External pH effects on the depolarization-activated K channels in guard cell protoplasts of Vicia faba

Nitza Ilan, Amnon Schwartz, Nava Moran*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

39 Scopus citations

Abstract

Previous studies reveal that the pH of the apoplastic solution in the guard cell walls may vary between 7.2 and 5.1 in closed and open stomata, respectively. During these aperture and pH changes, massive K+ fluxes cross the cellular plasma membrane driving the osmotic turgor and volume changes of guard cells. Therefore, we examined the effect of extracellular pH on the depolarization-activated K channels (K(D) channels), which constitute the K+ efflux pathway, in the plasma membrane of Vicia faba guard cell protoplasts. We used patch clamp, both in whole cells as well as in excised outside-out membrane patches. Approximately 500 K(D) channels, at least, could be activated by depolarization in one protoplast (density: ~0.6 μm-2). Acidification from pH 8.1 to 4.4 decreased markedly the whole-cell conductance, G(K), of the K(D) channels, shifted its voltage dependence, G(K)-E(M), to the right on the voltage axis, slowed the rate of activation and increased the rate of deactivation, whereas the single channel conductance was not affected significantly. Based on the G(K)-E(M) shifts, the estimated average negative surface charge spacing near the K(D) channel is 39 Å. To quantify the effects of protons on the rates of transitions between the hypothesized conformational states of the channels, we fitted the experimental macroscopic steady state conductance-voltage relationship and the voltage dependence of time constants of activation and deactivation, simultaneously, with a sequential three-state model CCO. In terms of this model, protonation affects the voltage-dependent properties via a decrease in localized, rather than homogeneous, surface charge sensed by the gating moieties. In terms of either the CO or CCO model, the protonation of a site with a pK(a) of 4.8 decreases the voltage-independent number of channels, N, that are available for activation by depolarization.

Original languageEnglish
Pages (from-to)807-831
Number of pages25
JournalJournal of General Physiology
Volume103
Issue number5
DOIs
StatePublished - May 1994

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