Molecular Properties of Oep21, an ATP-regulated Anion-selective Solute Channel from the Outer Chloroplast Membrane*
The flux of phosphorylated carbohydrates, the major export products of chloroplasts, is regulated at the level of the inner and presumably also at the level of the outer membrane. This is achieved through modulation of the outer membrane Oep21 channel currents and tuning of its ion selectivity. Refined analysis of the Oep21 channel properties by biochemical and electrophysiological methods revealed a channel formed by eight β-strands with a wider pore vestibule of dvest ∼2.4 nm at the intermembrane site and a narrower filter pore of drestr ∼1 nm. The Oep21 pore contains two high affinity sites for ATP, one located at a relative transmembrane electrical distance δ = 0.56 and the second close to the vestibule at the intermembrane site. The ATP-dependent current block and reduction in anion selectivity of the Oep21 channel is relieved by the competitive binding of phosphorylated metabolic intermediates like 3-phosphoglycerate and glycerinaldehyde 3-phosphate. Deletion of a C-terminal putative FX4K binding motif in Oep21 decreased the capability of the channel to tune its ion selectivity by about 50%, whereas current block remained unchanged.