Targeting and function of CAH1 Characterization of a novel protein pathway to the plant cell chloroplast
Abstract: The chloroplast is the organelle within a plant cell where photosynthesis takes place. This organelle originates from a cyanobacterium that was engulfed by a eukaryotic cell. During the transition from endosymbiont to organelle most of the cyanobacterial genes were transferred to the nuclear genome of the host cell, resulting in a chloroplast with a much reduced genome that requires massive import of gene products (proteins) back to the organelle. The majority of these proteins are translated in the cytosol as pre-proteins containing targeting information that directs them to a translocon complex in the chloroplast envelope, the Toc-Tic system, through which these proteins are transported.We have identified a protein in the model plant Arabidopsis thaliana, CAH1, that is trafficked via the endomembrane system (ER/Golgi apparatus) to the chloroplast instead of using the Toc-Tic machinery. This transport is partly mediated by canonical vesicle trafficking elements involved in ER to Golgi transport, such as Sar1 and RabD GTPases. Analysis of point mutated variants of CAH1 showed that both N-linked glycans and an intra-molecular disulphide bridge are required for correct folding, trafficking and function of the protein. Since chloroplasts lack N-glycosylation machinery, we propose that a route for chloroplast proteins that require endomembrane-specific post-translational modifications for their functionality exists as a complement to the Toc-Tic system.We also show that mutant plants with disrupted CAH1 gene expression have reduced rates of CO2 uptake and accumulate lower amounts of starch compared to wild-type plants, indicating an important function of the CAH1 protein for the photosynthetic capacity of Arabidopsis.Further study of CAH1 will not only be important to reveal its role in photosynthesis, but characterization of this novel protein pathway to the chloroplast can also shed light on how the plant cell evolved and clarify the purpose of keeping several chloroplast import pathways working in parallel. In addition, knowledge about this pathway could increase the opportunities for using plants as bio-factories for production of recombinant glycoproteins, which make up the vast majority of the bio-pharmaceutical molecules.
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