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  • Title: An immunohistochemical method that distinguishes free from complexed SNAP-25.
    Author: Xiao J, Xia Z, Pradhan A, Zhou Q, Liu Y.
    Journal: J Neurosci Res; 2004 Jan 01; 75(1):143-51. PubMed ID: 14689457.
    Abstract:
    Soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptor (SNARE) complexes composed of target (t-) SNAREs syntaxin and SNAP-25 and vesicle SNARE synaptobrevin play an essential role in neurosecretion. It is hypothesized that a transient intermediate complex between the t-SNAREs is formed during the assembly of the ternary complex. The existence of the t-SNARE binary complexes in vivo, however, has not been demonstrated. By using an affinity absorption scheme with preformed syntaxin-SNAP-25 complexes, we isolated antibodies capable of distinguishing free SNAP-25 from those associated with syntaxin. By semiquantitative immunohistochemistry, we estimated that, in cultured cerebellar neurons, the majority of SNAP-25 existed as complexes. Compared with the cultured neurons, PC12 cells expressed significantly less syntaxin, and we found that SNAP-25 was primarily in free forms. In contrast, a PC12 line that stably expressed a recombinant syntaxin showed a marked increase in SNAP-25 complexes. By using fluorescence resonance energy transfer (FRET) techniques, we observed FRET between cyan fluorescence protein-syntaxin and yellow fluorescence protein-SNAP-25 fusion proteins expressed in COS-7 and PC12 cells, suggesting a physiological interaction between syntaxin and SNAP-25. Our results demonstrate that, unlike what was previously hypothesized, syntaxin and SNAP-25 exist preferably as stable binary complexes in neurons. These findings offer novel insight into the mechanisms underlying the initiation and regulation of SNARE complex assembly.
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