NSC1: a novel high-current inward rectifier for cations in the plasma membrane of Saccharomyces cerevisiae

The plasma membrane of the yeast Saccharomyces cerevisiae possesses a non-specific cation 'channel', tentatively dubbed NSC1, which is blocked by normal (mM) calcium and other divalent metal ions, is unblocked by reduction of extracellular free divalents below approximately 10 microM, and is independent of the identified potassium channel and porters in yeast, Duk1p, Trk1p, and Trk2p. Ion currents through NSC1, observed by means of whole-cell patch recording, have the following characteristics: Large amplitude, often exceeding 1 nA of K+/ cell at -200 mV, in tetraploid yeast, sufficient to double the normal intracellular K+ concentration within 10 s; non-saturation at large negative voltages; complicated activation kinetics, in which approximately 50% of the total current arises nearly instantaneously with a voltage-clamp step, while the remainder develops as two components, with time constants of approximately 100 ms and approximately 1.3 s; and voltage independence of both the activation time constants and the associated fractional current amplitudes.