UNC-41/Stonin Functions with AP2 to Recycle Synaptic Vesicles in <em>Caenorhabditis elegans</em>

<div><p>The recycling of synaptic vesicles requires the recovery of vesicle proteins and membrane. Members of the stonin protein family (<em>Drosophila</em> Stoned B, mammalian stonin 2) have been shown to link the synaptic vesicle protein synaptotagmin to the endocytic machinery. Here we characterize the <em>unc-41</em> gene, which encodes the stonin ortholog in the nematode <em>Caenorhabditis elegans</em>. Transgenic expression of <em>Drosophila stonedB</em> rescues <em>unc-41</em> mutant phenotypes, demonstrating that UNC-41 is a <em>bona fide</em> member of the stonin family. In <em>unc-41</em> 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 <em>snt-1 unc-41</em> double mutants is stronger than <em>snt-1</em> mutants, suggesting that UNC-41 may have additional, synaptotagmin-independent functions. We also show that <em>unc-41</em> 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 <em>apm-2</em> mutants, which lack the µ2 subunit of the AP2 adaptor complex. However, no further reduction in synaptic vesicles was observed in <em>unc-41 apm-2</em> double mutants, suggesting that UNC-41 acts in the same endocytic pathway as µ2 adaptin.</p>