A homolog of voltage-gated Ca2+ channels stimulated by depletion of secretory Ca2+ in yeast

In animal cells, capacitative calcium entry (CCE) mechanisms become activat ed specifically in response to depletion of calcium ions (Ca2+) from secret ary organelles. CCE serves to replenish those organelles and to enhance sig naling pathways that respond to elevated free Ca2+ concentrations in the cy toplasm. The mechanism of CCE regulation is not understood because few of i ts essential components have been identified. We show here for the first ti me that the budding yeast Saccharomyces cerevisiae employs a CCE-like mecha nism to refill Ca2+ stores within the secretary pathway, Mutants lacking Pm r1p, a conserved Ca2+ pump in the secretary pathway, exhibit higher rates o f Ca2+ influx relative to wild-type cells due to the stimulation of a high- affinity Ca2+ uptake system. Stimulation of this Ca2+ uptake system nas blo cked in pmr1 mutants by expression of mammalian SERCA pumps. The high-affin ity Ca2+ uptake system was also stimulated in wild-type cells overexpressin g vacuolar Ca2+ transporters that competed with Pmr1p for substrate. A scre en for yeast mutants specifically defective in the high-affinity Ca2+ uptak e system revealed two genes, CCH1 and MID1, previously implicated in Ca2+ i nflux in response to mating pheromones. Cch1p and Mid1p were localized to t he plasma membrane, coimmunoprecipitated from solubilized membranes, and sh own to function together within a single pathway that ensures that adequate levels of Ca2+ are supplied to Pmr1p to sustain secretion and growth. Expr ession of Cch1p and Mid1p was not affected in pmr1 mutants. The evidence su pports the hypothesis that yeast maintains a homeostatic mechanism related to CCE in mammalian cells. The homology between Cch1p and the catalytic sub unit of voltage-gated Ca2+ channels raises the possibility that in same cir cumstances CCE in animal cells may involve homologs of Cch1p and a conserve d regulatory mechanism.