Omplete disulfide crosslinking between particular cysteines in the flanks of S0 and S4 (e.g., R17C and R201C) had remarkably small effects on V50, kact , and kdeact [22]. Although not all crosslinks between the flanks of S0 and S4 had small effects, that some did seemed inconsistent with more than a modest relative displacement during activation of the extracellular ends of S0 and S4. Because of the possibility that there might be sufficient flexibility in the flanks to confound both our structural and functional inferences, we CAL 120 custom synthesis mutated to Cys in pairs the four residues in the first helical turns in the membrane of S0, S4 and TM2. Compared to Cys in the flanks, these Cys in the membrane would likely be in a more constrained helical structure, albeit less accessible to water and to reagents and hence less reactive. Because of the structural constraints, disulfide crosslinking between these helices should strongly perturb activation if it involves relative movements of their extracellular ends. Recently, such relative movement was inferred from voltage-dependent perturbation of the fluorescence of fluorophore-labeled BK a [26]. We now describe the functional consequences of crosslinks of substituted Cys in the first helical turns in the membrane of S0 and S4 and the effects of the functional state of the VSD on the rates of crosslinking. We also demonstrate that the efficiency of recrosslinking between cysteines in S0 and S4 on the cell surface is greater in the presence of the b1 subunit than in its absence, consistent with b1 SPDP Crosslinker price acting through S0 to stabilize its interaction with S4.unreduced and reduced samples were electrophoresed, transferred to nitrocellulose, and immunoblotted with anti-BK a-C-terminalepitope antibody (BD Biosciences) and horseradish-peroxidase (HRP)-conjugated secondary antibody. Chemiluminescence was recorded with a CCD camera (Carestream) and quantitated with ImageQuant software (Molecular Dynamics). The fraction of crosslinked a in the unreduced aliquot was corrected for the efficiency of protease cleavage, determined from the DTT-reduced aliquot [22,25].Crosslinking of a and b subunitsWe determined the extent of crosslinking between Cyssubstituted pWT1 a and pWT b1 as previously described [23?25]. We calculated the extent of crosslinking 
from the integrated luminescence from the a-b band at apparent mass ,160 kDa, divided by the sum of the integrated luminescence of the bands at ,130 kDa (a) and ,160 kDa (a b).Reduction of disulfides and reoxidation of thiolsTransfected HEK293 cells were surface-biotinylated as above. Disulfides were reduced with 10 mM dithiothreitol (DTT) in a solution containing 137 mM NaCl, 2.7 mM KCl, 0.1 mM CaCl2, 0.1 mM MgCl2, and 40 mM HEPES (pH 8.0). The cells were washed with DPBS. Cys thiols were oxidized with 40 mM 4,4′-(azodicarbonyl)-bis-[1,1-dimethylpiperazinium, diiodide] [QPD], a bis-quaternary ammonium, piperazinium diamide [22,27] in 137 mM NaCl, 2.7 mM KCl, 0.9 mM CaCl2, and 0.49 mM MgCl2, 10 mM MOPS (pH 7.2). Cells were washed and lysed as detailed above.Materials and Methods ConstructsMutants of mouse BK asubunit (mSlo1, KCNMA1, Genbank/ EMBL/DDBJ accession no. NM_010610) were generated in a pseudo-wild-type a pWT1 acontaining the two extracellular Cys, C14 and C141, mutated to Ala, an N-terminal FLAG epitope (MDYKDDDDKSPGDS), and the human rhinovirus (HRV)-3C protease-consensus-cleavage site, LEVLFQGP, inserted in the S0S1 loop. This was created by the mutation A89L and the i.Omplete disulfide crosslinking between particular cysteines 
in the flanks of S0 and S4 (e.g., R17C and R201C) had remarkably small effects on V50, kact , and kdeact [22]. Although not all crosslinks between the flanks of S0 and S4 had small effects, that some did seemed inconsistent with more than a modest relative displacement during activation of the extracellular ends of S0 and S4. Because of the possibility that there might be sufficient flexibility in the flanks to confound both our structural and functional inferences, we mutated to Cys in pairs the four residues in the first helical turns in the membrane of S0, S4 and TM2. Compared to Cys in the flanks, these Cys in the membrane would likely be in a more constrained helical structure, albeit less accessible to water and to reagents and hence less reactive. Because of the structural constraints, disulfide crosslinking between these helices should strongly perturb activation if it involves relative movements of their extracellular ends. Recently, such relative movement was inferred from voltage-dependent perturbation of the fluorescence of fluorophore-labeled BK a [26]. We now describe the functional consequences of crosslinks of substituted Cys in the first helical turns in the membrane of S0 and S4 and the effects of the functional state of the VSD on the rates of crosslinking. We also demonstrate that the efficiency of recrosslinking between cysteines in S0 and S4 on the cell surface is greater in the presence of the b1 subunit than in its absence, consistent with b1 acting through S0 to stabilize its interaction with S4.unreduced and reduced samples were electrophoresed, transferred to nitrocellulose, and immunoblotted with anti-BK a-C-terminalepitope antibody (BD Biosciences) and horseradish-peroxidase (HRP)-conjugated secondary antibody. Chemiluminescence was recorded with a CCD camera (Carestream) and quantitated with ImageQuant software (Molecular Dynamics). The fraction of crosslinked a in the unreduced aliquot was corrected for the efficiency of protease cleavage, determined from the DTT-reduced aliquot [22,25].Crosslinking of a and b subunitsWe determined the extent of crosslinking between Cyssubstituted pWT1 a and pWT b1 as previously described [23?25]. We calculated the extent of crosslinking from the integrated luminescence from the a-b band at apparent mass ,160 kDa, divided by the sum of the integrated luminescence of the bands at ,130 kDa (a) and ,160 kDa (a b).Reduction of disulfides and reoxidation of thiolsTransfected HEK293 cells were surface-biotinylated as above. Disulfides were reduced with 10 mM dithiothreitol (DTT) in a solution containing 137 mM NaCl, 2.7 mM KCl, 0.1 mM CaCl2, 0.1 mM MgCl2, and 40 mM HEPES (pH 8.0). The cells were washed with DPBS. Cys thiols were oxidized with 40 mM 4,4′-(azodicarbonyl)-bis-[1,1-dimethylpiperazinium, diiodide] [QPD], a bis-quaternary ammonium, piperazinium diamide [22,27] in 137 mM NaCl, 2.7 mM KCl, 0.9 mM CaCl2, and 0.49 mM MgCl2, 10 mM MOPS (pH 7.2). Cells were washed and lysed as detailed above.Materials and Methods ConstructsMutants of mouse BK asubunit (mSlo1, KCNMA1, Genbank/ EMBL/DDBJ accession no. NM_010610) were generated in a pseudo-wild-type a pWT1 acontaining the two extracellular Cys, C14 and C141, mutated to Ala, an N-terminal FLAG epitope (MDYKDDDDKSPGDS), and the human rhinovirus (HRV)-3C protease-consensus-cleavage site, LEVLFQGP, inserted in the S0S1 loop. This was created by the mutation A89L and the i.
