MBE Advance Access published online on April 17, 2009
Molecular Biology and Evolution, doi:10.1093/molbev/msp079
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Research Article |
ERAD-derived Pre-Protein Transport across the 2nd Outermost Plastid Membrane of Diatoms
1 Department of Cell Biology, Philipps-University of Marburg, Karl-von-Frisch Strasse 8, 35032 Marburg, Germany
* Corresponding author: Uwe G. Maier, Phone number: +49 6421 28 21543; FAX number: +49 6421-2822057; e-mail: maier{at}staff.uni-marburg.de
Received for publication March 20, 2009. Revision received April 14, 2009. Accepted for publication April 14, 2009.
The diatom Phaeodactylum tricornutum harbours a plastid that is surrounded by four membranes and evolved by way of secondary endosymbiosis. Like land plants, most of its plastid proteins are encoded as pre-proteins on the nuclear genome of the host cell and are resultantly redirected into the organelle. Because two more membranes are present in diatoms than the one pair surrounding primary plastids, the targeting situation is obviously different and more complex. In this work, we focus on pre-protein transport across the second outermost plastid membrane – an issue that was experimentally inaccessible until now. We provide first indications that our hypothesis of an ERAD (ER-associated degradation)-derived pre-protein transport system might be correct. Our data demonstrate that the symbiont-specific Der1 proteins, sDer1-1 and sDer1-2, form an oligomeric complex within the second outermost membrane of the complex plastid. Moreover, we present first evidence that the complex interacts with transit peptides of pre-proteins being transported across this membrane into the periplastidal compartment, but not with transit peptides of stromal-targeted proteins. Thus, the sDer1-complex might have an additional role in discriminating pre-proteins that are transported across the two outermost membranes from pre-proteins directed across all four membranes of the complex plastid. Altogether, our studies of the symbiont-specific ERAD-like machinery of diatoms suggest that a pre-existing cellular machinery was recycled to fulfill a novel function during the transition of a former free-living eukaryote into a secondary endosymbiont.
Key Words: complex plastids • Der1 • Diatoms • ERAD • pre-protein transport • secondary endosymbiosis
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