Identification of a gephyrin-binding motif in the GDP/GTP exchange factor collybistin

The brain-specific GDP/GTP exchange factor collybistin interacts with the r eceptor-anchoring protein gephyrin and activates the Rho-like GTPase Cdc42, which is known to regulate actin cytoskeleton dynamics. Alternative splici ng creates two collybistin variants, I and II. In coexpression experiments, collybistin II has been shown to induce the formation of sub-membraneous g ephyrin aggregates which cluster with hetero-oligomeric glycine receptors ( GlyRs). Here we identified residues critical for interaction with gephyrin in the linker region between the SH3 and the DH domains of collybistin. Res pective collybistin deletion mutants failed to bind gephyrin upon coexpress ion in heterologous cells, in GST pull-down assays and in the yeast two-hyb rid system. Site-directed mutagenesis revealed polar amino acid residues as essential determinants of gephyrin binding. Furthermore, in vitro gephyrin bound simultaneously to both collybistin and the GlyR beta -subunit bindin g motif. Our data are consistent with collybistin-gephyrin interactions occ uring during inhibitory postsynaptic membrane formation.