25003:

Blue light activates the depolarization-dependent K+ channels in flexor cells from Samanea motor organs via two mechanisms.

Authors:	Suh, SuJeong(A)Moran, Nava(B)Lee, Youngsook(A)
Affiliations:	(A): Dept. Life Science, Pohang University of Science and Technology (B): Dept. Agricultural Botany, The Hebrew University of Jerusalem
Presenter:	Moran, Nava , morana@agri.huji.ac.il

Regulation of membrane transporters in motor cells is central in the mechanism of leaflet movement of Samanea. This nyctinastic legume opens its leaves and leaflets during daytime or in blue light (BL), and folds them together during night or in darkness. Leaflet movement results from oppositely-directed volume changes in 2 types of their subtending motor cells, flexors and extensors. When shrinking (as do the flexor cells under BL), these cells lose ions (mainly K+ and Cl-) and water. Because leaflet movement is easy to monitor, it has been used as a system to study the circadian rhythm and photoresponses. And since K+ concentration changes are the most dramatic, we use these motor cells to study the regulation of K channels - the pathway for the K+ fluxes in and out of the cells. We applied patch-clamp to investigate the mechanism of the K+ channels regulation by light. Isolated flexor cell protoplasts were patch-clamped in a cell-attached configuration, the least invasive recording approach, in a voltage-clamp mode. Depolarization elicited outward-directed currents through single open K+ channels (KD channels). We estimated the changes in cell membrane potential by applying voltage ramps and tracking the change of the apparent reversal potential of KD channel current. BL irradiation depolarized membrane potential (EM) by about 10 mV. Subsequent Red-Light (RL) pulse followed by Darkness shifted EM in an opposite, hyperpolarizing direction. The BL-induced shifts of EM were not observed in cells pre-treated with an H+-pump inhibitor, suggesting a contribution by H+-pump to the shift. The dependence of KD channels activation on depolarization is obligatory, more specifically, they are activated by the extent of depolarization relative to their prevailing reversal potential. To eliminate the effect of this voltage dependence, we compared the mean conductance of KD channels in the patch at a fixed distance (a depolarization of 40 mV) from their apparent reversal potential. BL increased this mean conductance, G@40, by about 12 pS, without a change of the single channel conductance. Subsequent RL-Darkness decreased G@40. This signifies a voltage-independent effect of BL light on the activity of the channels. Thus, the activity of the KD channels, and consequently, the K+ efflux, are regulated in a dual manner by membrane potential and by independent signaling pathway(s) originating in the alternate perception of BL and RL/Darkness by separate photoreceptors.