Crystallization and preliminary X-ray crystallographic analysis of the C-terminal domain of guanylate kinase-associated protein from Rattus norvegicus

Guanylate kinase-associated protein (GKAP) is a scaffolding protein that plays a role in protein–protein interactions at the synaptic junction such as linking the NMDA receptor–PSD-95 complex to the Shank–Homer complex. In this study, the C-terminal helical domain of GKAP from Rattus norvegicus was purified and crystallized by the vapour-diffusion method. To improve the diffraction quality of the GKAP crystals, a flexible loop in GKAP was truncated and an MBP (maltose-binding protein)-GKAP fusion was constructed in which the last C-terminal helix of MBP is fused to the N-terminus of the GKAP domain. The MBP-GKAP crystals diffracted to 2.0 Å resolution using synchrotron radiation. The crystal was orthorhombic, belonging to space group P2₁2₁2, with unit-cell parameters a = 99.1, b = 158.7, c = 65.5 Å. The Matthews coefficient was determined to be 2.44 ų Da⁻¹ (solvent content 49.5%) with two molecules in the asymmetric unit. Initial attempts to solve the structure by molecular replacement using the MBP structure were successful.