1.B.96. The Chloroplast Outer Envelope Porin 40 (OEP40) Family
OEP40 (Chloroplast 40 kDa outer membrane envelope protein) is a regulated glucose-permeable β-barrel solute channel in the chloroplast outer envelope membrane (Harsman et al. 2016). It is a a highly specific, regulated solute channel in the outer envelope of chloroplasts. Loss of OEP40 function in Arabidopsis thaliana results in early flowering under cold temperature. The reconstituted recombinant OEP40 protein forms a high conductance β-barrel ion channel with subconductant states in planar lipid bilayers. The OEP40 channel is slightly cation-selective PK+/PCl- ≈ 4:1 and rectifying (i⃗/i⃖ ≅ 2) with a slope conductance of Ḡmax ≅ 690 picosiemens. The OEP40 channel has a restriction zone diameter of ≅1.4 nm and is permeable for glucose, glucose 1-phosphate and glucose 6-phosphate, but not for maltose. Moreover, channel properties are regulated by trehalose 6-phosphate, which cannot permeate. Thus, OEP40 is a "glucose-gate" in the outer envelope membrane of chloroplasts, facilitating selective metabolite exchange between chloroplasts and the surrounding cytoplasm of the cell (Harsman et al. 2016).These porins and many others in eukaryotes have been reviewed and analyzed by Roumia et al. 2020.