title: The RCK2 domain of the human BKCa channel is a calcium sensor.
abstract: Large conductance voltage and Ca(2+)-dependent K(+) channels (BK(Ca)) are activated by both membrane depolarization and intracellular Ca(2+). Recent studies on bacterial channels have proposed that a Ca(2+)-induced conformational change within specialized regulators of K(+) conductance (RCK) domains is responsible for channel gating. Each pore-forming alpha subunit of the homotetrameric BK(Ca) channel is expected to contain two intracellular RCK domains. The first RCK domain in BK(Ca) channels (RCK1) has been shown to contain residues critical for Ca(2+) sensitivity, possibly participating in the formation of a Ca(2+)-binding site. The location and structure of the second RCK domain in the BK(Ca) channel (RCK2) is still being examined, and the presence of a high-affinity Ca(2+)-binding site within this region is not yet established. Here, we present a structure-based alignment of the C terminus of BK(Ca) and prokaryotic RCK domains that reveal the location of a second RCK domain in human BK(Ca) channels (hSloRCK2). hSloRCK2 includes a high-affinity Ca(2+)-binding site (Ca bowl) and contains similar secondary structural elements as the bacterial RCK domains. Using CD spectroscopy, we provide evidence that hSloRCK2 undergoes a Ca(2+)-induced change in conformation, associated with an alpha-to-beta structural transition. We also show that the Ca bowl is an essential element for the Ca(2+)-induced rearrangement of hSloRCK2. We speculate that the molecular rearrangements of RCK2 likely underlie the Ca(2+)-dependent gating mechanism of BK(Ca) channels. A structural model of the heterodimeric complex of hSloRCK1 and hSloRCK2 domains is discussed.
authors.affiliation: Division of Molecular Medicine, Department of Anesthesiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90095-7115, USA.
authors.forename: Taleh
authors.initials: T
authors.lastname: Yusifov
authors.forename: Nicoletta
authors.initials: N
authors.lastname: Savalli
authors.forename: Chris S
authors.initials: CS
authors.lastname: Gandhi
authors.forename: Michela
authors.initials: M
authors.lastname: Ottolia
authors.forename: Riccardo
authors.initials: R
authors.lastname: Olcese
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D002942:Circular Dichroism; D018360:Crystallography, X-Ray / Q000379:methods; D006801:Humans; D051667:Kv1.6 Potassium Channel / Q000737:chemistry*; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits / Q000737:chemistry*; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D013056:Spectrophotometry, Ultraviolet
chemical_list: C494841:KCNMA1 protein, human; D051667:Kv1.6 Potassium Channel; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D002118:Calcium