Der1-mediated Preprotein Import into the Periplastid Compartment of Chromalveolates?

Abstract

Phototrophic chromalveolates possess plastids surrounded by either 3 or 4 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 endoplasmic reticulum (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 (ERAD) machinery 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 noninvestigated exception of haptophytes a phylogenetically and mechanistically related system is apparently present in all chromalveolates with 4 membrane-bound plastids because 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 periplastid compartment of their secondary symbionts. Based on our findings, we suggest that all chromalveolates with 4 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 preproteins from the RER into the periplastid space.

Evidence for glycoprotein transport into complex plastids

“…Besides such synthetic constructs, we screened all known endogenous PPC proteins (9,10,13,16,(29)(30)(31) for potential N-glycosylation sites using the program NetNGlyc 1.0. Interestingly, 23% of the 52 proteins possess potential N-glycosylation sites with high confidence predictions (++) within the mature, soluble protein (Table S1).…”

“…Signal peptide and transit peptide together are called bipartite targeting signal (BTS) and represent the classical targeting signature for preproteins destined to complex plastids. Recently, it was proposed that transport across the second plastid membrane of diatoms is mediated by an ER-associated degradation (ERAD)-derived translocation machinery, which was termed symbiontspecific ERAD-like machinery (SELMA) (12,13). Phylogenetic data together with molecular analyses suggest that during evolution, core components of the red algal ERAD-transport system were relocalized from the symbiont's ER to the second plastid membrane to mediate preprotein translocation (12)(13)(14)(15)(16).…”

“…Recently, it was proposed that transport across the second plastid membrane of diatoms is mediated by an ER-associated degradation (ERAD)-derived translocation machinery, which was termed symbiontspecific ERAD-like machinery (SELMA) (12,13). Phylogenetic data together with molecular analyses suggest that during evolution, core components of the red algal ERAD-transport system were relocalized from the symbiont's ER to the second plastid membrane to mediate preprotein translocation (12)(13)(14)(15)(16). After transport across the second membrane, two main protein populations have to be discriminated: proteins that remain within the space between membrane two and threethe periplastidal compartment (PPC), which corresponds to the cytosol of the red algal endosymbiont-and proteins that are destined to the plastid stroma and have to be transported across two further membranes (Fig.…”

Protein targeting into secondary plastids of chlorarachniophytes

“…This functional compatibility is also supported by 2 previous findings. The existence of an ER-associated degradation (ERAD)-like system for transporting preproteins across the second-from-theoutside of the 4 plastid-surrounding membranes in cryptophytes, heterokonts, and apicomplexans (39); and the capability of cryptophyte plastid-targeting peptide to deliver GFP into the plastids of the heterokont Phaeodactylum tricornutum (17). It appears that photosynthetic chromalveolates with secondary plastids use, at least in part, the protein-targeting signals for plastids of the red algal endosymbiont, which eventually became their own plastids.…”

Genomic perspectives on the birth and spread of plastids

“…What about the remaining membrane? Maier and colleagues have shown that translocation across the second outermost membrane (originally the plasma membrane of the algal endosymbiont) in red-type four-membrane plastids is mediated by a multiprotein complex dubbed SELMA [symbiont-specific endoplasmic reticulum (ER)-associated degradation-like machinery (68)(69)(70)(71)]. The evolution of SELMA appears to represent a singularity, an intricate complex cobbled together from a dozen or more proteins at least some of which originally functioned as part of the ER-associated degradation machinery in the red algal endosymbiont.…”