Proteolytic activation of a hyperpolarization- and calcium-dependent potassium channel in Paramecium

The effects of proteolysis on a hyperpolarization- and Ca2+-dependent K channel from the surface membrane of Paramecium tetraurelia were examined in the inside-out excised patch mode. Treatment with trypsin, pronase or thermolysin removed the Ca2+-dependence of the channel activation, yielding an increase in channel activity greater than 2.5-fold at all Ca2+ concentrations between 10(-4) and 10(-8) M. Thermolysin additionally removed the voltage dependence of channel opening and gave the most activation among the three proteases tested. Proteolysis did not affect the single-channel conductance. In an analogy to the mechanism of activation of many Ca2+-dependent enzymes it is suggested that this Paramecium channel has a cytoplasmic inhibitory domain which can be removed by proteolysis, and that the physiological activation by Ca2+ is due to a temporary removal of this inhibition. Moreover, these findings indicate structural differences between depolarization-, Ca2+-dependent K channels (BK channels) and the hyperpolarization-, Ca2+-dependent K channels in Paramecium.