A junctophilin-caveolin interaction enables efficient coupling between ryanodine receptors and BK(Ca) channels in the Ca(2+) microdomain of vascular smooth muscle

Functional coupling between large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in the plasma membrane (PM) and ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR) is an essential mechanism for regulating mechanical force in most smooth muscle (SM) tissues. Spontaneous Ca(2+) release through RyRs (Ca(2+) sparks) and subsequent BK(Ca) channel activation occur within the PM-SR junctional sites. We report here that a molecular interaction of caveolin-1 (Cav1), a caveola-forming protein, with junctophilin-2 (JP2), a bridging protein between PM and SR, positions BK(Ca) channels near RyRs in SM cells (SMCs) and thereby contributes to the formation of a molecular complex essential for Ca(2+) microdomain function. Approximately half of all Ca(2+) sparks occurred within a close distance (<400 nm) from fluorescently labeled JP2 or Cav1 particles, when they were moderately expressed in primary SMCs from mouse mesenteric artery. The removal of caveolae by genetic Cav1 ablation or methyl-β-cyclodextrin treatments significantly reduced coupling efficiency between Ca(2+) sparks and BK(Ca) channel activity in SMCs, an effect also observed after JP2 knockdown in SMCs. A 20-amino acid-long region in JP2 appeared to be essential for the observed JP2-Cav1 interaction, and we also observed an interaction between JP2 and the BK(Ca) channel. It can be concluded that the JP2-Cav1 interaction provides a structural and functional basis for the Ca(2+) microdomain at PM-SR junctions and mediates cross-talk between RyRs and BK(Ca) channels, converts local Ca(2+) sparks into membrane hyperpolarization, and contributes to stabilizing resting tone in SMCs.