MBE Advance Access published online on January 22, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm008
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© 2007 The Authors
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Research Article |
Der1-Mediated Pre-Protein Import into the Periplastid Compartment of Chromalveolates?
1 Laboratory for Cell Biology, Philipps-University of Marburg, Karl-von-Frisch Strasse 8, D-35032 Marburg, Germany
2 present address: School of Botany, University of Melbourne, VIC-3010 Melbourne, Australia
3 Department of Biology, University of Konstanz, Universitätsstrasse 10, D-78457 Konstanz, Germany
4 Laboratory for Parasitology, Philipps-University of Marburg, Karl-von-Frisch Strasse 8, D-35032 Marburg, Germany
5 Correspondence to Uwe-G. Maier: maier{at}staff.uni-marburg.de, phone: +49 6421 28 21543.
Accepted for publication December 22, 2006.
Phototrophic chromalveolates possess plastids surrounded by either three or four membranes, revealing their secondary endosymbiotic origin from an engulfed eukaryotic alga. In cryptophytes, a member of the chromalveolates, the organelle is embedded within a designated region of the host's rough ER (RER). Its eukaryotic compartments other than the plastid were reduced to the mere remains of its former cytosol, the periplastid compartment (PPC, PP space), and its nucleus, the nucleomorph, separated from the RER by its former plasma membrane, the periplast membrane (PPM). In the nucleomorph genome of the cryptophyte Guillardia theta we identified several genes sharing homology with components of the ER-associated degradation machinery (ERAD) of yeast and higher eukaryotes, namely ORF201 and ORF477, homologs of membrane-bound proteins Der1p (Degradation in the ER protein 1) and the RING-finger ubiquitin-ligase Hrd1, and a truncated version of Udf1, a cofactor of Cdc48, a lumenal ATPase. Exemplarily studies on the Der1-homolog ORF201 showed that this protein partially rescued a yeast deletion mutant, indicating the existence of a functional PPC-specific ERAD-like system in cryptophytes. With the non-investigated exception of haptophytes a phylogenetically and mechanistically related system is apparently present in all chromalveolates with four-membrane bound plastids, since amongst others, PPC-specific Derlins (Der1-like proteins), CDC48 and its cofactor Ufd1 were identified in the nuclear genomes of diatoms and apicomplexa. These proteins are equipped with the required topogenic signals to direct them into the periplastidial compartment of their secondary symbions. Based on our findings, we suggest that all chromalveolates with four-membrane bound plastids express an ERAD-derived machinery in the PPM of their secondary plastid, coexisting physically and systematically adjacent to the host's own ERAD system.
We propose herewith that this system was functionally adapted to mediate transport of nucleus-encoded PPC/plastid pre-proteins from the rough ER into the periplastid space.
Key Words: secondary endosymbiosis • chromalveolates • protein-transport • Der1 • ERAD
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