Hamster diphtheria toxin receptor: A naturally occurring chimera of monkey and mouse HB-EGF precursors

The sensitivity of mammalian cell lines to diphtheria toxin (DT) varies between species. Monkey (Mk) Vero cells are highly sensitive to DT, whereas rat and mouse (Ms) cells are resistant; hamster (Hm) cells display moderate DT sensitivity. The precursor of the Mk heparin-binding epidermal growth factor-like growth factor (proHB-EGF) functions as a DT receptor but the Ms proHB-EGF does not. In this study we have cloned, expressed, and characterized the Hm proHB-EGF/DT receptor. The expression of Hm proHB-EGF confers moderate DT sensitivity to normally DT-resistant mouse cells. The amino acid sequence of Hm preproHB-EGF shows that, overall, it more closely resembles the Ms preproHB-EGF sequence, except in the DT-binding region where it more closely resembles the Mk sequence. In the DT-binding region the Hm proHB-EGF sequence differs from the Mk proHB-EGF in only four amino acid residues (124, 126, 133, and 147); one of these residues, Ile¹³³ in Mk proHB-EGF, has been previously reported to be important for DT binding and sensitivity. Analysis of Mk proHB-EGF mutants with residues substituted for Ile¹³³ suggests that Asn¹³³ in Hm proHB-EGF may be responsible for the moderate DT sensitivity of Hm proHB-EGF-expressing cells.