The Molecular Concept of Protein Translocation across the Outer Membrane of Chloroplasts

Chloroplasts, plant organelles which facilitate photosynthesis, originated when photosynthetic bacteria became a part of non-photosynthetic eukaryotic cell. Chloroplast proteins synthetised in the cytosol have to be transported into the organelle, which is facilitated by the Toe complex on the Outer envelope. The preprotein receptors Toc34 and Toc159 associate with the pore-forming Toc75 to form the Toe core complex. Toc64 and Toc 12 dynamically associate with the core complex and recruit chaperones, forming the intermembrane space complex. Describing Toc159 as integral membrane protein provides insight into its function. After receiving preproteins from Toc34, it uses GTP hydrolysis to push the precursor into the translocation channel. GTP hydrolysis by Toc34 controls its binding/handover cycle. Moreover, different isoforms of Toe components form complexes with different specificity for photosynthetic and non-photosynthetic preproteins. After initial translocation steps, a series of binding spots seems to constitute an affinity chain that guides preproteins further on their translocation pathway. This model parallels the "acid chain" described in mitochondrial import. Considering recent data, a new "big picture" of chloroplast import begins to emerge