Towards a molecular understanding of the interaction between Mint1 and Munc18a in the regulation of synaptic vesicle exocytosis

S Weeratunga1, E Livingstone1, J Martin1,2 and B Collins1

  1. Institute of Molecular Bioscience, The University of Queensland, Brisbane, Queensland, 4072, Australia
  2. The Eskitis Institute for Drug Discovery, Griffith University, Brisbane, Queensland, 4111, Australia

Munc-18-interacting (Mint) proteins are believed to act as adaptors and regulatory proteins involved in neuronal membrane trafficking. These are multi domain proteins composed of variable isoform specific disordered N-terminal regions, and a conserved C-terminal region that includes a PTB and two PDZ domains. Three mammalian Mint isoforms (Mint 1, 2 and 3) have been identified, but only the closely related Mint 1 and 2 are known to interact with the essential synaptic protein Munc18a and play redundant roles in the regulation of synaptic vesicle fusion during neurotransmission. Here, we identify the shortest region of Mint1 that is required for interacting with neuronal Munc18a, but not the Munc18c homologue found in other secretory cells. Munc18a controls the fusion of synaptic vesicles with the plasma membrane by binding the syntaxin-1 SNARE protein and regulating formation of the fusogenic SNARE complex. We find that the binding affinity of Munc18a and Syntaxin1 is significantly reduced in the presence of Mint1, suggesting an allosteric behaviour between Mint1 and Syntaxin1 upon binding to Munc18a. Moreover, our recent data show that the Mint1 peptide does not bind to Munc18a Δ317-333, where we have deleted a flexible loop within Munc18a proposed to regulate SNARE complex assembly. While speculative, our data suggests that Mint1 may function to control a key trigger point in SNARE complex assembly and vesicle fusion, and we are now pursuing further structural and functional studies to test this hypothesis.