Closing in on the toxic domain through analysis of a variant Clostridium difficile cytotoxin B

Strain 1470 is the standard typing strain for serogroup F of Clostridium difficile containing both toxin genes, toxA‐1470 and toxB‐1470. A polymerase chain reaction (PCR)‐based approach to the sequencing of the total toxB‐1470 gene identified an open reading frame (ORF) of 7104 nucleotides. In comparison with the previously sequenced toxB of C. difficile VPI10463, the toxB‐1470 gene has 16 additional nucleotides, 13 within the 5′‐untranslated region and three within the coding region. The Mr of ToxB‐1470 is 269 262, with an isoelectric point (IP) of 4.16. The equivalent values for ToxB are Mr 269 709 and IP 4.13. In comparison with ToxB, ToxB‐1470 differs primarily in the N‐terminal region between positions 1 and 868 where 148 amino acids residues are changed. The C‐terminal region between residues 869–2367 is highly conserved with only six amino acid alterations. Dot matrix comparison of ToxB‐1470 with ToxA and ToxB reveals the highest homology between ToxB‐1470 and ToxB. Thus ToxB‐1470 did not originate from recombination between ToxA and ToxB. On cultured endothelial cells, from porcine pulmonary artery, purified ToxB‐1470 is less potent than ToxB. The cytopathic effects of ToxB‐1470 are indistinguishable from those caused by the lethal toxin (LT) of Clostridium sordellii, but are clearly different from the patterns observed after exposure of endothelial cells to ToxA and ToxB of C. difficile (VPI10463) or α‐toxin (Tcnα) of Clostridium novyi. The LT‐like action of ToxB‐1470 was not due to altered internalization processes, as microinjection and addition to the medium induced identical effects on the cells. Since the differences between ToxB and ToxB‐1470 are clustered within the N‐terminal third of the respective proteins, we conclude that these domains carry the toxic determinants. A three‐domain structure is proposed for the family of large clostridal cytotoxins.