Voltage-dependence of virus-encoded miniature K+ channel Kcv

Kcv is a K+-selective channel encoded by the Paramecium bursaria Chlorella virus 1 (PBVC-1). Expression of this protein, so far the smallest known functional K+ channel, in Xenopus oocytes reveals an instantaneous and a time-dependent component during voltage-clamp steps. These two components have an identical sensitivity to the inhibitor amantadine, implying that they reflect distinct kinetic features of the same channel. About 70% of the channels are always open; at hyperpolarizing voltages the time-dependent channels (30%) open in a voltage-dependent manner reaching half-maximal activation at about -70 mV. At both extreme positive and negative voltages the open-channel conductance decreases in a voltage-dependent manner. To examine the mechanism underlying the voltage-dependence of Kcv we neutralized the two charged amino acids in the lipophilic N-terminus. However, this double mutation had no effect on the voltage-dependence of the channel, ruling against the possibility that these charged amino acids represent a membrane-embedded voltage sensor. We have considered whether a block by external divalent cations is involved in the voltage-dependence of the channel, The Kcv current was increased about 4-fold on reduction of external Ca2+ concentration by a factor of ten. This pronounced increase in current was observed on lowering Ca2+ but not Mg2+ and was voltage-independent. These data indicate a Ca2+-selective, but voltage-independent mechanism for regulation of channel conductance.