RED-LIGHT REGULATES CALCIUM-ACTIVATED POTASSIUM CHANNELS IN MOUGEOTIA PLASMA-MEMBRANE

The alga Mougeotia has a large central chloroplast whose positioning is regulated by photoactivation of phytochrome. possibly via modulation of cytosolic calcium (Serlin B, Roux SJ [1984] Proc Natl Acad Sci USA 81: 6368-6372). We used the patch clamp technique to examine the effects of red and far-red light on ion channel activity in the plasma membrane of Mougeotia protoplasts to determine if ion channels play a role in chloroplast movement. Patch clamping in the cell-attached mode reveals two channels of about 2 and 4 picoamperes amplitude at O millivolt (inside pipette) and estimated conductances of 30 and 65 picosiemens. They are activated by red light irradiation after a lag period of about 2 to 5 minutes. Far-red light, when applied immediately after red light irradiation, reverses this activation; otherwise it has no effect. This result implicates phytochrome. The addition of the calcium ionophore, A23187, also activates ion channel activity after a lag of a few minutes. The channels are not specific for calcium since they are present when calcium is removed from the external and pipette media. They are inhibited by quaternary ammonium ions. Thus, we believe they are calciumactivated potassium channels. Their possible role in chloroplast positioning is discussed.