Graphical Abstract Highlights d Delivery of synaptic vesicle precursors occurs with high precision d Presynaptic sites are hotspots of dynamic GTP-rich microtubule plus ends d KIF1A binds more weakly to the GTP lattice, rapidly detaching from plus ends d A human KIF1A mutation perturbs lattice sensing and reduces synaptic strength SUMMARYNeurons in the CNS establish thousands of en passant synapses along their axons. Robust neurotransmission depends on the replenishment of synaptic components in a spatially precise manner.Using live-cell microscopy and single-molecule reconstitution assays, we find that the delivery of synaptic vesicle precursors (SVPs) to en passant synapses in hippocampal neurons is specified by an interplay between the kinesin-3 KIF1A motor and presynaptic microtubules. Presynaptic sites are hotspots of dynamic microtubules rich in GTPtubulin. KIF1A binds more weakly to GTP-tubulin than GDP-tubulin and competes with end-binding (EB) proteins for binding to the microtubule plus end. A disease-causing mutation within KIF1A that reduces preferential binding to GDP-versus GTPrich microtubules disrupts SVP delivery and reduces presynaptic release upon neuronal stimulation. Thus, the localized enrichment of dynamic microtubules along the axon specifies a localized unloading zone that ensures the accurate delivery of SVPs, controlling presynaptic strength in hippocampal neurons.

The GABA B receptor is one of the principal inhibitory neurotransmitter receptors in the brain, and it signals through heterotrimeric G proteins to activate a variety of effectors including G proteincoupled inwardly-rectifying potassium channels (GIRKs) 1,2 . GABA B receptor signaling is tightly regulated by auxiliary subunits called KCTDs, which control the kinetics of GIRK activation and desensitization [3][4][5] . However, the mechanistic basis for KCTD modulation of GABA B signaling remains incompletely understood. Here, using a combination of X-ray crystallography, electron microscopy, functional and biochemical experiments we reveal the molecular details of KCTD binding to both GABA B receptors and Gβγ subunits. KCTDs associate with the receptor by forming an asymmetric pentameric ring around a region of the receptor C-terminal tail, while a second KCTD domain, H1, engages in a symmetric interaction with five copies of Gβγ in which the G protein subunits also directly interact with one another. We further show that KCTD binding to Gβγ is highly cooperative, defining a model in which KCTDs cooperatively strip G proteins from GIRK channels to induce rapid desensitization following receptor activation. These results provide a framework for understanding the molecular basis for the precise temporal control of GABA B signaling by KCTD proteins.The GABA B receptor (GABA B R) is expressed in excitatory and inhibitory synapses throughout the brain, and it is an important target for anxiolytic and antispastic drugs 1,2 . The receptor is a heterodimer composed of GABA B1 and GABA B2 subunits, both of which are essential for the formation and trafficking of a functional receptor 6 . Upon activation, the receptor catalyzes dissociation of heterotrimeric G proteins into Gα i/o and Gβγ subunits to initiate downstream signaling 7,8 . Following activation of presynaptic GABA B Rs, the Gβγ heterodimer typically inhibits voltage-gated calcium channels (VGCCs) to suppress Reprints and permissions information is available at http://www.nature.com/reprints.Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

Highlights d GRKs and arrestins mediate the fast and slow desensitization of a subset of mGluRs d TIRF imaging reveals scaffold and catalytic coupling between mGluR3 and b-arrestins d Different patterns of Ser and Thr control the desensitization of group II mGluRs d Mutational analysis reveals G protein-and b-arrestin-biased mGluR3 variants

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