Sac1p of Saccharomyces cerevisiae is not involved in ATP release to the extracellular fluid.

One activity ascribed to Sac1p is the transport of ATP into the lumen of the endoplasmic reticulum of Saccharomyces cerevisiae; therefore, the question of whether this protein plays a role in ATP efflux from yeast was addressed. Preliminary results suggested that deletion of the SAC1 gene eliminated nigericin-stimulated ATP efflux. However, further experimentation revealed that this result was caused by a pronounced extracellular ATPase activity for sac1delta cells at alkaline pH, conditions required to measure extracellular ATP in wild type cells. At acid pH, sac1delta cells exhibit glucose-dependent, nigericin-stimulated ATP efflux. sac1delta cells express less acid phosphatase activity in the periplasm than do wild type cells, thus increasing the stability of extracellular ATP. At alkaline pH, however, sac1delta cells tend to lose structural integrity and release lactate dehydrogenase as well as an unidentified ATPase activity to the extracellular fluid. Therefore, Sac1p is not involved in ATP efflux from S. cerevisiae.