DIPHTHERIA-TOXIN AT LOW PH DEPOLARIZES THE MEMBRANE, INCREASES THE MEMBRANE CONDUCTANCE AND INDUCES A NEW-TYPE OF ION-CHANNEL IN VERO CELLS

Receptor-dependent translocation of diphtheria toxin across the surfac e membrane of Vero cells was studied using patch clamp techniques. Tra nslocation was induced by exposing cells with surface-bound toxin to l ow pH. Whole cell current and voltage clamp recordings showed that tox in translocation was associated with membrane depolarization and incre ased membrane conductance. The conductance increase was voltage indepe ndent, with a reversal potential of similar to 15 mV. This value was u naffected by changing the Cl- gradient across the membrane and microfl uorometric measurements showed that the cytosolic Ca2+ concentration w as only marginally elevated by the translocation. The conductance incr ease is thus mainly due to monovalent cations. Exposing outside-out an d cell-attached patches with bound toxin to low pH induced a new type of ion channel in the membrane. The channel current was inward at nega tive membrane potentials and the single channel conductance was simila r to 30 pS. This value is about three times larger than for receptor-i ndependent channels induced by diphtheria toxin or toxin fragments in artificial lipid membranes.