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-IP3receptor antibody and were unchanged when the endoplasmic reticulum was segregated to the hemisphere opposite the injection site by centrifugation. Confocal microscopy revealed that [Ca2+]iincreases were most pronounced at the periphery of the oocyte. The patterns of [Ca2+]iincreases were replicated by computer simulations based on a diffusible messenger of about 700 Da that directly activates Ca2+influx. In addition, ICRAC, 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.