Functional Study Of Genes Related To Calcium Homeostasis In The Genome Of Saccharomyces Cerevisiae

From a genome-scale genetic screen, we have identified120calcium-sensitivegene mutations in Saccharomyces cerevisiae. These mutations are of genes involvedin metabolism, protein sorting, ion homeostasis, transcription, cellular transport,cellcycle and osmotic stress response. Seven of these genes encode components ofESCRT-I,-II, and-III complexes, which function to form multivesicular bodies(MVBs) for sorting of proteins destined for the vacuole or the lysosome. In responseto calcium stress, these ESCRT mutants show elevated levels of intracellular calciumand activated calcium/calcineurin signaling, but the expression level of PMR1,encoding the ER/Golgi calcium pump, is reduced in these mutants independent ofcalcium stress. Consistent with previous observations that these ESCRT mutants havea defect in proteolytic processing of Rim101, we demonstrate that their calciumhypersensitivity is partially due to repressed expression of PMR1through elevatedlevels of Nrg1and suppressed by expression of a constitutively active form ofRim101. Nrg1binds to two motifs in the promoter of PMR1in vitro and in vivo.Furthermore, we have identified two Crz1-depedent response elements in thepromoter of PMR1, which are required for its calcium-induced expression. Inresponse to alkaline stress, a proper level of PMR1expression is maintained by bothCrz1and Nrg1, and lack of either of them breaks the balance of its expression.However, mutation of CRZ1is epistatic to mutation of NRG1in calcium stress, wherethey are antagonistic to each other in saline stress.