LRET Determination of Molecular Distances during pH Gating of the Mammalian Inward Rectifier Kir1.1b

Figures

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  Figure 1. Possible tetrameric configurations during labeling of C189-Kir1.1b, single-Cys dimers. (A) Dimers labeled with terbium chelate donor, (B) dimers labeled with BODIPY-FL maleimide SE. Upon mixing, dimers (A and B) self-assemble into tetramers having one donor and one acceptor ((C) ∼50%), or tetramers having only donors or only acceptors ((D) ∼25% each). Donors and acceptors on adjacent subunits (E) are not structurally possible. To see this figure in color, go online.

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  Figure 2. Optical setup for LRET measurements from Kir1.1b proteoliposomes, resting on a fused silica coverslip in a controlled pH solution. The data acquisition photomultiplier is gated to open 50 μs after the laser pulse, to avoid saturation by the 330-nm excitation signal. To see this figure in color, go online.

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  Figure 3. Current-voltage data for an A161C-Kir1.1b oocyte at internal pH 6, controlled with a 100 mM K acetate buffer. Whole-cell oocyte currents were recorded with a two-electrode voltage clamp. Conductance (slope) at 0 mV clamp potential indicates a predominance of closed channels at pH 6. To see this figure in color, go online.

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  Figure 4. Current-voltage data for the same A161C-Kir1.1b oocyte as in Fig 3, but with internal pH 8, controlled with a 100 mM K acetate buffer. Conductance (slope) at 0 mV clamp potential indicates a predominance of open channels at pH 8. To see this figure in color, go online.

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  Figure 5. pH dependence of wild-type Kir1.1b and cysteine mutants. Whole-cell inward conductance measured with two-electrode voltage clamp in oocytes at the indicated membrane potentials. To see this figure in color, go online.

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  Figure 6. Normalized LRET lifetimes for C189-Kir1.1b dimer proteoliposomes in 158 mM KMOPS. (A) Closed state at pH 6. (B) Open state at pH 8. C189 dimers were labeled with Tb chelate donor and BODIPY-FL acceptor. Black tracings denote Tb donor-only (DO) signal, fitted with fast and slow Tb time constants (red lines). Green tracings are the corresponding sensitized acceptor (SE) lifetimes at (A) pH 6 (closed state) and (B) pH 8 (open state), together with exponential fits (red lines). Time constants are listed in Table 1, and additional calculations are in Supporting Material. To see this figure in color, go online.

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  Figure 7. Normalized LRET lifetimes for C289-Kir1.1b dimer proteoliposomes in 158 mM KMOPS. (A) Closed state at pH 6. (B) Open state at pH 8. C289 dimers were labeled with Tb chelate donor and BODIPY-FL acceptor. Black tracings denote Tb DO signal, fitted with fast and slow Tb time constants (red lines). Green tracings are the corresponding SE lifetimes at (A) pH 6 (closed state) and (B) pH 8 (open state), together with exponential fits (red lines). Time constants are listed in Table 2, and additional calculations are in Supporting Material. To see this figure in color, go online.

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  Figure 8. Normalized LRET lifetimes for A161C-Kir1.1b dimer proteoliposomes in 158 mM KMOPS. (A) Closed state at pH 6. (B) Open state at pH 8. A161C dimers were labeled with Tb chelate donor and Atto465 acceptor. Black tracings denote Tb DO signal, fitted with fast and slow Tb time constants (red lines). Green tracings are the corresponding sensitized acceptor (SE) lifetimes at (A) pH 6 (closed state) and (B) pH 8 (open state), together with exponential fits (red lines). Time constants are listed in Table 3, and additional calculations are in Supporting Material. To see this figure in color, go online.

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  Figure 9. Contribution of background proteins to the terbium donor signal. Comparison of Tb DO signals from Cys-free Kir1.1b and A161C-Kir1.1b proteoliposomes. Both samples were produced using identical CF translation and labeling protocols. Red: Tb luminescence from the Cys-free tetrameric sample, representing background (non-Kir) contamination. Blue: Tb luminescence from the A161C tetrameric channel. To see this figure in color, go online.

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  Figure 10. State-dependent Kir1.1b dimensions measured by LRET between paired Cys on opposite subunits. C189: 33 Å (closed) to 34 Å (open). C289: 27 Å (closed) to 27 Å (open). A161C: 14 Å (closed) to 23 Å (open). Solid bar denotes width of actual HBC gate: 6.3 Å (closed, pH 6) and 15.1 Å (open, pH 8), derived from A161C distances. Open and closed state structures were drawn in Pymol from our homology model (Methods).