The recycling of synaptic vesicles requires the recovery of vesicle proteins and membrane. Members of the stonin protein family (Drosophila Stoned B, mammalian stonin 2) have been shown to link the synaptic vesicle protein synaptotagmin to the endocytic machinery. Here we characterize the
unc-41 gene, which encodes the stonin ortholog in the nematode Caenorhabditis elegans. Transgenic expression of Drosophila stonedB rescues
unc-41 mutant phenotypes, demonstrating that UNC-41 is a bona fide member of the stonin family. In
unc-41 mutants, synaptotagmin is present in axons, but is mislocalized and diffuse. In contrast, UNC-41 is localized normally in synaptotagmin mutants, demonstrating a unidirectional relationship for localization. The phenotype of
snt-1 unc-41 double mutants is stronger than
snt-1 mutants, suggesting that UNC-41 may have additional, synaptotagmin-independent functions. We also show that
unc-41 mutants have defects in synaptic vesicle membrane endocytosis, including a 50% reduction of vesicles in both acetylcholine and GABA motor neurons. These endocytic defects are similar to those observed in
apm-2 mutants, which lack the 2 subunit of the AP2 adaptor complex. However, no further reduction in synaptic vesicles was observed in
unc-41 apm-2 double mutants, suggesting that UNC-41 acts in the same endocytic pathway as 2 adaptin.