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Secondary plastids of chromalveolates are, in most cases, bound by four surrounding membranes, hence revealing their (secondary) symbiogenetic origin from an engulfed formerly free-living, phototrophic eukaryote (an ancestral rhodophyte). In the cryptophytes, which possess the most primordial plastids of this group, the nucleomorph (a remnant of the rhodophytic nucleus) bears additional witness to this process.
One major task in establishing secondary symbiosis was the massive transfer of genetic material from the symbionts' genomes to the host nucleus. This was subsequently followed by a loss of the original loci, and accounted ex ante for the evolution of adequate mechanisms for a re-import of the encoded proteins. Present models merely explain the transport processes at the outermost and innermost of the four membranes (see above). The underling mechanisms at the remaining membranes, especially the second outermost (the periplastid membrane, PPM) continue to be elusive.
Starting with Gt_ORF201 (homolog to Der1p) from the model cryptophyte Guillardia theta, a number of nucleomorph-encoded factors were identified with homology to components of the ER-associated degradation system (ERAD) from Saccharomyces cerevisiae, the core of which composes a machinery for the elimination (dislocation) of misfolded proteins from the ER and their subsequental degradation at the proteasome. These were complimented by nucleomorph-encoded homologues of Hrd1p (Gt_ORF477), the AAA-ATPase Cdc48p (Gt_sCdc48) and its cofactor Ufd1p (Gt_sUfd1). Exemplarily, ORF201 was shown to be a functional ortholog to Der1p, most likely located in the periplastid membrane of Guillardia theta.
In exhaustive genome analyses of previously sequenced chromalveolates (amongst others the genomes of the heterokont Phaeodactylum tricornutum and the apicomplexan human pathogen Plasmodium falciparum), a conservation of the identified symbiont-specific factors was shown (as being nuclear-encoded pre-proteins with functional symbiont-specific targeting signals in these cases).
Considering that the symbiont-specific "ERAD"-factors exist in parallel to the host-specific ERAD-machinery in all investigated organisms for protein degradation, and that these factors are exclusively associated with the pre-degradative dislocation of ERAD-substrates from the ER, a novel ERAD-independent role is suggested: The import of nucleus-encoded symbiont- and plastid-localised proteins through the periplastid membrane of four-membrane bound plastids of chromalveolates.
Based on current knowledge of ERAD associated substrate dislocation, this thesis provides the first experimentally testable mechanistic model for protein transport across the PPM.