8.A.76. The Gephyrin (GPHN) Family
Gephyrin (GPHN: GPH) is a microtubule-associated protein involved in membrane protein-cytoskeleton interactions. It is thought to anchor the inhibitory glycine receptor (GLYR) to subsynaptic microtubules. It also acts at inhibitory synapses, where it clusters GABA type A receptors (Dejanovic et al. 2014, Dejanovic et al. 2015). Gephyrin is essential for the formation of inhibitory synapses. It binds directly to the large cytoplasmic loop located between transmembrane helices three and four of the beta-subunit of the glycine receptor (GlyR) and to microtubules, thus promoting GlyR anchoring to the cytoskeleton and clustering in the postsynaptic membrane. Besides its structural role, gephyrin is involved in the biosynthesis of the molybdenum cofactor that is essential for all molybdenum-dependent enzymes in mammals.
Gephyrin can be divided into an N-terminal trimeric G domain and a C-terminal E domain, which are connected by a central linker region. Schrader et al. 2004 studied the in vitro interaction of gephyrin and its domains with the large cytoplasmic loop of the GlyR beta-subunit (GlyRbeta-loop). Binding of gephyrin to the GlyR is exclusively mediated by the E domain, and the binding site was mapped to one of its sub-domains (residues 496-654). A high affinity (Kd = 0.2-0.4 μM) and low affinity (Kd = 11-30 μM) binding sites for the GlyRbeta-loop were found on holo-gephyrin and the E domain, respectively, with a binding stoichiometry of two GlyRbeta-loops per E domain in both cases. Binding of the GlyRβ-loop does not change the oligomeric state of either full-length gephyrin or the isolated E domain (Schrader et al. 2004). Gephyrin dynamically regulates synaptic strength and plasticity of GABA (and probably Glycine) receptors (Pizzarelli et al. 2020). GPHN anchors synaptic GABA(A)Rs and glycine receptors (GlyRs), which are crucial for maintaining the architecture of the inhibitory postsynaptic density (iPSD) (Kasaragod and Schindelin 2019).