title: Pore- and state-dependent cadmium block of I(Ks) channels formed with MinK-55C and wild-type KCNQ1 subunits.
abstract: Human MinK and KCNQ1 subunits assemble to form I(Ks) channels. When MinK position 55 is mutated to cysteine (MinK-55C), I(Ks) channels can be blocked by external cadmium (Cd(2+)). We have supported a pore-associated location for MinK-55C because Cd(2+) block is sensitive to voltage, permeant ions on the opposite side of the membrane (trans-ions), and external tetraethylammonium (TEA), an I(Ks) pore-blocker. Two recent reports argue that MinK-55C is distant from the pore: one finds TEA does not affect Cd(2+) block if channels are formed with a KCNQ1 mutant (K318I, V319Y) that increases TEA affinity; the second proposes that Cd(2+) binds between MinK-55C and a cysteine in KCNQ1 that is posited to lie toward the channel periphery. Here, these discrepancies are considered. First, Cd(2+) block of MinK-55C channels formed with wild-type KCNQ1 is shown to depend not only on voltage and trans-ions but state (showing decreased on-rate with increased open time and blocker trapping on channel closure). Conversely, MinK-55C channels with K318I, V319Y KCNQ1 are found to demonstrate Cd(2+) block that is independent of voltage, trans-ions and state (and to have a lower unitary conductance): thus, the KCNQ1 mutations alter the process under study, yielding Cd(2+) inhibition that is pore-independent and, perforce, TEA-insensitive. Second, MinK-55C channels are found to remain sensitive to Cd(2+) despite mutation of any single native cysteine in KCNQ1 or all nine simultaneously; this suggests no KCNQ1 cysteine binds Cd(2+) and can serve to localize MinK-55C. Despite many concerns that are enumerated, we remain obliged to conclude that Cd(2+) enters and leaves the pore to reach MinK-55C, placing that residue in or near the pore.
authors.affiliation: Department of Pediatrics, Boyer Center for Molecular Medicine, Yale University School of Medicine, New Haven, Connecticut, USA.
authors.forename: Haijun
authors.initials: H
authors.lastname: Chen
authors.forename: Federico
authors.initials: F
authors.lastname: Sesti
authors.forename: Steve A N
authors.initials: SA
authors.lastname: Goldstein
mesh_terms: D000818:Animals; D001665:Binding Sites; D002104:Cadmium / Q000737:chemistry* / Q000494:pharmacology*; D002478:Cells, Cultured; D004558:Electric Stimulation; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D051656:KCNQ Potassium Channels; D051657:KCNQ1 Potassium Channel; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009154:Mutation; D009865:Oocytes / Q000737:chemistry / Q000187:drug effects / Q000502:physiology; D016062:Porosity / Q000187:drug effects; D015221:Potassium Channels / Q000737:chemistry* / Q000187:drug effects / Q000502:physiology*; D024642:Potassium Channels, Voltage-Gated; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D013329:Structure-Activity Relationship; D014982:Xenopus laevis / Q000235:genetics
chemical_list: D051656:KCNQ Potassium Channels; D051657:KCNQ1 Potassium Channel; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D011994:Recombinant Proteins; C059983:potassium channel protein I(sk); D002104:Cadmium