Closure of the yeast mitochondria unspecific channel (YMUC) unmasks a Mg2+ and quinine sensitive K+ uptake pathway in Saccharomyces cerevisiae.

The K+ uptake pathways in yeast mitochondria are still undefined. Nonetheless, the K+-mediated mitochondrial swelling observed in the absence of phosphate (PO4) and in the presence of a respiratory substrate has led to propose that large K+ movements occur in yeast mitochondria. Thus, the uptake of K+ by isolated yeast mitochondria was evaluated. Two parallel experiments were conducted to evaluate K+ transport; these were mitochondrial swelling and the uptake of the radioactive K+ analog 86Rb+. The opening of the yeast mitochondrial unspecific channel (YMUC) was regulated by different PO4 concentrations. The high protein concentrations used to measure 86Rb+ uptake resulted in a slight stabilization of the transmembrane potential at 0.4 mM PO4 but not at 0 or 4 mM PO4. At 4 mM PO4 swelling was inhibited while, in contrast, 86Rb+ uptake was still observed. The results suggest that an energy-dependent K+ uptake mechanism was unmasked when the YMUC was closed. To further analyze the properties of this K+ uptake system, the Mg2+ and quinine sensitivity of both swelling and 86Rb+ uptake were evaluated. Under the conditions where the unspecific pore was closed, K+ transport sensitivity to Mg2+ and quinine increased. In addition, when Zn2+ was added as an antiport inhibitor, uptake of 86Rb+ increased. It is suggested that in yeast mitochondria, the K+ concentration is highly regulated by the equilibrium of uptake and exit of this cation through two specific transporters.