ISOLATION AND CHARACTERIZATION OF AN AMINO ACID-SELECTIVE CHANNEL PROTEIN PRESENT IN THE CHLOROPLASTIC OUTER ENVELOPE MEMBRANE

The reconstituted pea chloroplastic outer envelope protein of 16 kDa ( OEP16) forms a slightly cation-selective, high-conductance channel wit h a conductance of ii = 1,2 nS (in 1 M KCl), The open probability of O EP16 channel is highest at 0 mV (P-open = 0.8), decreasing exponential ly with higher potentials. Transport studies using reconstituted recom binant OEP16 protein show that the OEP16 channel is selective for amin o acids but excludes triosephosphates or uncharged sugars, Crosslinkin g indicates that OEP16 forms a homodimer in the membrane, According to its primary sequence and predicted secondary structure, OEP16 shows n either sequence nor structural homologies to classical porins. The res ults indicate that the intermembrane space between the two envelope me mbranes might not be as freely accessible as previously thought.