MitoBK channel is functionally associated with its regulatory β1 subunit in cardiac mitochondria.

Key Points: Association of plasma membrane BK channels with BK-β subunits shapes their biophysical properties and physiological roles; however, functional modulation of the mitochondrial BK channel (mitoBK ) by BK-β subunits is not established. MitoBK -α and the regulatory BK-β1 subunit associate in mouse cardiac mitochondria. A large fraction of mitoBK display properties similar to that of plasma membrane BK when associated with BK-β1 (left-shifted voltage dependence of activation, V  = -55 mV, 12 µm matrix Ca ). In BK-β1 knockout mice, cardiac mitoBK displayed a low P and a depolarized V of activation (+47 mV at 12 µm matrix Ca ) Co-expression of BK with the BK-β1 subunit in HeLa cells doubled the density of BK in mitochondria. The present study supports the view that the cardiac mitoBK channel is functionally modulated by the BK-β1 subunit; proper targeting and activation of mitoBK shapes mitochondrial Ca handling.

Abstract: Association of the plasma membrane BK channel with auxiliary BK-β1-4 subunits profoundly affects the regulatory mechanisms and physiological processes in which this channel participates. However, functional association of mitochondrial BK (mitoBK ) with regulatory subunits is unknown. We report that mitoBK functionally associates with its regulatory subunit BK-β1 in adult rodent cardiomyocytes. Cardiac mitoBK is a calcium- and voltage-activated channel that is sensitive to paxilline with a large conductance for K of 300 pS. Additionally, mitoBK displays a high open probability (P ) and voltage half-activation (V  = -55 mV, n = 7) resembling that of plasma membrane BK when associated with its regulatory BK-β1 subunit. Immunochemistry assays demonstrated an interaction between mitochondrial BK -α and its BK-β1 subunit. Mitochondria from the BK-β1 knockout (KO) mice showed sparse mitoBK currents (five patches with mitoBK activity out of 28 total patches from n = 5 different hearts), displaying a depolarized V of activation (+47 mV in 12 µm matrix Ca ). The reduced activity of mitoBK was accompanied by a high expression of BK transcript in the BK-β1 KO, suggesting a lower abundance of mitoBK channels in this genotype. Accordingly, BK-β1subunit increased the localization of BKDEC (i.e. the splice variant of BK that specifically targets mitochondria) into mitochondria by two-fold. Importantly, both paxilline-treated and BK-β1 KO mitochondria displayed a more rapid Ca overload, featuring an early opening of the mitochondrial transition pore. We provide strong evidence that mitoBK associates with its regulatory BK-β1 subunit in cardiac mitochondria, ensuring proper targeting and activation of the mitoBK channel that helps to maintain mitochondrial Ca homeostasis.