Large conductance, calcium-sensitive K+ channels (BKCa channels) contribute to the control of membrane potential in a variety of tissues, including smooth muscle, where they act as the target effector for intracellular "calcium sparks" and the endothelium-derived vasodilator nitric oxide. Various signal transduction pathways, including protein phosphorylation can regulate the activity of BKCa channels, along with many other membrane ion channels. In our study, we have examined the regulation of BKCa channels by the cellular Src gene product (cSrc), a soluble tyrosine kinase that has been implicated in the regulation of both voltage and ligand-gated ion channels. Using a heterologous expression system, we observed that co-expression of murine BKCa channel and the human cSrc tyrosine kinase in HEK 293 cells led to a calcium-sensitive enhancement of BKCa, channel activity in excised membrane patches. In contrast, co-expression with a catalytically inactive cSrc mutant produced no change in BKCa, channel activity, demonstrating the requirement for a functional cSrc molecule. Furthermore, we observed that BKCa channels underwent direct tyrosine phosphorylation in cells co-transfected with BKCa, channels and active cSre but not in cells co-transfected with the kinase inactive form of the enzyme. A single Tyr to Phe substitution in the C-terminal half of the channel largely prevented this observed phosphorylation, Given that cSre may become activated by receptor tyrosine kinases or G-protein-coupled receptors, these findings suggest that cSrc-dependent tyrosine phosphorylation of BKCa, channels in situ may represent a novel regulatory mechanism for altering membrane potential and calcium entry.
Enhanced activity of a large conductance, calcium-sensitive K+ channel in the presence of Src tyrosine kinase
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