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Keywords:

  • calcium channel;
  • presynaptic;
  • release site;
  • scaffold;
  • transmitter release

Abstract

Neurotransmitter release sites at the freeze-fractured frog neuromuscular junction are composed of inner and outer paired rows of large membrane particles, the putative calcium channels, anchored by the ribs of an underlying protein scaffold. We analysed the locations of the release site particles as a reflection of the scaffold structure, comparing particle distributions in secreting terminals with those where secretion was blocked with botulinum toxin A, which cleaves a small segment off SNAP-25, or botulinum toxin C1, which cleaves the cytoplasmic domain of syntaxin. In the idle terminal the inner and outer paired rows were located ≈25 and ≈44 nm, respectively, from the release site midline. However, adjacent to vesicular fusion sites both particle rows were displaced towards the midline by ≈25%. The intervals between the particles along each row were examined by a nearest-neighbour approach. In control terminals the peak interval along the inner row was ≈17 nm, consistent with previous reports and the spacing of the scaffold ribs. While the average distance between particles in the outer row was also ≈17 nm, a detailed analysis revealed short ‘linear clusters’ with a ≈14 nm interval. These clusters were enriched at vesicle fusion sites, suggesting an association with the docking sites, and were eliminated by botulinum C1, but not A. Our findings suggest, first, that the release site scaffold ribs undergo a predictable, and possibly active, shortening during exocytosis and, second, that at the vesicle docking site syntaxin plays a role in the cross-linking of the rib tips to form the vesicle docking sites.