Rim1 was previously identified as a Rab3 effector localized to the presynaptic active zone in vertebrates. We cloned a C. elegans homolog of Rim and demonstrate that it is encoded by the gene
unc-10 . Null mutants lacking the entire Rim coding sequence are viable, but exhibit a variety of behavioral defects consistent with synaptic transmission deficits. These defects are more severe than those of
rab-3 mutants suggesting Rim may play roles beyond
rab3 signaling. Electrophysiological analysis of the null mutant revealed a 3-fold decrease in the amplitude of evoked release events and a five-fold decrease in the frequency of spontaneous miniature events at the neuromuscular junction. Despite the physiological defects, neuromuscular junctions of Rim null mutants exhibited normal presynaptic densities and normal levels of docked vesicles. These data suggest that Rim acts after docking, perhaps in regulating priming of synaptic vesicles for exocytosis. Vertebrate Rim interacts with several proteins: UNC-13, cAMP-GEFII and
rab3. To further our understanding of the molecular mechanism of Rim function, we are also determining if these interactions are conserved using two hybrid and in vitro biochemical techniques. Like the vertebrate protein, C. elegans Rim is localized to a discrete sub-domain of the synapse. Antibodies against the N-terminal domain exhibit bright punctate staining in wild type animals. The staining pattern is more restricted than that obtained using antibodies directed against synaptic vesicle proteins. Specifically, individual punctum can be resolved in both the ventral and dorsal nerve cord. We are currently assessing which domains of Rim are required for function and synaptic localization. In another approach to identify other potential Rim interacting proteins, we created a functional Rim-GFP fusion that localizes similarly, though slightly less discretely than the antibody staining. Using this fusion expressed under the Rim promoter, we screened for mutants which disrupts normal localization.
js569 , a mutant with sub-lethal and uncoordinated phenotypes was isolated in this screen. Characterization of this interesting mutation is in progress.