At the C. elegans neuromuscular junction (NMJ), fast synaptic transmission is mediated by acetylcholine (ACh). A specific subset of acetylcholine receptors (AChR) at the postsynaptic site can selectively activated by the agonist levamisole. UNC-50, an integral membrane protein, was found in screens for mutants that are resistant to levamisole. In
unc-50 mutants, levamisole sensitive AChRs do not reach the synapse at the NMJ. Instead, they are sorted to the lysosomal system and rapidly degraded. As a consequence of the lack of levamisole sensitive AChRs at the NMJ,
unc-50 mutants exhibit an uncoordinated slow movement. Interestingly, a second type of muscle nAChR, which is insensitive to levamisole, and the muscle GABAA receptor are normally transported to the cell surface. Based on its selectivity, UNC-50 could be assumed to be a trafficking or sorting factor specific for levamisole sensitive AChRs. In accordance, UNC-50 localizes in endosome and late Golgi. Despite its specificity, UNC-50 is evolutionarily conserved from yeast to human. However, there are no AChRs in yeast or plants. Therefore, it''s likely that the UNC-50 protein family may have more general functions during intracellular transport. However,
unc-50 mutants are viable and show almost no visible phenotype. Therefore, redundant or parallel pathways with UNC-50 may exist. To find redundant or parallel pathways with UNC-50, we did synthetic lethality screens in C. elegans. We found that
vps-52 and
vps-54 mutations are lethal in combination with
unc-50 mutation in C. elegans. In addition, we show that this synthetic lethal interaction also exists in yeast. Vps52p and Vps54p are components of the GARP (Golgi-Associated-Retrograde-Protein) complex which is a tethering factor involved in retrograde transport from endosome to Golgi. However, the GARP complex has not been characterized in C. elegans. Our work shows that in C. elegans, the GARP complex consists of three subunits: VPS-52, VPS-53 and VPS-54. GARP complex is ubiquitously expressed in all tissues and localized to endosome and late Golgi, largely overlapping with UNC-50 localization. The C. elegans GARP complex also colocalizes with RAB-6 and likely to interact with it, which has already been shown in yeast and human. Our results indicate that UNC-50, like the GARP complex, is also involved in retrograde transport from endosome to Golgi. Further mechanistic insights of UNC-50 functions will be presented at the meeting.