Fig. 1.

Mechanical stabilization of SNARE complexes by complexin. a SNARE–DNA hybrid construct in magnetic tweezers setup. b A predicted three-dimensional structure of the neuronal SNARE complex bound to a molecule of Cpx (PDB 3IPD⁴⁹ aligned to 1KIL²⁰). The N-terminal domain of Cpx not determined in the crystal structure (1–25) is represented with a blue dashed line. The letters “S” denote the thiol groups of artificial cysteine residues. The numbers below the structure indicate the leucine-zipper layers. c Typical force–extension curves from a single SNARE complex in the absence (black) and presence (blue) of Cpx. The black and blue traces show the stretching period that features “unzipping” events from the fully zippered to the unzipped state, while the orange traces show the relaxation period with the opposite “rezipping” events. In gray, a curve for the unfolded SNAREs that fail to re-zip is shown. Dashed lines indicate the models of extension for the corresponding conformations of SNARE complex (Supplementary Note 3). d, e Force–extension curves from a single SNARE complex in the absence (d) or presence (e, same graph as in c) of Cpx. f Unzipping force distributions from a single SNARE complex in the absence (black, N = 51) and presence (blue, N = 44) of 5 μM Cpx