The 25-kDa synaptosome-associated protein (SNAP-25) binds and inhibits delayed rectifier potassium channels in secretory cells

Delayed-rectifier K⁺ channels (KDR) are important regulators of membrane excitability in neurons and neuroendocrine cells. Opening of these voltage-dependent K⁺ channels results in membrane repolarization, leading to the closure of the Ca²⁺ channels and cessation of insulin secretion in neuroendocrine islet β cells. Using patch clamp techniques, we have demonstrated that the activity of the K_DR channel subtype, K_v1.1, identified by its specific blocker dendrodotoxin-K, is inhibited by SNAP-25 in insulinoma HIT-T15 β cells. A coprecipitation study of rat brain confirmed that SNAP-25 interacts with the K_v1.1 protein. Cleavage of SNAP-25 by expression of botulinum neurotoxin A in HIT-T15 cells relieved this SNAP-25-mediated inhibition of K_DR. This inhibitory effect of SNAP-25 is mediated by the N terminus of K_v1.1, likely by direct interactions with K_vα1.1 and/or Kvβ subunits, as revealed by co-immunoprecipitation performed in the Xenopus oocyte expression system and in vitro binding. Taken together we have concluded that SNAP-25 mediates secretion not only through its participation in the exocytotic SNARE complex but also by regulating membrane potential and calcium entry through its interaction with K_DR channels.