Fast-activating channel controls cation fluxes across the native chloroplast envelope.
A prerequisite for photosynthetic CO(2) fixation is the maintenance of alkaline pH in the stroma. This is achieved by H(+) pumping from the stroma to the cytosol, electrically balanced by an influx of cations through some unidentified non-selective envelope channels. In this study, the patch-clamp technique was applied to isolated Pisum sativum L. (pea) chloroplasts, and a fast-activating chloroplast cation (FACC) channel was discovered in the native envelope. This channel opens within a few milliseconds upon voltage steps to large positive or negative potentials. Remarkably, the single-channel conductance increased fivefold, from approximately 40 pS to approximately 200 pS (symmetric 250 mM KCl), upon a potential change from zero to +/- 200 mV. The FACC channel conducts all physiologically essential inorganic cations (K(+), Na(+), Ca(2+), Mg(2+)) with little preference. An increase of stromal pH from 7.3 to 8.0, mimicking dark-light transition, caused about a 2-fold decrease of the FACC channel activity within a physiologically relevant potential range. The FACC channel was completely and irreversibly blocked by Gd(3+). Based on the estimated transport capacity of the whole chloroplast population of FACC channels together with the envelope H(+)-ATPases, these channels can mediate electroneutral K(+)/H(+) exchange across the envelope, enabling stroma alkalinization, thereby allowing an optimal photosynthetic performance.
Pottosin
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II
Biomedical Center, University of Colima, 28045, Colima, Mexico.
Muñiz
J
J
Shabala
S
S
D017920:Antiporters / Q000502:physiology*; D001693:Biological Transport, Active / Q000502:physiology; D002412:Cations; D002478:Cells, Cultured; D002736:Chloroplasts / Q000502:physiology*; D007425:Intracellular Membranes / Q000502:physiology*; D015640:Ion Channel Gating / Q000502:physiology*; D007473:Ion Channels / Q000502:physiology*; D018532:Pisum sativum / Q000502:physiology*; D010940:Plant Proteins / Q000378:metabolism*
D017920:Antiporters; D002412:Cations; D007473:Ion Channels; D010940:Plant Proteins