Sal1p, a novel Ca(2+)-dependent ATP-Mg/Pi carrier, is essential in yeast lacking all adenine nucleotide translocases. By targeting luciferase to the mitochondrial matrix to monitor mitochondrial ATP levels, we show in isolated mitochondria that both ATP-Mg and free ADP are taken up by Sal1p with a K(m) of 0.20 +/- 0.03 mM and 0.28 +/- 0.06 mM, respectively. Nucleotide transport along Sal1p is strictly Ca(2+) dependent. Ca(2+) increases the V(max) with a S(0.5 )of 15 microM, and no changes in the K(m) for ATP-Mg. Glucose sensing in yeast generates Ca(2+) transients involving Ca(2+) influx from the external medium. We find that carbon-deprived cells respond to glucose with an immediate increase in mitochondrial ATP levels which is not observed in the presence of EGTA or in Sal1p-deficient cells. Moreover, we now report that during normal aerobic growth on glucose, yeast mitochondria import ATP from the cytosol and hydrolyze it through H(+)-ATP synthase. We identify two pathways for ATP uptake in mitochondria, the ADP/ATP carriers and Sal1p. Thus, during exponential growth on glucose, mitochondria are ATP consumers, as those from cells growing in anaerobic conditions or deprived of mitochondrial DNA which depend on cytosolic ATP and mitochondrial ATPase working in reverse to generate a mitochondrial membrane potential. In conclusion, the results show that growth on glucose requires ATP hydrolysis in mitochondria and recruits Sal1p as a Ca(2+)-dependent mechanism to import ATP-Mg from the cytosol. Whether this mechanism is used under similar settings in higher eukaryotes is an open question.
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