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To study the role of the inositol 1,3,4,5-trisphosphate-binding protein GAP1(IP4BP) in store-operated Ca2+ entry, we established a human erythroleukemia (HEL) cell line in which the expression of GAP1(IP4BP) was substantially reduced by transfection with a vector containing antisense DNA under control of a Rous Sarcoma virus promoter and the Escherichia coli LacI repressor (AS-HEL cells). Control cells were transfected with vector lacking antisense DNA (V-HEL cells). GAP1(IP4BP) protein, which is a member of the GTPase-activating protein (GAP1) family, was reduced by 85% in AS-HEL cells and was further reduced by 96% by treatment with isopropylthio-beta-D- galactoside to relieve LacI repression. The loss of GAP1(IP4BP) was associated with both a membrane hyperpolarization and a substantially increased Ca2+ entry induced by thrombin or thapsigargin. The activation of intermediate conductance Ca2+-activated K+ channels in AS-HEL cells (not seen in V-HEL cells) was responsible for the membrane hyperpolarization and the enhanced Ca2+ entry, and both were blocked by charybdotoxin. Stimulated V-HEL cells did not hyperpolarize and basal Ca2+ influx was unaffected by charybdotoxin. In V-HEL cells hyperpolarized by removal of extracellular K+, the thapsigargin-stimulated Ca2+ influx was increased. Expression of mRNA for the human Ca2+-activated intermediate conductance channel KCa4 was equivalent in both AS-HEL and V-HEL cells, suggesting that the specific appearance of calcium-activated potassium current (IK(Ca)) in AS-HEL cells was possibly due to modulation of preexisting channels. Our results demonstrate that GAP1(IP4BP), likely working through a signaling pathway dependent on a small GTP-binding protein, can regulate the function of K(Ca) channels that produce a hyperpolarizing current that substantially enhances the magnitude and time course of Ca2+ entry subsequent to the release of internal Ca2+ stores.

the journal of general physiology/the journal of general physiology

Antisense Knock Out of the Inositol 1,3,4,5-Tetrakisphosphate Receptor GAP1IP4BP in the Human Erythroleukemia Cell Line Leads to the Appearance of Intermediate Conductance K(Ca) Channels that Hyperpolarize the Membrane and Enhance Calcium Influx