Synaptic transmission between neurons relies on the exact spatial organization of postsynaptic transmitter receptors, which are recruited and positioned by dedicated scaffolding and regulatory proteins. At GABAergic synapses, the regulatory protein collybistin (Cb) interacts with small GTPases, cell-adhesion proteins, and phosphoinositides to recruit the scaffolding protein gephyrin and GABAA-receptors to nascent synapses. We dissected the interaction of Cb with the small Rho-like GTPase TC10 and phospholipids. Our data define a protein-lipid interaction network that controls the clustering of gephyrin at synapses. Within this network, TC10 and monophosphorylated phosphoinositides, particulary phosphatidylinositol-3-phosphate (PI3P), provide a coincidence detection platform that allows the accumulation and activation of Cb in endomembranes. Upon activation, TC10 induces a phospholipid affinity switch in Cb, which allows Cb to specifically interact with phosphoinositide species present at the plasma membrane. We propose that this GTPase-based regulatory switch mechanism represents an important step in the process of tethering of Cb-dependent scaffolds and receptors at nascent postsynapses.