title: The Sushi domains of GABAB receptors function as axonal targeting signals.
abstract: GABA(B) receptors are the G-protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the brain. Two receptor subtypes, GABA(B(1a,2)) and GABA(B(1b,2)), are formed by the assembly of GABA(B1a) and GABA(B1b) subunits with GABA(B2) subunits. The GABA(B1b) subunit is a shorter isoform of the GABA(B1a) subunit lacking two N-terminal protein interaction motifs, the sushi domains. Selectively GABA(B1a) protein traffics into the axons of glutamatergic neurons, whereas both the GABA(B1a) and GABA(B1b) proteins traffic into the dendrites. The mechanism(s) and targeting signal(s) responsible for the selective trafficking of GABA(B1a) protein into axons are unknown. Here, we provide evidence that the sushi domains are axonal targeting signals that redirect GABA(B1a) protein from its default dendritic localization to axons. Specifically, we show that mutations in the sushi domains preventing protein interactions preclude axonal localization of GABA(B1a). When fused to CD8alpha, the sushi domains polarize this uniformly distributed protein to axons. Likewise, when fused to mGluR1a the sushi domains redirect this somatodendritic protein to axons, showing that the sushi domains can override dendritic targeting information in a heterologous protein. Cell surface expression of the sushi domains is not required for axonal localization of GABA(B1a). Altogether, our findings are consistent with the sushi domains functioning as axonal targeting signals by interacting with axonally bound proteins along intracellular sorting pathways. Our data provide a mechanistic explanation for the selective trafficking of GABA(B(1a,2)) receptors into axons while at the same time identifying a well defined axonal delivery module that can be used as an experimental tool.
authors.affiliation: Department of Biomedicine, Institute of Physiology, Pharmazentrum, University of Basel, CH-4056 Basel, Switzerland.
authors.forename: Barbara
authors.initials: B
authors.lastname: Biermann
authors.forename: Klara
authors.initials: K
authors.lastname: Ivankova-Susankova
authors.forename: Amyaouch
authors.initials: A
authors.lastname: Bradaia
authors.forename: Said
authors.initials: S
authors.lastname: Abdel Aziz
authors.forename: Valerie
authors.initials: V
authors.lastname: Besseyrias
authors.forename: Josef P
authors.initials: JP
authors.lastname: Kapfhammer
authors.forename: Markus
authors.initials: M
authors.lastname: Missler
authors.forename: Martin
authors.initials: M
authors.lastname: Gassmann
authors.forename: Bernhard
authors.initials: B
authors.lastname: Bettler
mesh_terms: D000818:Animals; D001370:Axonal Transport / Q000502:physiology*; D001369:Axons / Q000378:metabolism* / Q000648:ultrastructure; D016827:CD8 Antigens / Q000235:genetics / Q000378:metabolism; D016764:Cell Polarity / Q000502:physiology; D002478:Cells, Cultured; D003712:Dendrites / Q000378:metabolism / Q000648:ultrastructure; D006624:Hippocampus / Q000378:metabolism* / Q000648:ultrastructure; D051379:Mice; D008807:Mice, Inbred BALB C; D018345:Mice, Knockout; D009154:Mutation / Q000235:genetics; D009433:Neural Inhibition / Q000502:physiology*; D018408:Patch-Clamp Techniques; D017434:Protein Structure, Tertiary / Q000235:genetics; D021122:Protein Subunits / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D018079:Receptors, GABA / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011963:Receptors, GABA-A / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D018080:Receptors, GABA-B / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology*
chemical_list: D016827:CD8 Antigens; C546266:Gabrb1 protein, mouse; C470436:Gabrb2 protein, mouse; D021122:Protein Subunits; D018079:Receptors, GABA; D011963:Receptors, GABA-A; D018080:Receptors, GABA-B; D011993:Recombinant Fusion Proteins