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Neuronal Ca2+ Sensor-1/Frequenin Functions in an Autocrine Pathway Regulating Ca2+ Channels in Bovine Adrenal Chromaffin Cells*

NCS-1/frequenin belongs to a family of EF-hand-containing Ca2+ sensors expressed mainly in neurons. Overexpression of NCS-1/frequenin has been shown to stimulate neurotransmitter release but little else is known of its cellular roles. We have constructed an EF-hand mutant, NCS-1(E120Q), as a likely dominant inhibitor of cellular NCS-1 function. Recombinant NCS-1(E120Q) showed an impaired Ca2+-dependent conformational change but could still bind to cellular proteins. Transient expression of this mutant, but not NCS-1, in bovine adrenal chromaffin cells increased non-L-type Ca2+ channel currents. Cells expressing NCS-1(E120Q) no longer responded effectively to the removal of autocrine purinergic/opioid inhibition of Ca2+ currents but still showed voltage-dependent facilitation. These data are consistent with the existence of both voltage-dependent and voltage-independent pathways for Ca2+ channel inhibition in chromaffin cells. Our results suggest a novel function for NCS-1 specific for the voltage-independent autocrine pathway that negatively regulates non-L-type Ca2+ channels in chromaffin cells.