Abstract: The present invention provides a preparation of botulinum toxin light chain type A or E, wherein the preparation is both catalytically active and soluble. Preferably, the preparation consists essentially of amino acid residues 1 through 425 of the botulinum toxin light chain type A. A method of screening inhibitors is also provided, wherein the method comprises exposing a test inhibitor to the preparation of botulinum toxin light chain type A and evaluating the biological activity of the preparation. In another embodiment, a method of providing a catalytically active, soluble preparation of botulinum toxin light chain, type A is provided, wherein the method comprises obtaining an expression vector comprising a DNA sequence encoding amino acid residues 1-425 and expressing a polypeptide.