CA2+ TRANSPORT IN SACCHAROMYCES-CEREVISIAE

Cytosolic free Ca2+ is maintained at submicromolar levels in budding y east by the activity of Ca2+ pumps and antiporters. We have recently i dentified the structural genes for two Ca2+ pumps, PCM1 and PMR1, whic h are required for Ca2+ sequestration into the vacuole and secretory o rganelles, respectively. The function of either Ca2+ pump is sufficien t for yeast viability, but deletion of both genes is lethal because of elevation of cytosolic [Ca2+] and activation of calcineurin, a Ca2+- and calmodulin-dependent protein phosphatase. Calcineurin activation d ecreases Ca2+ sequestration in the vacuole by a putative Ca2+ antiport er and may also increase Ca2+ pump activity. These regulatory processe s can affect the ability of yeast strains to tolerate high extracellul ar [Ca2+]. We propose a model in which the cellular response to change s in the environmental levels of Ca2+ is mediated by calmodulin and ca lcineurin which, in turn, modulate the various types of Ca2+ transport ers.