RIM and RIM-binding protein localize synaptic Cav2 channels in a differential manner to regulate transmission in neuronal circuits

Synapses are intricately organized subcellular compartments in which molecular machines cooperate to ensure spatiotemporally precise transmission of chemical signals. Key components of this machinery are voltage-gated Ca2+-channels (VGCCs), that translate electrical signals into a trigger for fusion of synaptic vesicles (SVs) with the plasma membrane. The VGCCs and the Ca2+ microdomains they generate must be located in the right distance to the primed SV, to elicit transmitter release without delay. Rab3 interacting molecule (RIM) and RIM-binding protein (RIM-BP) were shown in different systems to contribute to the spatial organization of the active zone protein scaffold, and to localize VGCCs next to docked SVs by binding to each other and to the C-terminal region of the Cav2 VGCC α-subunit. We asked how this machinery is organized at the neuromuscular junction (NMJ) of Caenorhabditis elegans , and whether it can differentially regulate transmission in circuits composed of different neuron types. rimb-1 mutants had mild synaptic defects, through loosening the anchoring of the UNC-2 VGCC and delaying the onset of SV fusion, while RIM deletion had much more severe defects. rimb-1 mutants caused increased cholinergic but reduced GABAergic transmission, while overall transmission at the NMJ was reduced, as shown by voltage imaging. The UNC-2 channel could further be untethered by removing its C-terminal PDZ binding motif, and this untethering could be exacerbated by combining the ΔPDZ mutant with the rimb-1 mutation. Similar phenotypes resulted from acute degradation of the UNC-2 β-subunit, indicating that destabilization of the VGCC complex causes the same phenotypes as its untethering. ### Competing Interest Statement The authors have declared no competing interest.