Original scientific paper

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

Aleksandar Vojta ; LMU, Department of Biology I, VW-Research group, München, Germany
Hrvoje Fulgosi ; Ruđer Bošković Institute, Zagreb, Croatia
Enrico Schleiff ; LMU, Department of Biology I, VW-Research group, München, Germany

Abstract

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 Toc complex
on the outer envelope. The preprotein receptors Toc34 and Toc159 associate with the poreforming
Toc75 to form the Toc core complex. Toc64 and Toc12 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 Toc 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.

Keywords

chloroplasts; envelope membranes; GTPase; protein receptors; translocation channel; phosphorylation

Hrčak ID:

31138

URI

https://hrcak.srce.hr/31138

Publication date:

15.11.2008.

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