THE VOLTAGE-DEPENDENT POTASSIUM-UPTAKE CHANNEL OF CORN COLEOPTILES HAS PERMEATION PROPERTIES DIFFERENT FROM OTHER K+ CHANNELS

The initial response of coleoptile cells to growth hormones and light is a rapid change in plasmamembrane polarization. We have isolated pro toplasts from the cortex of maize (Zea mays L.) coleoptiles to study t he electrical properties of their plasma membrane by the patch-clamp t echnique. Using the whole-cell configuration and cell-free membrane pa tches we could identify an H+-ATPase, hyperpolarizing the membrane pot ential often more negative than -150 mV, and a voltage-dependent, inwa rd-rectifying K+ channel (unit conductance approximate to 5-7 pS) as t he major membrane conductances. Potassium currents through this channe l named CKC1(in) (for Coleoptile K+ Channel inward rectifier) were eli cited upon voltage steps negative to -80 mV, characterized by a half-a ctivation potential of -112 mV. The kinetics of activation, well descr ibed by a double-exponential process, were strongly dependent on the d egree of hyperpolarization and the cytoplasmic Ca2+ level. Whereas at nanomolar Ca2+ concentrations K+ currents increased with a t(1/2)=16 m s (at -180 mV), higher calcium levels slowed the activation process ab out four- to fivefold. Upon changes in the extracellular K+ concentrat ion the reversal potential of the K+ channel followed the Nernst poten tial for potassium with a 56-mV shift for a tenfold increase. The abse nce of a measurable conductance for Na+, Rb+, Cs+ and a permeability r atio P-NH4+/P-K+ around 0.25 underlines the high selectivity of CKClin for K+. In contrast to Cs+, which at submillimolar concentration bloc ks the channel in a voltage-dependent manner, Rb+, often used as a tra cer for K+, does not permeate this type of K+ channel. The lack of Rb permeability is unique with respect to other K+ transporters. Therefo re, future molecular analysis of CKC1(in) considered as a unique varia tion of plant inward rectifiers, might help to understand the permeati on properties of K+ channels in general.