Effect of colicins Ia and E1 on ion permeability of liposomes

Colicins Ia and E1 are shown to inhibit the formation and bring about the collapse of a potassium diffusion potential imposed across the membrane of liposomes prepared from soybean or Escherichia coli phospholipids. Such depolarization results from a colicin-induced increase in membrane ion permeability. Colicins E2 and E3 do not depolarize such membranes. In addition to the colicin Ia-induced rapid efflux of preloaded rubidium, sodium, phosphate, or choline from liposomes, a slower efflux of preloaded sucrose or glucose 6-phosphate occurs. However, treated liposomes do not leak inulin or dextran, demonstrating that the effects of E1 and Ia are not due to a general disruption of membrane structure. The fact that colicin-induced ion efflux is observed in the complete absence of a membrane potential shows that the action of these colicins on liposomes is not voltage dependent. These results provide strong evidence that the depolarization of E. coli cells by colicins Ia and E1 results from a colicin-induced increase in membrane permeability to ions. It is proposed that this is brought about by the direct interaction of the colicin molecules with the bacterial cytoplasmic membrane.