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Summary: The excitability of CA1 hippocampal pyramidal cells is mediated by a slow AHP (sAHP) that responds to calcium increases by Cav1 calcium channels and ryanodine receptors (RyR). We used super-resolution and FRET microscopy to investigate the proximity and functional coupling among Cav1.3/Cav1.2, RyR2, and KCa3.1 potassium channels that contribute to the sAHP. dSTORM and FRET imaging shows that Cav1.3, RyR2, and KCa3.1 are organized as a triprotein complex that colocalizes with junctophilin (JPH) 3 and 4 proteins that tether the plasma membrane to the endoplasmic reticulum. JPH3 and JPH4 shRNAs dissociated a Cav1.3-RyR2-KCa3.1 complex and reduced the IsAHP. Infusing JPH3 and JPH4 antibodies into CA1 cells reduced IsAHP and spike accommodation. These data indicate that JPH3 and JPH4 proteins maintain a Cav1-RyR2-KCa3.1 complex that allows two calcium sources to act in tandem to define the activation properties of KCa3.1 channels and the IsAHP. : Sahu et al. establish that junctophilin proteins maintain a triprotein complex that assembles a voltage-gated plasma membrane calcium channel and endoplasmic reticular ryanodine receptors with a calcium-gated potassium channel to generate the long-duration slow afterhyperpolarization that regulates spike output in hippocampal pyramidal cells. Keywords: CaV1.3, Cav1.2, KCa3.1, ryanodine receptor, RyR2, junctophilin, sAHP, hippocampus, dSTORM, FRET