This invention relates to isolated Clostridium botulinum propeptides and neurotoxins, isolated nucleic acid molecules encoding Clostridium botulinum propeptides and neurotoxins, methods of expression, treatment methods, and methods of detecting neurotoxin trafficking. The isolated Clostridium botulinum propeptides have a light chain region； a heavy chain region, where the light and heavy chain regions are linked by a disulfide bond； an intermediate region connecting the light and heavy chain regions and comprising a highly specific protease cleavage site； and an S6 peptide sequence according to SEQ ID NO:2 positioned upstream from, but not attached directly to, the N-terminus of the neurotoxin propeptide at the light chain region to enable site specific attachment of cargo.
