Diphtheria toxin. Effect of substituting aspartic acid for glutamic acid 148 on ADP-ribosyltransferase activity

Photoaffinity labeling experiments with diphtheria toxin fragment A have implicated glutamic acid 148 as a constituent of the NAD binding site. To evaluate the role of this residue in ADP-ribosylation of elongation factor 2, we replaced it with aspartic acid by in vitro mutagenesis of a toxin gene fragment cloned in Escherichia coli. Fragment A containing aspartic acid at position 148 had less than 0.6% the ADP-ribosylation activity of wild-type fragment A. The mutation produced no change in sensitivity of fragment A to trypsin and little, if any, reduction in affinity of fragment A for NAD. These results indicate that glutamic acid 148 is essential for the ADP-ribosylation of elongation factor 2 and are consistent with other data suggesting that this residue may be at or near the catalytic center of the toxin.