title: The contribution of RCK domains to human BK channel allosteric activation.
abstract: Large conductance voltage- and Ca(2+)-activated K(+) (BK) channels are potent regulators of cellular processes including neuronal firing, synaptic transmission, cochlear hair cell tuning, insulin release, and smooth muscle tone. Their unique activation pathway relies on structurally distinct regulatory domains including one transmembrane voltage-sensing domain (VSD) and two intracellular high affinity Ca(2+)-sensing sites per subunit (located in the RCK1 and RCK2 domains). Four pairs of RCK1 and RCK2 domains form a Ca(2+)-sensing apparatus known as the "gating ring." The allosteric interplay between voltage- and Ca(2+)-sensing apparati is a fundamental mechanism of BK channel function. Using voltage-clamp fluorometry and UV photolysis of intracellular caged Ca(2+), we optically resolved VSD activation prompted by Ca(2+) binding to the gating ring. The sudden increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) induced a hyperpolarizing shift in the voltage dependence of both channel opening and VSD activation, reported by a fluorophore labeling position 202, located in the upper side of the S4 transmembrane segment. The neutralization of the Ca(2+) sensor located in the RCK2 domain abolished the effect of [Ca(2+)](i) increase on the VSD rearrangements. On the other hand, the mutation of RCK1 residues involved in Ca(2+) sensing did not prevent the effect of Ca(2+) release on the VSD, revealing a functionally distinct interaction between RCK1 and RCK2 and the VSD. A statistical-mechanical model quantifies the complex thermodynamics interplay between Ca(2+) association in two distinct sites, voltage sensor activation, and BK channel opening.
authors.affiliation: Department of Anesthesiology, Division of Molecular Medicine, David Geffen School of Medicine, University of California at Los Angeles, Los Angeles, California 90075, USA.
authors.forename: Nicoletta
authors.initials: N
authors.lastname: Savalli
authors.forename: Antonios
authors.initials: A
authors.lastname: Pantazis
authors.forename: Taleh
authors.initials: T
authors.lastname: Yusifov
authors.forename: Daniel
authors.initials: D
authors.lastname: Sigg
authors.forename: Riccardo
authors.initials: R
authors.lastname: Olcese
mesh_terms: D000495:Allosteric Site; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry; D015220:Calcium Channels / Q000737:chemistry*; D004594:Electrophysiology / Q000379:methods; D005470:Fluorometry / Q000379:methods; D006801:Humans; D015640:Ion Channel Gating / Q000502:physiology*; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000737:chemistry*; D008564:Membrane Potentials / Q000502:physiology; D008958:Models, Molecular; D008968:Molecular Conformation; D010782:Photolysis; D011485:Protein Binding; D013816:Thermodynamics; D014466:Ultraviolet Rays
chemical_list: D015220:Calcium Channels; D051036:Large-Conductance Calcium-Activated Potassium Channels; D002118:Calcium