Molecular characterization of a protein, insoluble at low temperature, produced by Clostridium botulinum type G

A preliminary study of a low-toxicity protein, called cryoprotein, produced by Clostridium botulinum type G, led to a better characterization of this substance and to discriminate its relationship with type G botulinum toxin. This sparingly soluble protein has been characterized as an aggregated form of a soluble precursor with an Mr of 170,000. This phenomenon is temperature-dependent. The monomeric protein is usually contaminated with a lower Mr form (150,000) quite probably originated by a limited proteolytic process. The amino acid composition of this protein is relatively analogous to that of the botulinum toxins A and B, the only notable exception being the absence of cysteine. The N-terminal amino acid is alanine and the C-terminal sequence is Val-Ala-Leu-OH. The low toxicity which is usually demonstrable in samples of this protein disappears after a reductive treatment, strongly suggesting that it is not an intrinsic property. Taking into account that some of its physiochemical properties are similar to those of the known botulinal toxins, it is quite probable that this substance accompanies G toxin preparations currently obtained by routine methods, increasing its non-toxic antigenic mass. This fact could be critical to the sensitivity and specificity of G toxin immunological detection methods.