Plasma membrane calcium channels in human carcinoma A431 cells are functionally coupled to inositol 1,4,5-trisphosphate receptor- phosphatidylinositol 4,5-bisphosphate complexes

In most nonexcitable cells, calcium (Ca²⁺) release from inositol 1,4,5-trisphosphate (InsP₃)-sensitive intracellular Ca²⁺ stores is coupled to Ca²⁺ influx (calcium release-activated channels (I(CRAC))) pathway. Despite intense investigation, the molecular identity of I(CRAC) and the mechanism of its activation remain poorly understood. InsP₃-dependent miniature calcium channels (I(min)) display functional properties characteristic for I(CRAC). Here we used patch clamp recordings of I(min) channels in human carcinoma A431 cells to demonstrate that I(min) activity was greatly enhanced in the presence of anti-phosphatidylinositol 4,5- bisphosphate antibody (PIP₂Ab) and diminished in the presence of PIP₂. AntiPIP₂ antibody induced a greater than 6-fold increase in I(min) sensitivity for InsP₃ activation and an almost 4-fold change in I(min) maximal open probability. The addition of exogenous PIP₂ vesicles to the cytosolic surface of inside-out patches inhibited I(min) activity. These results lead us to propose an existence of a Ca²⁺ influx pathway in nonexcitable cells activated via direct conformational coupling with a selected population of InsP₃ receptors, located just underneath the plasma membrane and coupled to PIP₂. The described pathway provides for a highly compartmentalized Ca²⁺ influx and intracellular Ca²⁺ store refilling mechanism.