GISP: a novel brain‐specific protein that promotes surface expression and function of GABA_Breceptors

Abstract

Synaptic transmission depends on the regulated surface expression of neurotransmitter receptors, but many of the cellular processes required to achieve this remain poorly understood. To better define specific mechanisms for the GABA B receptor (GABA B R) trafficking, we screened for proteins that bind to the carboxy-terminus of the GABA B1 subunit. We report the identification and characterization of a novel 130-kDa protein, GPCR interacting scaffolding protein (GISP), that interacts directly with the GABA B1 subunit via a coiled-coil domain. GISP co-fractionates with GABA B R and with the postsynaptic density and co-immunoprecipitates with GABA B1 and GABA B2 from rat brain. In cultured hippocampal neurons, GISP displays a punctate dendritic distribution and has an overlapping localization with GABA B Rs. When co-expressed with GABA B Rs in human embryonic kidney cells, GISP promotes GABA B R surface expression and enhances both baclofen-evoked extracellular signal-regulated kinase (ERK) phosphorylation and G-protein inwardly rectifying potassium channel (GIRK) currents. These results suggest that GISP is involved in the forward trafficking and stabilization of functional GABA B Rs.

GABA-B1 Receptors are Coupled to the ERK1/2 MAP Kinase Pathway in the Absence of GABA-B2 Subunits

“…This interaction is unlikely to occur between Cterminal coil-coil domains, since their masking of an ER retention sequence would presumably also result in cellsurface targetting of the GABA-B 1 multimers, which is not observed. Other putative GABA-B 1 partners, such as the recently identified GISP, might conceivably play a role in trafficking these receptors to the cell membrane in the absence of GABA-B 2 (Kantamneni et al 2007), but we have not detected surface expression at all. Co-immunoprecipitation of the splice variant GABA-B 1E (consisting only of the GABA-B 1 extracellular domain) or of a truncated GABA-B 1B receptor, with GABA-B 2 , already indicated that the C-terminal coil-coil domains of GABA-B subunits are not the sole elements involved in formation of the heterodimeric GABA-B 1/2 receptor (Schwarz et al 2000;Calver et al 2001).…”

Direct Interaction of GABABReceptors with M2Muscarinic Receptors Enhances Muscarinic Signaling

“…GBR1 contains two sushi domain repeats in the N-terminal domain that bind extracellular matrix proteins and could be important for targeting (Blein et al, 2004). In neuronal PC12 cells, it is also possible that a neuron-specific protein promotes targeting of GBR2-M2R, such as through a related GABAB receptor (Calver et al, 2003), a GPCR scaffolding interacting protein (GISP) (Kantamneni et al, 2007) or a modulator of GABAB receptors (e.g. RAMPs) (Parameswaran and Spielman, 2006).…”

GABAB receptor modulation of synaptic function

“…It was thus surprising when only two isoforms were ultimately discovered [17], which have similar agonist binding and signaling properties [2]. This discrepancy was recently resolved by the discovery of auxiliary binding proteins including KCTD (potassium channel tetramerization domain-containing) proteins [18,19], Mupp1 [20], and GISP [21], which together help confer the diversity observed in earlier studies. Understanding the roles of different GABA B -R isoforms and auxiliary binding proteins in synaptic modulation remains an exciting topic for future study.…”