An attempt to form mutant CRM197 proteins of diphtheria toxin with neutralizing activity to heregulin.

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We have found that diphtheria toxin (DT) bind to the EGF-like domain of diphtheria toxin receptor/membrane-anchored form of heparin-binding EGF-like growth factor (proHB-EGF). DT,or its non-toxic mutant CRM197, inhibits the binding of HB-EGF to EGF receptor, resulting in inhibition of mitogenic activity of HB-EGF.The aim of this work is to produce mutant forms of CRM197 of diphtheria toxin with neutralizing activity to heregulin. We have obtained the following results.(1) Analysis of the DT-binding site on the EGF-like domain of HB-EGFTo form heregulin-binding CRM197, DT-binding site on EGF-like domain of HB-EGF was precisely determined. We introduced single point mutations for 10 amino acids within the EGF-like domain. Mutation at F115, L127 or E141 greatly decreased the DT-binding activityof HB-EGF,but mutations for the remaining 7 amino acids did not show large effect.(2) Construction of three-dimentional structure model of the EGF-like domain of HB-EGFThree-dimentional structure model of the EGF-like domain of HB-EGF was constructed based on the NMR data of EGF and TGFalpha. The model indicated that amino acid residues F115, L127 and E141 exist on the same side of the molecule, suggesting that HB-EGF interacts with DT at this side.These results is critical to construct heregulin-binding CRM197. Further studies are in progress.