R. Hedrich et al., INWARD RECTIFIER POTASSIUM CHANNELS IN PLANTS DIFFER FROM THEIR ANIMAL COUNTERPARTS IN RESPONSE TO VOLTAGE AND CHANNEL MODULATORS, European biophysics journal, 24(2), 1995, pp. 107-115
We have investigated the electrophysiological basis of potassium inwar
d rectification of the KAT1 gene product from Arabidopsis thaliana exp
ressed in Xenopus oocytes and of functionally related Kf channels in t
he plasmamembrane of guard and root cells from Vicia faba and Zen mays
. The whole-cell currents passed by these channels activate, following
steps to membrane potentials more negative than -100 mV, with half ac
tivation times of tens of milliseconds. This voltage dependence was un
affected by the removal of cytoplasmic magnesium. Consequently, unlike
inward rectifier channels of animals, inward rectification of plant p
otassium channels is an intrinsic property of the channel protein itse
lf. We also found that the activation kinetics of KAT1 were modulated
by external pH. Decreasing the pH in the range 8.5 to 4.5 hastened act
ivation and shifted the steady state activation curve by 19 mV per pH
unit. This indicates that the activity of these K+ channels and the ac
tivity of the plasma membrane H+-ATPase may not only be coordinated by
membrane potential but also by pH. The instantaneous current-voltage
relationship, on the other hand, did not depend on pH, indicating that
H+ do not block the channel. In addition to sensitivity towards proto
ns, the channels showed a high affinity voltage dependent block in the
presence of cesium, but were less sensitive to barium. Recordings fro
m membrane patches of KAT1 injected oocytes in symmetric, Mg2+-free, 1
00 mM-K+, solutions allowed measurements of the current-voltage relati
on of single open KAT1 channels with a unitary conductance of 5 pS. We
conclude that the inward rectification of the currents mediated by th
e KAT1 gene product, or the related endogenous channels of plant cells
, results from voltage-modulated structural changes within the channel
proteins. The voltage-sensing or the gating-structures appear to inte
ract with a titratable acidic residue exposed to the extracellular med
ium.