The anion channel protein from
Clavibacter michiganense ssp. nebraskense (Schürholz, Th. et al. 1991,
J. Membrane Biol. 123: 1-8) was analyzed at different concentrations of KCl and KF. At 0.8 M KCl the conductance
G(
Vm) increases exponentially from 21 pS at 50 mV up to 53 pS at
Vm = 200 mV, 20°C. The concentration dependence of
G(
Vm) corresponds to a Michaelis-Menten type saturation function at all membrane voltage values applied (0-200 mV). The anion concentration
K0.5, where
G(
Vm) has its half-maximum value, increases from 0.12 M at 50 mV to 0.24 M at 175 mV for channels in a soybean phospholipid bilayer. The voltage dependence of the single channel conductance, which is different for charged and neutral lipid bilayers, can be described either by a two-state flicker (2SF) model and the Nernst-Planck continuum theory, or by a two barrier, one-site (2B1S) model with asymmetric barriers. The increase in the number of open channels after a voltage jump from 50 mV to 150 mV has a time constant of 0.8 s. The changes of the single-channel conductance are much faster (<1 ms). The electric part of the gating process is characterized by the (reversible) molar electrical work Δ
Gθel = ρZ
gFV
m ≈ -1.3 RT, which corresponds to the movement of one charge of the gating charge number |
Zg| = 1 across the fraction ρ = Δ
Vm/
Vm = 0.15 of the membrane voltage V
m = 200 mV. Unlike with chloride, the single channel conductance of fluoride has a maximum at about 150 mV in the presence of the buffer PIPES (≥5 mM, pH 6.8) with
K0.5 ≈ 1 M. It is shown that the decrease in conductance is due to a blocking of the channel by the PIPES anion. In summary, the results indicate that the anion transport by the
Clavibacter anion channel (CAC) does not require a voltage dependent conformation change of the CAC.
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