Effect of killer toxin K1 on yeast membrane potential reported by the diS-C-3(3) probe reflects strain- and physiological state-dependent variations

The rate and extent of uptake of the fluorescent probe diS-C-3(3) reporting on membrane potential in S. cerevisiae is affected by the strain under stu dy, cell-growth phase, starvation and by the concentration of glucose both in the growth medium and in the monitored cell suspension under non-growth conditions. Killer toxin K1 brings about changes in membrane potential. In all types of cells tested, viz. in glucose-supplied stationary or exponenti al cells of the killer-sensitive strain S6/1 or a conventional strain RXII, or in glucose-free exponential cells of both strains, both active and heat -inactivated toxin slow down the potential-dependent uptake of diS-C-3(3) i nto the cells. This may reflect "clogging" of pores in the cell wall that h inders, but does not prevent, probe passage to the plasma membrane and its equilibration. The clogging effect of heat-inactivated toxin is stronger th an that exerted by active toxin. In susceptible cells, ie. in exponential-p hase glucose-supplied cells of the sensitive strain S6/1, this phase of pro be uptake retardation is followed by an irreversible red shift in probe flu orescence maximum lambda(max) indicating damage to membrane integrity and c ell permeabilization. A similar fast red shift in IZ,, signifying lethal ce ll damage was found in heat-killed or nystatin-treated cells.