Details

Autor(en) / Beteiligte

• Brock, I.W. (University of Warwick, Coventry.)
• Mills, J.D
• Robinson, D
• Robinson, C

Titel

The delta pH-driven, ATP-independent protein translocation mechanism in the chloroplast thylakoid membrane

Ist Teil von

• The Journal of biological chemistry, 1995-01, Vol.270 (4)

Erscheinungsjahr

1995

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Previous studies have shown that proteins are transported across the chloroplast thylakoid membrane by two very different mechanisms, one of which requires stromal factors and ATP, whereas the other mechanism is ATP independent but completely reliant on the thylakoidal deltapH. We have examined the role of the deltapH in the latter mechanism by simultaneously monitoring the magnitude of deltapH (by 9-aminoacridine fluorescence quenching) and the rate of import of the 23-kDa photosystem II protein into isolated pea thylakoids. We show that protein import can take place, at low but significant rates, at very low values of deltapH (in the region of 1.2-1.4), and that plots of the rate of protein import against proton concentration gradient are probably hyperbolic in nature. There is no evidence for a threshold level of deltapH which is required to drive translocation of the 23-kDa protein. Addition of uncouplers midway during import incubations results in a rapid and complete inhibition of translocation, showing that the continuous presence of the deltapH is required for translocation to take place. During import into intact chloroplasts, the intermediate-size 23-kDa protein substrate for the thylakoidal protein transport machinery is found only in the stromal fraction at all values of deltapH, suggesting that the initial interaction with the machinery is relatively weak, reversible and pH-independent. We therefore propose that the deltapH is required for both the initiation and completion of translocation; these roles are in marked contrast to the roles of protonmotive force in mitochondrial and sec-dependent bacterial protein transport