Basic properties of large-conductance, voltage-, and Ca2+-sensitive (maxi-KCa) channels. A: diagram of slo channels. S0–S6 mark NH2-terminal transmembrane segments. S0 is unique for maxi-KCa channels, and S1–S6 are homologous to voltage-dependent ion channels. S7–S10 are hydrophobic regions at the COOH terminus (two-thirds of the protein). Two separable domains, the “core” and “tail,” are marked. Triangles 1–6 mark alternative splice sites that lead to the insertion of 3–59 amino acids into the primary sequence. Putative sites for phosphorylation (~P) are in stars (protein kinase G) and in circles (protein kinase C). B: bar graph comparing the amino acid (aa) identity of human α- (slo) and β-subunits with other species. GenBank accession numbers for α- and β-subunits, respectively, are as follows: h (human), U11058 and U25138; b (bovine), U60105 and L26101; r (rat), U55995 and AF020712; m (mouse), L16912 and AF020711; c (canine), U41001 and U41002; ch (chicken), U73189; n (nematode, C. elegans), see Ref. 14; d (D. melanogaster), JH0697. C: blockade of Hslo, but not of Dslo, by 100 nM iberiotoxin (IbTX). Dissociation constant was ~1 nM (6).
Two voltage-gated K+ channels of the S4 superfamily: Kv and maxi-KCa channels. A, left: Kv channels have intracellular NH2 and COOH termini. Kv modulatory β-subunit(s) is cytosolic. Some of them carry sequences resembling the inactivation “ball” and are thought to interact with the pore region. Right: maxi-KCa channels α- and β-subunits. βKv,Ca, Transmembranal β-subunit. Ψ, glycosylation sites. S0–S6, α-subunit 7 transmembrane segments. S0 and part of the exoplasmic NH2 terminus determine functional coupling between α- and β-subunits. ?, Regions with uncertain topology (S8-S7, shaded box) and uncertain Ca2+ binding site. Hatched box, cytosolic “tail” region with hydrophobic S9–S10 regions. B: voltage-dependent gating of Shaker K (typical Kv) and maxi-KCa channels. Kv and maxi-KCa channels have an intrinsic voltage sensor whose movement generates gating currents. Increasing evidence suggests that depolarization induces the outward movement of S4 (arrows in A), leading to pore opening. Inset: maxi-KCa currents in the practical absence of Ca2+ (5 nM).
Maxi-KCa channel unique NH2 terminus. A: extracellular NH2 terminus. Diagram depicts a cell expressing maxi-KCa channels with a c-myc epitope tag at the NH2 terminus. Bead coated with secondary antibody (Ab) binds to the cell through a bridge formed by the primary antibody (anti-c-myc antibody). Picture at right illustrates intact cells expressing c-myc-tagged (at the NH2 terminus) maxi-KCa channel profusely labeled with beads, as in diagram. B: extracellular NH2 terminus and S0 are structural determinants of β-subunit modulation. Coexpression of Hslo (α, bold lines, filled segments) and β-subunit shifts the voltage activation curves of maxi-KCa currents by ~100 mV (V1/2, half-maximal voltage). Dslo (thin lines, open segments) currents are not affected by coexpression with β-subunit. Hslo NH2 terminus and S0 in the backbone of Dslo (HD8) restore β-subunit modulation. Experiments were done in isotonic 140 mM K+ and 10 μM Ca2+.
Physiological role of maxi-KCa channels. A: iberiotoxin (IbTX) that blocks maxi-KCa channels induces contractions of human uterine strips and increases the frequency of contractions of rat uterine strips. B: possible models of how maxi-KCa channels open in response to localized increases in Ca2+. Colocalization with voltage-dependent Ca2+ channels (VDCC) and/or close approximation with subsarcolemmal Ca2+ stores allows a local rise in Ca2+ that induces maxi-KCa channel opening and hyperpolarization. In smooth muscle, spontaneous Ca2+ release from ryanodine receptors (RR), “Ca2+ sparks,” open maxi-KCa channels, generating spontaneous transient outward currents (STOCs) (7). This causes hyperpolarization and relaxation, which can be reversed by maxi-KCa channels blockers (e.g., iberiotoxin). Inositol trisphosphate (IP3) receptors as an alternative Ca2+ store with similar function need to be explored. [Modified from K. Anwer et al. Am. J. Physiol. 265 (Cell Physiol. 34): C976–C985, 1993.]