Abstract: The bakers yeast Saccharomyces cerevisiae utilizes a high-affinity Ca2+ influx system (HACS) to survive assaults by mating pheromones, tunicamycin, and azole-class antifungal agents. HACS consists of two known subunits, Cch1 and Mid1, which are homologous and analogous to the catalytic alpha-subunits and regulatory alpha2delta-subunits of mammalian voltage-gated calcium channels (VGCCs), respectively. To search for additional subunits and regulators of HACS, a collection of gene knockout mutants was screened for abnormal uptake of Ca2+ after exposure to mating pheromone or to tunicamycin. The screen revealed that Ecm7 is required for HACS function in most conditions. Cycloheximide chase experiments showed that Ecm7 was stabilized by Mid1 and Mid1 was stabilized by Cch1 in non-signaling conditions, suggesting they all interact. Ecm7 is a member of the PMP-22/EMP/MP20/Claudin superfamily of transmembrane proteins, which includes gamma-subunits of VGCCs. Eleven additional members of this superfamily were identified in yeast, but none were required for HACS activity in response to the stimuli. Remarkably, many dozens of genes involved in vesicle-mediated trafficking and protein secretion were required to prevent spontaneous activation of HACS. Taken together, the findings suggest that HACS and calcineurin monitor performance of the membrane trafficking system in yeasts and coordinate compensatory processes. Conservation of this quality control system in Candida glabrata suggests that many pathogenic species of fungi may utilize HACS and calcineurin to resist azole-class antibiotics and other compounds that target membrane biosynthesis.