The present invention provides a nucleic acid molecule which contains a nucleotide sequence encoding a SNAP-25 substrate which includes (i) a green fluorescent protein; (ii) a first partner of an affinity couple; and (iii) a portion of SNAP-25 that includes a BoNT/A, BoNT/C1 or BoNT/E recognition sequence containing a cleavage site, where the cleavage site intervenes between the green fluorescent protein and the first partner of the affinity couple. Further provided herein is a nucleic acid molecule which contains a nucleotide sequence encoding a tagged toxin substrate which includes (i) a fluorescent protein; (ii) a first partner of an affinity couple; and (iii) a clostridial toxin recognition sequence containing a cleavage site, where the cleavage site intervenes between the fluorescent protein and the first partner of the affinity couple.