CATION-SELECTIVE CHANNELS IN THE VACUOLAR MEMBRANE OF SACCHAROMYCES - DEPENDENCE ON CALCIUM, REDOX STATE, AND VOLTAGE

The vacuolar membrane of the yeast Saccharomyces cerevisiae, which is proposed as a system for functional expression of membrane proteins, was examined by patch-clamp techniques. Its most conspicuous feature, in the absence of energizing substrates, is a cation channel with a characteristic conductance of ≃120 pS for symmetric 100 mM KCl solutions and with little selectivity between K+ and Na+ (P(Na+)/P(K+) ≃ 1) but strong selectivity for cations over anions (P(Cl-)/P(K+) < 0.1). Channel gating is voltage-dependent; open probability, P(o), reaches maximum (≃0.7) at a transmembrane voltage of -80 mV (cytoplasmic surface negative) and declines at both more negative and more positive voltages (i.e., to 0 around +80 mV). The time-averaged current-voltage curve shows strong rectification, with negative currents (positive charges flowing from vacuolar side to cytoplasmic side) much larger than positive currents. The open probability also depends strongly on cytoplasmic Ca2+ concentration but, for ordinary recording conditions, is high only at unphysiologically high (≥1 mM) Ca2+. However, reducing agents such as dithiothreitol and 2-mercaptoethanol poise the channels so that they can be activated by micromolar cytoplasmic Ca2+. The channels are blocked irreversibly by chloramine T, which is known to oxidize exposed methionine and cysteine residues specifically.