Abstract
The single transmembrane-spanning Ca(2+)-binding protein, STIM1, has been proposed to function as a Ca(2+) sensor that links the endoplasmic reticulum to the activation of store-operated Ca(2+) channels. In this study, the presence, subcellular localization and function of STIM1 in store-operated Ca(2+) entry in oocytes was investigated using the pig as a model. Cloning and sequence analysis revealed the presence of porcine STIM1 with a coding sequence of 2058 bp. In oocytes with full cytoplasmic Ca(2+) stores, STIM1 was localized predominantly in the inner cytoplasm as indicated by immunocytochemistry or overexpression of human STIM1 conjugated to the yellow fluorescent protein. Depletion of the Ca(2+) stores was associated with redistribution of STIM1 along the plasma membrane. Increasing STIM1 expression resulted in enhanced Ca(2+) influx after store depletion and subsequent Ca(2+) add-back; the influx was inhibited when the oocytes were pretreated with lanthanum, a specific inhibitor of store-operated Ca(2+) channels. When STIM1 expression was suppressed using siRNAs, there was no change in cytosolic free Ca(2+) levels in the store-depleted oocytes after Ca(2+) add-back. The findings suggest that in oocytes, STIM1 serves as a sensor of Ca(2+) store content that after store depletion moves to the plasma membrane to stimulate store-operated Ca(2+) entry.