SNAP-25 (synaptosomal associated protein of 25kD) is a SNARE protein essential for exocytosis in neurotransmitter release. Elevated levels of SNAP-25 fragments in cerebrospinal fluid are correlated with neurodegenerative diseases like Huntington’s and Alzheimer’s (Zhang, et al. Clinica Chimica Acta , 571, 2025). We observe fragmentation of this protein in vitro after purification from bacteria using a GST tag and thrombin. Protein fragmentation patterns were characterized using gel electrophoresis, mass spectrometry, and circular dichroism (CD) spectroscopy. To suppress this fragmentation, we incubated SNAP-25 (several days at 37°C) under altered environmental conditions-pH, salt, temperature, or metal ions-and then analyzed the resulting clipping patterns. Utilizing gel electrophoresis, we observed that fragmentation was reduced by Ni 2+ or Zn 2+ (1 mM), high salt (>100 mM), low pH (80°C). In contrast, Ca 2+ or Mg 2+ (1 mM) enhanced the fragmentation. Using mass spectrometry, all protein fragments were identified as SNAP-25 with no external protease present, and four common clip sites were observed. CD spectroscopy revealed that Ni 2+ , Zn 2+ , high salt, and low pH increased secondary structure. Incubation with Ni 2+ and heating the protein induced oxidation which may be the structural change that inhibits proteolysis. These data support the hypothesis that, rather than being fragmented by a contaminating protease, SNAP-25 is self-proteolytic. Further research is needed to identify the mechanism that causes fragmentation. Ongoing work seeks to identify the SNAP-25 domain(s) responsible for this fragmentation.
Bates et al. (2026) studied this question.