Abstract
Diphtheria toxin inhibits protein synthesis in eukaryotic cells by catalyzing inactivation of elongation factor 2. The 10,000-fold greater sensitivity in vitro to diphtheria toxin of human cells as compared to mouse cells seems to be attributable to a difference at the level of the cell membrane. Mouse-human cell hybrids are as sensitive to diphtheria toxin as human cells. We have shown that the sensitivity of the hybrid cells is due to a gene or genes located on human chromosome 5. Mouse-human hybrid cells in which chromosome 5 is present are as sensitive to the toxin as human cells, which hybrids without chromosome 5 are as resistant as mouse cells. Entry of toxin into cells seems to be a two-step process involvin, (1) binding of toxin to the cell surface and (2) endocytotic uptake of toxin. The difference in sensitivity between human and mouse cells and between hybrid cells with and without chromosome 5 does not appear to be due to a difference in endocytotic activity and may be due to presence or absence of toxin-specific receptor.