Calcium influx factor is synthesized by yeast and mammalian cells depleted of organellar calcium stores

Depletion of endoplasmic reticulum Ca²⁺ stores leads to the entry of extracellular Ca²⁺ into the cytoplasm, a process termed capacitative or store-operated Ca²⁺ entry. Partially purified extracts were prepared from the human Jurkat T lymphocyte cell line and yeast in which Ca²⁺ stores were depleted by chemical and genetic means, respectively. After microinjection into Xenopus laevis oocytes, the extracts elicited a wave of increased cytoplasmic free Ca²⁺ ([Ca²⁺]_i) that spread from the point of injection across the oocyte. Extracts from cells with replete organellar Ca²⁺ stores were inactive. The increases depended on extracellular Ca²⁺, were unaffected by the inositol 1,4,5-trisphosphate (IP₃) inhibitor heparin or an anti-IP₃ receptor antibody and were unchanged when the endoplasmic reticulum was segregated to the hemisphere opposite the injection site by centrifugation. Confocal microscopy revealed that [Ca²⁺]_i increases were most pronounced at the periphery of the oocyte. The patterns of [Ca²⁺]_i increases were replicated by computer simulations based on a diffusible messenger of about 700 Da that directly activates Ca²⁺ influx. In addition, I_CRAC, a Ca²⁺ release-activated Ca²⁺ current monitored in Jurkat cells by whole-cell patch clamp recordings, was more rapidly activated when active extracts were included in the patch pipette than by the inclusion of a Ca²⁺ chelator or IP₃. These data support the existence in yeast and mammalian cells depleted of Ca²⁺ stores of a functionally conserved diffusible calcium influx factor that directly activates Ca²⁺ influx.