|
Figure 1. Schematic of the structure of spHCN channel subunits. (A) Schematic of the putative topology of an spHCN subunit. Locations of the charged S4 voltage sensor, the S4–S5 linker, the post-S6/C-linker, the CNBD, and the binding site for cyclic nucleotide monophosphate (cNMP) are shown. (B) Sequence alignments of the S4, S4–S5 linker, pore-lining S6, and post-S6/C-linker regions of spHCN1 (GenBank accession no. CAA76493), with human Kv1.2 (NCBI Protein database accession no. NP_004965), rat Kv1.2 (NCBI Protein database accession no. NP_037102), and KcsA (UniProtKB/Swiss-Prot sequence P0A334) channel subunits. Alignments were made using ClustalW2. Mutations of spHCN1 discussed in Results and Discussion are shown as bold and underlined.
|
|
Figure 2. Metal bridges can form between the S4–S5 linker and C-linker in the open state. Representative current traces of selected single C-linker residue mutants and double mutants with F359C. Currents were elicited by 500-ms voltage steps to −120 mV from a holding potential of 0 mV, followed by a voltage step to +50 mV. Currents were recorded in the absence (black) or presence (red) of 1 µM Cd2+ from excised inside-out patches expressing the various mutants as indicated. Current traces recorded from the WT partial cysteine-less background, which contains C211Y, C224I, C254V, C266M, C369F, and C373G in addition to the M349I and H462Y mutations, are shown. Current traces recorded from the single F359C mutant are also shown, as well as current traces recorded from single S472C, S473C, S474C, Q476C, R478C, E479C, K482C, and E485C mutants. Current traces from double mutants containing F359C and each of the C-linker mutations are also shown.
|
|
Figure 3. Metal bridges can form between the S4–S5 linker and C-linker in both open and closed states. Representative current traces of selected C-linker mutants with V361C and A364C. Currents were elicited by voltage steps to −120 mV from a holding potential of 0 mV, followed by a voltage step to +50 mV. Currents were recorded in the absence (black) or presence (red) of 1 µM Cd2+ from excised inside-out patches expressing the various mutants as indicated. For the V361CQ476C and A364CS473C mutants, currents recorded in response to a 1-s hyperpolarizing pulse from 0 to −120 mV are also shown to highlight the slowing of activation by Cd2+. For the A364CS474C, A364CQ476C, and A364CR478C mutants, currents recorded in response to a 2-s hyperpolarizing pulse from 0 to −120 mV are also shown to highlight the slowing of activation by Cd2+.
|
|
Figure 4. Summary of the effects of Cd2+ on spHCN channels containing introduced cysteine residues. (A) The lock-open effects of 1 µM Cd2+, as indicated by the proportion of the nondecaying tail current, in the single C-linker controls (left-most panel) and the double mutants containing one of the F359C, V361C, or A364C mutations are shown. Gray bars indicate values for control conditions, and green bars indicate values after the addition of Cd2+. The mutants giving rise to significant lock-open effects are clustered in two separate regions. (B) The lock-closed effects of 1 µM Cd2+, as indicated by the prolongation of the rise time between 10 and 50% of the maximum current in a 500-ms pulse, in the single C-linker controls (left-most panel) and the double mutants containing F359C, V361C, or A364C mutations are shown. Gray bars indicate values for control conditions, and red bars indicate values after the addition of Cd2+. (C) A summary of the effects of 1 µM Cd2+ on the double mutants tested in this study.
|
|
Figure 5. Use of concatenated subunits shows that in the open state metal bridges can form between A364C and S472C, possibly within the same subunit. (A) Representative current traces were recorded in the absence (black) or presence (red) of 1 µM Cd2+ from inside-out patches expressing one of the tandem dimers (364C472C_364C472C, WT_364C472C, or 364C_472C). Currents were elicited by 500-ms voltage steps to −120 mV from a holding potential of 0 mV, followed by a voltage step to +50 mV. (B) Effects of Cd2+ on the average proportions of nondecaying tail current produced by the three tandem–dimeric constructs.
|
|
Figure 6. Intersubunit metal bridges can form between A364C and E482C in the open state. (A) Representative current traces were recorded in the absence (black) or presence (red) of 1 µM Cd2+ from excised inside-out patches expressing one of the tandem dimers (364C482C_364C482C, WT_364C482C, or 364C_482C). Currents were elicited by 500-ms voltage steps to −120 mV from a holding potential of 0 mV, followed by a voltage step to +50 mV. (B) Effects of Cd2+ on the average proportions of nondecaying tail currents produced by the three tandem dimers.
|
|
Figure 7. Intersubunit metal bridges can form between A364C and Q476C in the closed state. (A) Representative current traces were recorded in the absence (black) or presence (red) of 1 µM Cd2+ from excised inside-out patches expressing one of the tandem dimers (364C476C_364C476C, WT_364C476C, or 364C_476C). Currents were elicited by voltage steps to −120 mV from a holding potential of 0 mV, followed by a voltage step to +50 mV. Currents from a 2-s (364C476C_364C476C) or 1-s (WT_364C476C and 364C_476C) hyperpolarizing pulse in the presence of Cd2+ are superimposed on the control current traces to highlight the slowing of the activation kinetics by Cd2+. (B) Effects of Cd2+ on the average rise time of currents produced by the intersubunit (364C_476C) and intrasubunit (WT_364C476C) tandem dimers. (C) The time course of activation was fit to a double-exponential function, and the time constants for the two components, along with the relative proportion of the slow component, are shown. The slowing of activation kinetics was mainly caused by an increase in the proportion of the slow time constant (tau).
|
|
Figure 8. Model of the structure and gating motions of an HCN channel based on high affinity metal bridges. (A) Stereo view of a model of the cAMP-bound closed state of the spHCN channel, based on the crystal structures of KcsA (Protein Data Bank accession no. 1K4C) and the cAMP-bound CNBD of spHCN (Protein Data Bank accession no. 2PTM). Residue D471 of spHCN is in approximately the same position as KcsA-118, based on the equivalence of spHCN-468 to KcsA-115 (Rothberg et al., 2003). The uncoiled strands between the S6 helices and the A′ helices include the spHCN residues 472SSS474. (B) Stereo view of the locations of C-linker residues that when mutated to cysteine form metal bridges with 364C to produce lock-open (green spheres) and lock-closed (red spheres) effects. Spheres of ∼6-Å diameter are centered around the β carbon of each C-linker residue involved in bridges with 364C. (C) Model of possible motions occurring during HCN channel gating. A view from the extracellular side of the channel, showing the locations of lock-open (green) and lock-closed (red) effects with 364C, superimposed on the closed-state model. Cylinders represent each S4–S5 linker, and asterisks indicate the positions of 364C residues. The proposed motions of 364C residues (dashed arrows) and the lower ends of the S6 helices (solid arrows) during channel opening are indicated. The color of the numbers for the C-linker residues studied matches the ribbon color for the corresponding subunit.
|
