Ubiquitin‐modulated intracellular processing as a determinant of the potency of tumor‐targeted anthrax toxins

Anthrax toxin proteins constitute a highly efficient system for delivering cytotoxic enzymes to the cytosol of tumor cells. However, the ultimate anti‐tumor efficacy of these agents depends on their resistance to inactivation and degradation within the cells. It is evident from the low lysine to arginine ratios in many bacterial toxins that avoidance of ubiquitinylation is a key factor controlling their potency. We created fusion proteins containing modified ubiquitins having C‐terminal fusions to the catalytic domain of Pseudomonas Exotoxin A (PEIII). The potency of these proteins was highly dependent on the number of lysines retained with the ubiquitin domain, and on whether they retained the C‐terminal ubiquitin sequence cleaved by cytosolic deubiquitinating proteases (DUBs). Thus, fusions in which all seven native lysines of ubiquitin were retained whereas the DUB site was removed were non‐toxic, apparently due to rapid ubiquitination and proteasomal degradation. In contrast, fusions in which all lysines of ubiquitin were substituted by arginine and the DUB site was retained had high potency, exceeding that of a simple fusion lacking ubiquitin. It appears that rapid cytosolic release of a cytotoxic enzyme (e.g., PEIII) that is itself resistant to ubiquitination, is an effective strategy for enhancing the potency of tumor‐targeting toxins.