The X-ray Crystal Structure and Putative Ligand-derived Peptide Binding Properties of γ-Aminobutyric Acid Receptor Type A Receptor-associated Protein

The γ-aminobutyric acid receptor type A (GABA A ) receptor-associated protein (GABARAP) has been reported to mediate the interaction between the GABA A receptor and microtubules. We present the three-dimensional structure of GABARAP obtained by x-ray diffraction at 1.75 à resolution. The structure was determined by molecular replacement using the structure of the homologous protein GATE-16. NMR spectroscopy of isotope-labeled GABARAP showed the structure in solution to be compatible with the overall fold but showed evidence of conformation heterogeneity that is not apparent in the crystal structure. We assessed the binding of GABARAP to peptides derived from reported binding partner proteins, including the M3-M4 loop of the γ2 subunit of the GABA A receptor and the acidic carboxyl-terminal tails of human α- and β-tubulin. There is a small area of concentrated positive charge on one surface of GABARAP, which we found interacts weakly with all peptides tested, but we found no evidence for specific binding to the proposed physiological target peptides. These results are compatible with a more general role in membrane targeting and transportation for the GABARAP family of proteins.