Patent ID: 7749759

Claim:
A cell comprising (a) at least one receptor that binds a clostridial toxin, and (b) a clostridial toxin substrate, said clostridial toxin substrate comprising: (i) a donor fluorophore; (ii) an acceptor having an absorbance spectrum overlapping the emission spectrum of said donor fluorophore; and (iii) a clostridial toxin recognition sequence comprising a clostridial toxin recognition sequence comprising a clostridial toxin P 5 -P 4 -P 3 -P 2 -P 1 -P 1 ′-P 2 ′-P 3 ′-P 4 ′-P 5 ′ cleavage site sequence, said clostridial toxin P 5 -P 4 -P 3 -P 2 -P 1 -P 1 ′-P 2 ′-P 3 ′-P 4 ′-P 5 ′ cleavage site sequence intervening between said donor fluorophore and said acceptor, wherein, under the appropriate conditions, resonance energy transfer is exhibited between said donor fluorophore and said acceptor.