The neuronal t-SNARE complex is a parallel four-helix bundle

Assembly of the soluble N-ethylmaleimide sensitive factor attachment protei n receptor (SNARE) complex is an essential step for neurotransmitter releas e in synapses. The presynaptic plasma membrane associated-proteins (t-SNARE s), SNAP-25 (synaptosome-associated protein of 25,000 Da) and syntaxin 1A m ay form an intermediate complex that later binds to vesicle-associated memb rane protein 2 (VAMP2), Using spin labeling electron paramagnetic resonance (EPR), we found that the two t-SNARE proteins assemble into a parallel fou r-helix bundle that consists of two identical syntaxin IA components and th e N-terminal and C-terminal domains of SNAP-25, Although the structure is g enerally similar to that of the final SNARE complex, the middle region of t he helical bundle appears more flexible in the t-SNARE complex, Such flexib ility might facilitate interactions between VAMP2 and the t-SNARE complex.