Co-localization of calcium-modulating cyclophilin ligand with intracellular calcium pools

The calcium-modulating cyclophilin ligand (CAML) protein activates Ca²⁺ influx signaling when overexpressed in Jurkat T cells. Although CAML appears to directly participate in Ca²⁺-dependent signaling initiated by the transmembrane activator and CAML interactor cell surface receptor, its mechanism of action is unknown. To address this issue, we have determined its membrane topology, subcellular localization, and ability to mobilize intracellular Ca²⁺ pools. Fractionation of cell extracts on discontinuous sucrose gradients and indirect immunofluorescence indicate that CAML colocalizes with sarcoplasmic/endoplasmic reticulum calcium/ATPase-2 and calreticulin at membrane-bound cytosolic vesicles. Limited trypsin digests indicate that the hydrophilic NH₂-terminal domain of CAML is directed toward the cytoplasm. Functionally, CAML overexpression was shown to deplete thapsigargin-sensitive intracellular Ca²⁺ pools. These data suggest that CAML may initiate Ca²⁺ signaling through activation of a capacitative Ca²⁺ influx pathway.