| Summary: | Summary: Munc18-1 is a critical component of the core machinery controlling neuroexocytosis. Recently, mutations in Munc18-1 leading to the development of early infantile epileptic encephalopathy have been discovered. However, which degradative pathway controls Munc18-1 levels and how it impacts on neuroexocytosis in this pathology is unknown. Using neurosecretory cells deficient in Munc18, we show that a disease-linked mutation, C180Y, renders the protein unstable at 37°C. Although the mutated protein retains its function as t-SNARE chaperone, neuroexocytosis is impaired, a defect that can be rescued at a lower permissive temperature. We reveal that Munc18-1 undergoes K48-linked polyubiquitination, which is highly increased by the mutation, leading to proteasomal, but not lysosomal, degradation. Our data demonstrate that functional Munc18-1 levels are controlled through polyubiquitination and proteasomal degradation. The C180Y disease-causing mutation greatly potentiates this degradative pathway, rendering Munc18-1 unable to facilitate neuroexocytosis, a phenotype that is reversed at a permissive temperature. : Mutations in Munc18-1, an essential component of the machinery controlling neurotransmission, are linked to the development of early infantile epileptic encephalopathy (EIEE). In this study, Martin et al. show that one of these mutations, C180Y, results in a thermolabile protein with a strong propensity to aggregate. The level of Munc18-1C180Y is regulated by K48-linked polyubiquitination and proteasomal degradation. The impaired exocytic function of Munc18-1C180Y is rescued by growth at a permissive temperature. An imbalance in exocytosis could therefore underpin EIEE.
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