UEV-1 and Glutamate Receptors different genetic pathways. By contrast, GLR-1 trafficking defects in rab-10 uev-1 double mutants are not significantly greater than in uev-1 single mutants, suggesting that RAB-10 and UEV-1 might function in the same genetic pathway. RAB-10 mediates the recycling of clathrin-independent cargo [55]; thus, if UEV-1 and RAB-10 work in the same pathway, this would position UEV-1 as a mediator of GLR-1 recycling in the clathrin-independent endocytosis pathway from endosomes back to synaptic mem- branes. Consistent with this hypothesis, uev-1 mutants, like rab-10 mutants, have a reversal phenotype indicative of reduced GLR-1 synaptic function. It should be noted that uev-1 mutants have a stronger GLR-1 accumulation defect than rab-10 mutants, suggesting either that UEV-1 is a more critical component than RAB-10 in this recycling pathway, or UEV-1 regulates GLR-1 trafficking by one or more additional mechanisms than just the clathrin-independent recycling pathway. This leads us to entertain a model in which UEV-1 functions in GLR-1-expressing interneurons to facilitate the exit of GLR-1 from early endosomes (Figure 10A). A fraction of GLR-1 receptors is normally endocytosed by a clathrin-independent pathway and sent to RAB-10-containing early endosomes [55]. Subsequently, GLR-1 receptors exit these endosomes and are either recycled to the synapse or sent to lysosomes for turnover. In the absence of UEV-1 function, a fraction of GLR-1 receptors become endocytosed into these endosomes, but cannot exit them, resulting in a net build up of receptor in the neurons while simultaneously depleting the synaptic surface of receptor and resulting in decreased GLR-1 signaling (Figure 10B). Interestingly, we observed a spatial bias in the localization defects in uev-1 mutants, suggesting that these endosomes might lie more proximal along neurites. This finding implies that neurons can control the spatial localization of glutamate receptors along neurites by regulating the balance of the different endocytosis pathways that regulate the receptors. Our model is consistent with work showing a role for K63-linked polyubiquitination in the trafficking of renal aqua- porin-2 water channels [27], the epidermal growth factor receptor [41], Gap1 permease [30], and other proteins [40], with the novel feature that the GLR-1 receptor itself is not the likely K63-linked ubiquitination target. Finally, this model raises the possibility that UEV proteins are important for regulating AMPAR function, Figure 10. A model for UEV-1 function. Previous results indicate that GLR-1 is endocytosed and recycled by two pathways: a clathrin-dependent pathway (which involves ITSN-1 and RAB-5) that requires LIN-10 for recycling, and a clathrin-independent pathway that requires RAB-10 for recycling. Based on genetic and cell biological data, we suggest that UEV-1 functions in the clathrin independent pathway to move receptors out of endosomes for recycling and for turnover. (A) Cartoon illustrating both pathways at different synapses along the ventral cord of wild-type animals. GLR-1 receptors (red) are being endocytosed and recycled by either the clathrin-independent endocytosis pathway (CIE) and RAB-10 (the synapse on the left) or the clathrin-dependent endocytosis pathway (CDE) and LIN-10 (the synapse on the right). Our results suggest that the more proximal synapses favor the clathrin-independent, UEV-1 dependent pathway. (B) Cartoon illustrating that the clathrin-independent pathway is disrupted in uev-1 mutants, resulting in the internalization of GLR-1 receptors into accretions due to their failure to exit endosomal compartments. doi:10.1371/journal.pone.0014291.g010 PLoS ONE | www.plosone.org 16 December 2010 | Volume 5 | Issue 12 | e14291