Structure/function analysis of interleukin-2-toxin (DAB486-IL-2). Fragment B sequences required for the delivery of fragment A to the cytosol of target cells.

We have used cassette and deletion mutagenesis to analyze the structural features of fragment B-related sequences in the fusion toxin DAB486-IL-2 (where IL-2 represents interleukin-2) that are necessary for the efficient delivery of fragment A to the cytosol of target cells. We demonstrate that whereas an intact disulfide bond between Cys461 and Cys471 may be required for the cytotoxic action of native diphtheria toxin, this bond is not required for the cytotoxic action of DAB486-IL-2. The in-frame deletion of the 97 amino acids from Thr387 to His485 of DAB486-IL-2 increases both the potency and the apparent dissociation constant (Kd) of the resulting fusion toxin for high affinity interleukin-2 receptor-bearing target cells. In contrast, the inframe deletion of either the 191 amino acids between Asp291 and Gly483 or the 85 amino acids between Asn204 and Ile290 results in a 1000-fold loss in potency. These regions contain the putative membrane-spanning regions and the amphipathic membrane surface-associating regions of fragment B, respectively. These results indicate that the efficient delivery of the ADP-ribosyltransferase from DAB486-IL-2 to the cytosol requires the membrane-associating domains of fragment B. This function has been postulated to play a role in the diphtherial intoxication of eukaryotic cells. However, unlike native diphtheria toxin, fragment B sequences distal to Thr387 do not enhance the potency of DAB486-IL-2.