Guanylate-kinase-associated protein (GKAP) is a scaffolding protein that links NMDA receptor-PSD-95 to Shank-Homer complexes by protein-protein interactions at the synaptic junction. GKAP family proteins are characterized by the presence of a C-terminal conserved GKAP homology domain 1 (GH1) of unknown structure and function. In this study, crystal structure of the GH1 domain of GKAP from Rattus norvegicus was determined in fusion with an N-terminal maltose-binding protein at 2.0 angstrom resolution. The structure of GKAP GH1 displays a three-helix bundle connected by short flexible loops. The predicted helix alpha 4 which was not visible in the crystal structure associates weakly with the helix alpha 3 suggesting dynamic nature of the GH1 domain. The strict conservation of GH1 domain across GKAP family members and the lack of a catalytic active site required for enzyme activity imply that the GH1 domain might serve as a protein-protein interaction module for the synaptic protein clustering. (C) 2014 Elsevier Inc. All rights reserved.