Intrinsic Disorder of the Neuronal SNARE Protein SNAP25a in its Pre-fusion Conformation

The neuronal Q-SNARE protein SNAP25a (isoform 2) forms part of the SNARE complex eliciting synaptic vesicle fusion during neuronal exocytosis. While the post-fusion cis-SNARE complex has been studied extensively, little is known about the pre-fusion conformation of SNAP25a. Here we analyze SNAP25a in its monomeric pre-fusion conformation by NMR spectroscopy and find it intrinsically disordered and highly dynamic. While from residue L35 onwards, intrinsic disorder dominates (including the second SNARE motif, SN2), region A5 to L35 (comprising the N-terminus of the first SNARE motif, SN1) shows strong α-helical propensity. Our findings suggest that the N-terminus of SN1 may act as a nucleation site for SNARE acceptor complex assembly. As a side note, our result of an intrinsic disorder of SNAP25 contrasts with a recent prediction of AlphaFold2 that predicted SN1 and SN2 to be entirely α-helical with high confidence.