Identification of clathrin heavy chain as a direct interaction partner for the gamma-aminobutyric acid type A receptor associated protein.

Gamma-aminobutyric acid type A receptors (GABAA receptors) are the major sites of GABA-mediated fast synaptic inhibition in the central nervous system. Variation of the cell surface receptor count is postulated to be of importance in modulating inhibitory synaptic transmission. The GABAA receptor associated protein (GABARAP) is a ubiquitin-like modifier, implicated in GABAA receptor clustering, trafficking, and turnover. GABARAP pull-down experiments with brain lysate identified clathrin heavy chain to be GABARAP-associated. Phage display screening of a randomized peptide library for GABARAP ligands yielded a sequence motif which characterizes the peptide binding specificity of GABARAP. Sequence database searches with this motif revealed clathrin heavy chain as a protein containing the identified sequence motif within its residues 510-522, supporting the result of the pull-down experiments. Calreticulin, which was identified recently as a GABARAP ligand, contains a very similar sequence motif. We demonstrate that calreticulin indeed competes with clathrin heavy chain for GABARAP binding. Finally, employing nuclear magnetic resonance spectroscopy, we mapped the GABARAP residues responsible for binding to clathrin. The hereby mapped GABARAP regions overlap very well with the homologue residues in yeast Atg8 that were recently shown to be important for autophagy. Together with the knowledge that GABARAP and clathrin are known to be involved in GABAA receptor trafficking within the cell, this strongly suggests a clear physiological relevance of the direct interaction of GABARAP with clathrin heavy chain.