Crystal structure and biophysical properties of a complex between the N-terminal SNARE region of SNAP25 and syntaxin 1a.

SNARE proteins are required for intracellular membrane fusion. In the neuro n, the plasma membrane SNARES syntaxin 1a and SNAP25 bind to VAMP2 found on neurotransmitter-containing vesicles. These three proteins contain "SNARE regions" that mediate their association into stable tetrameric coiled-coil structures. Syntaxin la contributes one such region, designated H3, and SNA P25 contributes two SNARE regions to the fusogenic complex with VAMP2. Synt axin la H3 (synlaH3) and SNAP25 can form a stable assembly, which can then be bound by VAMP2 to form the full SNARE complex. Here we show that syn1aH3 can also form a stable but kinetically trapped complex with the N-terminal SNARE region of SNAP25 (S25N). The crystal structure of this complex revea ls an extended parallel four-helix bundle similar to that of the core SNARE and the syn1aH3(.)SNAP25 complexes. The inherent ability of synlaH3 and S2 5N to associate stably in vitro implies that the intracellular fusion machi nery must prevent formation of, or remove, any nonproductive complexes. Com parison with the syn1aH3(.)SNAP25 complex suggests that the linkage of the N- and C-terminal SNAP25 SNARE regions is kinetically advantageous in preve nting formation of the non-productive syn1aH3(.)S25N complex. We also demon strate that the syn1aH3(.)S25N complex can be disassembled by a-SNAP and N- ethylmaleimide-sensitive factor.