The Effect of Partially Conserved Residues of hTGF-alpha C Domain in Its Structure and Function

There is a highly homologous region in the C domain of the EGF family, some of its residues are semi-conserved. We constructed three hTGF-alpha mutants, hTGF-alphaV35, hTGF-alphaQ44, hTGF-alphaY45R46, by site-directed mutagenesis to replace the semi-conserved residues in the C domain of hTGF-alpha with the corresponding residues of hEGF. We observed that although the binding affinity of hEGF to hEGF receptor was about two fold that of hTGF-alpha, but the receptor binding affinity of the three mutants was respectively decreased to about 22 %, 13.4 % and 25 % compared of that of hTGF-alpha. On the other hand, the stimulating action of hEGF on NRK-49F cell proliferation was only 10 % that of hTGF-alpha, but those of the threemutants was about 4 fold, 10 fold and 5 fold more active than hTGF-alpha. Thus, the three mutants did not become more similar to hEGF in function. The functional difference between hEGF and hTGF-alpha was not simply determined by any single semi-conserved residue, but substitution at those sites in the C domain have altered the characters of hTGF-alpha sharply.