ABSTRACT
Bacterial toxins such as diphtheria toxin and Pseudomonas exotoxin A and plant toxins such as ricin and abrin are able to kill their target cells by delivering a catalytically active polypeptide into the cell cytosol. In the bacterial toxins, the ADP-ribosylating peptide and the cell-binding peptide are initially synthesized as distinct domains within a single chain protein molecule. The chimeric proteins are endocytosed by their target cells and are converted through the action of intracellular proteases into their cytotoxic heterodimeric forms. The approach of making functionally cytotoxic, single-chain chimeric proteins by the generation and expression of novel DNA fusions cannot be directly extended to the plant toxins. The alternative approach we have successfully used is to employ site-directed mutagenesis to alter residues within the ricin linker sequence to generate a specific protease recognition site. One advantage of recombinant chimeras containing the diphtheria toxin loop is that target cell protease(s) can process the molecule intracellularly to a cytotoxic heterodimeric form.
