Protein import into chloroplasts involves redox-regulated proteins

Pre-protein translocation into chloroplasts is accomplished by two distinct translocation machineries in the outer and inner envelope, respectively. We have isolated the translocon at the inner envelope membrane (Tic complex) by blue-native PAGE and describe a new Tic subunit, Tic62. Tic62, together with Tic110 and Tic55, forms a core translocation unit. The N-terminus of Tic62 shows strong homologies to NAD(H) dehydrogenases in eukaryotes and to Ycf39-like proteins present in cyanobacteria and non-green algae. The stromal-facing C-terminus of Tic62 contains a novel, repetitive module that interacts with a ferredoxin-NAD(P)(+) oxidoreductase. Ferredoxin-NAD(P)(+) oxidoreductase catalyses the final electron transfer of oxygenic photosynthesis from ferredoxin to NAD(P). Substrates that interfere with either NAD binding, such as deamino-NAD, or influence the ratio of NAD(P)/NAD(P)H, such as ruthenium hexamine trichloride, modulate the import characteristics of leaf-specific ferredoxin-NAD(P)(+) oxidoreductase isologues differently. We conclude that the Tic complex can regulate protein import into chloroplasts by sensing and reacting to the redox state of the organelle.