Activation of phospholipase C in nonexcitable cells causes the release of calcium (Ca 2؉ ) from intracellular stores and activation of Ca 2؉ influx by means of Ca 2؉ release-activated channels (ICRAC) in the plasma membrane. The molecular identity and the mechanism of I CRAC channel activation are poorly understood. Using the patch-clamp technique, here we describe the plasma membrane Ca 2؉ channels in human carcinoma A431 cells, which can be activated by extracellular UTP, by depletion of intracellular Ca 2؉ stores after exposure to the Ca 2؉ -pump inhibitor thapsigargin, or by loading the cells with Ca 2؉ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetate. The observed channels display the same conductance and gating properties as previously described I min channels, but have significantly lower conductance for monovalent cations than the I CRAC channels. Thus, we concluded that the depletion-activated Ca 2؉ entry processes (1-3). These processes are mediated by plasma membrane Ca 2ϩ channels termed ''Ca 2ϩ release activated channels'' (I CRAC ) (4-7). The molecular identity of I CRAC remains unclear, with mammalian trp channels (mTrp) usually considered the most likely candidate for the role of I CRAC (1-3, 8, 9). When compared with I CRAC , mTrp channels display relatively low selectivity for divalent cations, higher single channel conductance, and different kinetic and pharmacological properties. In experiments with a human carcinoma A431 cell line, we previously described plasma membrane Ca 2ϩ channels (I min ) that are activated by application of uridine triphosphate and bradykinin to cell-attached patches or by application of inositol (1,4,5)-trisphosphate (IP 3 ) to excised inside-out (i͞o) patches (10-12). IP 3 -gated channels that share some common properties with I min have been also observed in experiments with human T cells (13), rat macrophages (12), and endothelial cells (14,15). Major functional properties of I min channels, such as small conductance (1 pS for divalent cations), high selectivity for divalent cations (P Ca/K Ͼ 1,000), inward rectification, and sensitivity to block by SKF95365 are similar to I CRAC channels (12, 16). Thus, we previously suggested that I min and I CRAC may in fact be the same channels (17).The mechanism of I CRAC activation remains similarly controversial (1-3). When studied in a heterologous expression system, activation of mTrp channels by IP 3 appear to be mediated by direct conformational coupling between the cytosolic carboxylterminal tail of mTrp and the amino-terminal ligand-binding domain of intracellular IP 3 receptor (IP 3 R) (18-21). However, whether mTrp can serve as an appropriate model system for understanding I CRAC activation is unresolved (18,21,22). In previous studies, we demonstrated that activity of I min in i͞o patches is potentiated by addition of IP 3 R-enriched microsomes as predicted by an I min -IP 3 R conformational coupling model (16). More recently, we discovered that anti-PIP 2 antibody (PIP 2 Ab) sensitizes I min to IP 3 a...