CHARACTERIZATION OF A CHLOROPLAST INNER ENVELOPE K+ CHANNEL

A K+-conducting protein of the chloroplast inner envelope was characte rized as a K+ channel. Studies of this transport protein in the native membrane documented its sensitivity to K+ channel blockers. Further s tudies of native membranes demonstrated a sensitivity of K+ conductanc e to divalent cations such as Mg2+, which modulate ion conduction thro ugh interaction with negative surface charges on the inner-envelope me mbrane. Purified chloroplast inner-envelope vesicles were fused into a n artificial planar lipid bilayer to facilitate recording of single-ch annel K+ currents. These single-channel K+ currents had a slope conduc tance of 160 picosiemens. Antibodies generated against the conserved a mino acid sequence that serves as a selectivity filter in the pore of K+ channels immunoreacted with a 62-kD polypeptide derived from the ch loroplast inner envelope. This polypeptide was fractionated using dens ity gradient centrifugation. Comigration of this immunoreactive polype ptide and K+ channel activity in sucrose density gradients further sug gested that this polypeptide is the protein facilitating K+ conductanc e across the chloroplast inner envelope.