Fig. 5. Proteolytic ablation of syntaxin: average density <1 copy/CV. Using a calibration curve derived from serial dilution of purified glutathione S-transferase (GST)–syntaxin (Coorssen et al., 2001) 0.73 fmoles, corresponding to 113 molecules/CV, were detected in 0 hour controls. No syntaxin was detected in papain-treated (2000 units/ml, replaced every hour for 3 hours) samples. The limit of detection for this calibration was <0.039 fmoles or 0.3 Treated Equivalent (T.E.) molecules/CV for the same protein load (CV number) used to visualize the papain-treated sample. The maximal syntaxin density in this sample is 0.3 molecules/CV. The plot of normalized fluorescence vs. Western Blot Coordinate shows how the signal associated with 0.039 fmoles is easily quantified. The curves are drawn with an offset for visual comparison.