Transport of CtpA Protein from the Cyanobacterium Synechocystis 6803 Across the Thylakoid Membrane in Chloroplasts

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Abstract
The CtpA protein in the cyanobacterium Synechocystis 6803 is a C‐terminal processing protease that is essential for the assembly of the manganese cluster of the photosystem II complex. When fused to different chloroplast‐targeting transit peptides, CtpA can be imported into isolated spinach chloroplasts and is subsequently translocated into the thylakoid lumen. Thylakoid transport is mediated by the cyanobacterial signal peptide which demonstrates that the protein transport machinery in thylakoid membranes is functionally conserved between chloroplasts and cyanobacteria. Transport of CtpA across spinach thylakoid membranes is affected by both nigericin and sodium azide indicating that the SecA protein and a transthylakoidal proton gradient are involved in this process. Saturation of the Sec‐dependent thylakoid transport route by high concentrations of the precursor of the 33‐kDa subunit of the oxygen‐evolving system leads to a strongly reduced rate of thylakoid translocation of CtpA which demonstrates transport by the Sec pathway. However, thylakoid transport of CtpA is affected also by excess amounts of the 23‐kDa subunit of the oxygen‐evolving system, though to a lesser extent. This suggests that the cyanobacterial protein is capable of also interacing with components of the pH‐dependent route and that transport of a protein across the thylakoid membrane may not always be restricted to a single pathway.

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