The Drosophila epsin 1 is required for ubiquitin-dependent synaptic growth and function but not for synaptic vesicle recycling

The ubiquitin-proteasome system plays an important role in synaptic development and function. However, many components of this system, and how they act to affect synapses, are still not well understood. In this study, we use the Drosophila neuromuscular junction to study the in vivo function of Liquid facets (Lqf), a homolog of mammalian epsin 1. Our data show that Lqf plays a novel role in synapse development and function. Contrary to prior models, Lqf is not required for clathrin-mediated endocytosis of synaptic vesicles. Lqf is required to maintain bouton size and shape and to sustain synapse growth by acting as a specific substrate of the deubiquitinating enzyme Fat facets. However, Lqf is not a substrate of the Highwire (Hiw) E3 ubiquitin ligase; neither is it required for synapse overgrowth in hiw mutants. Interestingly, Lqf converges on the Hiw pathway by negatively regulating transmitter release in the hiw mutant. These observations demonstrate that Lqf plays distinct roles in two ubiquitin pathways to regulate structural and functional plasticity of the synapse.