RIMB-1/RIM-Binding Protein and UNC-10/RIM Redundantly Regulate Presynaptic Localization of the Voltage-Gated Calcium Channel in Caenorhabditis elegans

Presynaptic active zones (AZs) contain many molecules essential for neurotransmitter release and are assembled in a highly organized manner. A network of adaptor proteins known as cytomatrix at the AZ (CAZ) is important for shaping the structural characteristics of AZ. Rab3-interacting molecule (RIM)-binding protein (RBP) family are binding partners of the CAZ protein RIM and also bind the voltagegated calcium channels (VGCCs) in mice and flies. Here, we investigated the physiological roles of RIMB-1, the homolog of RBPs in the nematode Caenorhabditis elegans. RIMB-1 is expressed broadly in neurons and predominantly localized at presynaptic sites. Loss-of-function animals of rimb-1 displayed slight defects in motility and response to pharmacological inhibition of synaptic transmission, suggesting a modest involvement of rimb-1 in synapse function. We analyzed genetic interactions of rimb-1 by testing candidate genes and by an unbiased forward genetic screen for rimb-1 enhancer. Both analyses identified the RIM homolog UNC-10 that acts together with RIMB-1 to regulate presynaptic localization of the P/Q-type VGCC UNC-2/Ca(v)2. We also find that the precise localization of RIMB-1 to presynaptic sites requires presynaptic UNC-2/Ca(v)2. RIMB-1 has multiple FN3 and SH3 domains. Our transgenic rescue analysis with RIM B-1 deletion constructs revealed a functional requirement of a C-terminal SH3 in regulating UNC-2/Ca(v)2 localization. Together, these findings suggest a redundant role of RIMB-1/RBP and UNC-10/RIM to regulate the abundance of UNC-2/Ca(v)2 at the presynaptic AZ in C. elegans, depending on the bidirectional interplay between CAZ adaptor and channel proteins.