Functional analysis of domains II, Ib, and III of Pseudomonas exotoxin

Pseudomonas exotoxin is composed of three structural domains that are responsible for cell recognition, membrane translocation, and ADP-ribosylation. The substitution of the cell recognition domain (domain Ia) with a growth factor such as transforming growth factor alpha (TGF alpha), creates a cell-specific cytotoxic agent, TGF alpha-PE40, which kills cells bearing epidermal growth factor (EGF) receptors. We have used TGF alpha-PE40 to define the role of sequences in domains II, Ib, and III. Various mutations were made in these domains and mutant forms of TGF alpha-PE40 expressed in Escherichia coli. Mutant proteins were then tested for their ADP-ribosylation, EGF receptor-binding, and cell-killing activities. Additionally, the amino boundary of domain III, which contains the ADP-ribosylation activity, was determined by deletion analysis. Data indicate that (i) the functional amino terminus of domain III is near amino acid 400; (ii) deletion of various regions in domain II or conversion of cysteines 265 and 268 to serines results in a loss of cytotoxicity which ranged from 10-fold to more than 150-fold, indicating that domain II is essential for full expression of cytotoxicity; (iii) deletion of the amino terminus of domain Ib results in a molecule with somewhat increased cytotoxic activity, indicating that domain Ib is not essential for the cytotoxic effect of TGF alpha-PE40; and (iv) TGF alpha-PE40, produced by denaturing and refolding of insoluble material from inclusion bodies, binds better to EGF receptors and is about 10-fold more cytotoxic to cells bearing EGF receptors than is the secreted form of soluble TGF alpha-PE40.