Tic62 (gold grains) may be a redox sensor at the inner envelope of chloroplasts.


Chloroplasts communicate their redox needs by altering protein import, according to Michael Küchler, Jürgen Soll, and colleagues (Munich University, Munich, Germany). They found that an electron transport component and an import protein bind to each other and may communicate to control the rate of protein import.

The group's original aim was to find other components in the complex that translocates proteins across the chloroplast inner membrane. Tic62 fit the bill, as it copurified, colocalized, and coimmunoprecipitated with known translocase components. It also bound to two interesting molecules: NAD, and a ferredoxin-NAD(P)+ oxidoreductase (FNR). FNR acts at the end of the photosynthetic electron transport chain to transfer electrons from ferredoxin to NAD(P), thus creating NAD(P)H.

Increasing the level of NAD with either an analogue or an alternative electron sink resulted in increased translocation of FNR-L1, one isoform of FNR. Thus, if the last step of electron transport is insufficient to meet the demands of the chloroplast, it is capable of replenishing itself. Soll is now testing to see whether the Tic62-bound FNR regulates this process directly by tranferring electrons to the NAD bound to Tic62. ▪


Küchler, M., et al.