Cellular regulation of diphtheria toxin cell surface receptors.

The cellular regulation of diphtheria toxin cell surface receptors was studied. Treatment of Vero cells with cycloheximide reduced their diphtheria toxin binding capacity, while cells treated with actinomycin D did not lose their ability to bind diphtheria toxin. A non-toxic analogue of diphtheria toxin, CRM 197, produced a dose-related depletion of cell surface diphtheria toxin binding capacity that was reversible upon washing the cells. Vero cells depleted of toxin receptors by CRM 197 did not restore their ability to bind diphtheria toxin in the presence of cycloheximide. Phospholipase C treatment of Vero cells reduced their diphtheria toxin binding capacity in a dose-dependent manner. The loss of diphtheria toxin binding capacity was recovered within 2 hr after removal of the enzyme. Protein synthesis inhibition blocked this recovery while actinomycin D partially inhibited it. Receptors prebound with toxin were resistant to phospholipase C treatment, suggesting that the action of the enzyme was directly on the receptor. Inhibition of glycosylation with tunicamycin did not prevent reappearance of toxin receptors after CRM 197 or phospholipase C treatment. These data establish the requirement of a continuous protein synthesis for the maintenance of diphtheria toxin cell surface receptors and also suggest that these receptors do not recycle after binding ligand. A hypothesis is put forward that the diphtheria toxin receptor might be a lipid-linked cell surface protein.