessible binding cavity, which has not been observed in other structures of a family of proteins that bind hydrophobic ligands.

Original languageEnglish (US)
Pages (from-to)433-446
Number of pages14
JournalJournal of Molecular Biology
Issue number4
StatePublished - Sep 29 1995

Bibliographical note

Funding Information:
We are grateful to Dr Ellen Li at Washington University for providing us with the strain of E. coli containing the expression plasmid for murine CRABPI. The authors thank Ed Hoeffner for maintaining the X-ray diffraction instrumentation, and the computing hardware and software. We thank Jeremia Ory and Bob Ledford for help in protein purification. We also gratefully acknowledge research support from the NIH (GM 13925) and the Minnesota Supercomputer Institute.


  • Cellular retinoic acid binding protein I
  • Lipid binding protein
  • Retinoic acid
  • Vitamin A


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