 Solution

Partial proteolysis is one plausible explanation: intact SNAP25 and a slightly shortened form would migrate as two nearby bands in SDS-PAGE. Heterogeneous processing, a retained affinity tag, or an anomalously migrating species are other possibilities. The gel alone does not identify the chemical difference. Because the proteins were expressed in Escherichia coli, a mammalian post-translational modification should not simply be assumed. A Western blot establishing that both bands contain SNAP25, followed by mass spectrometry, could distinguish these explanations.