Calcium Activation of Mougeotia Potassium Channels

Abstract

Phytochrome mediates chloroplast movement in the alga Mougeotia, possibly via changes in cytosolic calcium. It is known to regulate a calcium-activated potassium channel in the algal plasma membrane. As part of a characterization of the potassium channel, we examined the properties of calcium activation. The alga Mougeotia is a classic system for the study of phytochrome since photoactivation of phytochrome regulates the positioning of its single large chloroplast, which is easily monitored (5). It is known that calcium plays a role in the movement of the chloroplast (1-3, 13). Phytochrome may cause cytosolic calcium to increase either by calcium release from internal calcium stores (2) or entry of external calcium into the cell (1), and this in turn results in chloroplast repositioning (13).We recently discovered a phytochrome-activated potassium channel in Mougeotia which is also activated by the calcium ionophore A23 187 (8). It is not known what function, if any, the potassium channel has in chloroplast positioning, though we hypothesize that it allows entry of extracellular calcium into the cell. In this paper, we study calcium-activation of the channel, and present evidence that it is activated by internal calcium stores.

Phytochrome Activation of K+ Channels and Chloroplast Rotation in Mougeotia

“…Even with low resistance seals (less than 500 MU), there was little short-term noise, so channels with amplitudes greater than 0.25 pA were readily identified. In the experiments shown here, the channels open in the negative direction, whereas in previous work (8,9) …”

“…Research has concentrated on the effects of light upon the positioning of the single large chloroplast (5). The discovery that red light activates K+ channels in the plasma membrane, activation that is reversed by far-red light (8), and that both chloroplast positioning ( 1 2) and K+ channel activity (9) are also activated by Ca2 , suggests that both may share part or all of the same transduction pathway. Phytochrome activation could lead to increases in cytosolic Ca2' that in turn could directly activate the K+ channel as well as induce chloroplast rotation.…”

Red Light Regulates Calcium-Activated Potassium Channels in Mougeotia Plasma Membrane

“…When the solvent for the ionophore, dimethyl sulfoxide, was added alone, channel activity decreased (data not shown). In the absence of extracellular Ca2", A23187 causes no channel activation (8 unaffected by the addition of A23187. In the experiment shown, the increase in number of channel events was paralleled by a slight increase in channel open time from a halftime of 13 msec, to a half-time of 23 msec.…”

“…We are presently exploring this possibility by determining if the escape time (the amount of time after red light irradiation during which far-red light can reverse its effect) for channel activation is the same as the escape time for chloroplast movement (BS Serlin, RR Lew, CL Schauf, ME Stockton, unpublished data). We have also characterized the calcium-activation in detail (8).…”

Substrate Regulation of Single Potassium and Chloride Ion Channels in Arabidopsis Plasma Membrane

“…Prior to the experiment, a 500 giL aliquot of cells was added to about 2. (8,11,15). Typical gains were 10 mV/pA for monitoring seal formation and 100 mV/pA for channel measurements.…”