Recruitment of Plasma Membrane GABA-A Receptors by Submembranous Gephyrin/Collybistin Clusters.

It has been shown that subunit composition is the main determinant of the synaptic or extrasynaptic localization of GABA receptors (GABARs). Synaptic and extrasynaptic GABARs are involved in phasic and tonic inhibition, respectively. It has been proposed that synaptic GABARs bind to the postsynaptic gephyrin/collybistin (Geph/CB) lattice, but not the typically extrasynaptic GABARs. Nevertheless, there are no studies of the direct binding of various types of GABARs with the submembranous Geph/CB lattice in the absence of other synaptic proteins, some of which are known to interact with GABARs. We have reconstituted GABARs of various subunit compositions, together with the Geph/CB scaffold, in HEK293 cells, and have investigated the recruitment of surface GABARs by submembranous Geph/CB clusters. Results show that the typically synaptic α1β3γ2 GABARs were trapped by submembranous Geph/CB clusters. The α5β3γ2 GABARs, which are both synaptic and extrasynaptic, were also trapped by Geph/CB clusters. Extrasynaptic α4β3δ GABARs consistently showed little or no trapping by the Geph/CB clusters. However, the extrasynaptic α6β3δ, α1β3, α6β3 (and less α4β3) GABARs were highly trapped by the Geph/CB clusters. AMPA and NMDA glutamate receptors were not trapped. The results suggest: (I) in the absence of other synaptic molecules, the Geph/CB lattice has the capacity to trap not only synaptic but also several typically extrasynaptic GABARs; (II) the Geph/CB lattice is important but does not play a decisive role in the synaptic localization of GABARs; and (III) in neurons there must be mechanisms preventing the trapping of several typically extrasynaptic GABARs by the postsynaptic Geph/CB lattice.