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

Abstract

Association of plasma membrane BK _Ca channel with auxiliary BK-β (1–4) subunits profoundly affects regulatory mechanisms and physiological processes in which this channel participate. However, functional association of mitochondrial BK (mitoBK _Ca) with regulatory subunits is unknown. We report that mitoBK _Ca functionally associates with its regulatory subunit BK-β1 in adult rodent cardiomyocytes. Cardiac mitoBK _Ca is a calcium and voltage activated channel, sensitive to paxilline with a large conductance for K ⁺ of 300 pS. Additionally, mitoBK _Ca displays a high open probability (P _o) and voltage half of activation ( V _1/2 = −55 mV, n=7) that resembles that of plasma membrane BK _Ca when associated with its regulatory BK-β1 subunit. Immunochemistry assays demonstrated an interaction between mitochondrial BK _Ca-α and its BK-β1 subunit. Mitochondria from the BK-β1 KO mice showed sparse mitoBK _Ca currents (5 patches with mitoBK _Ca activity out of 28 total patches from n=5 different hearts), displaying a depolarized V _1/2 activation (+47 mV in 12 µM matrix Ca ²⁺). The reduced activity of mitoBK _Ca was accompanied with a high expression of BK _Ca transcript in the BK-β1 KO, suggesting less abundance of mitoBK _Ca channels in this genotype. Accordingly, BK-β1subunit increased two-fold the localization of BKDEC (the splice variant of BK _Ca that specifically targets mitochondria) into mitochondria. Importantly, both paxilline treated and BK-β1 KO mitochondria displayed a more rapid Ca ²⁺ overload, featuring an early opening of the mitochondrial transition pore (mPTP). We provide strong evidence that mitoBK _Ca associates with its regulatory BK-β1 subunit in cardiac mitochondria, ensuring proper targeting and activation of the mitoBK _Ca channel helps to maintain mitochondrial Ca ²⁺ homeostasis.