<i>Toxoplasma gondii</i>
            Tic20 is essential for apicoplast protein import

Dooren, Giel G. van; Tomova, Cveta; Agrawal, Swati; Humbel, Bruno M.; Striepen, Boris

doi:10.1073/pnas.0803862105

Cited by 192 publications

(281 citation statements)

References 27 publications

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“…Here, the 11-stranded β-barrel of GFP was split into saGFP1-10, comprising β-strands 1-10 (short 1-10), and a saGFP11 (short 11) fragment, containing β-strand 11. Neither fragment was fluorescent by itself, but when located in the same cellular subcompartment, the two fragments assembled into fluorescent GFP (35).…”

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confidence: 99%

“…Because of the paucity of data concerning the topology of chloroplast Omp85 proteins and the importance of the POTRA domain as a central functional element in the models of chloroplast OMP integration/assembly and preprotein import, we decided to investigate the topology of the atToc75-III (AT3G46740) and atToc75-V (AT5G19620). We applied the recently established self-assembling split GFP (saGFP) (34), which has been used to determine membrane protein topology in vivo before (35). Here, the 11-stranded β-barrel of GFP was split into saGFP1-10, comprising β-strands 1-10 (short 1-10), and a saGFP11 (short 11) fragment, containing β-strand 11.…”

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confidence: 99%

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Chloroplast Omp85 proteins change orientation during evolution

Sommer

Daum

Groß

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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The majority of outer membrane proteins (OMPs) from Gramnegative bacteria and many of mitochondria and chloroplasts are β-barrels. Insertion and assembly of these proteins are catalyzed by the Omp85 protein family in a seemingly conserved process. All members of this family exhibit a characteristic N-terminal polypeptide-transport-associated (POTRA) and a C-terminal 16-stranded β-barrel domain. In plants, two phylogenetically distinct and essential Omp85's exist in the chloroplast outer membrane, namely Toc75-III and Toc75-V. Whereas Toc75-V, similar to the mitochondrial Sam50, is thought to possess the original bacterial function, its homolog, Toc75-III, evolved to the pore-forming unit of the TOC translocon for preprotein import. In all current models of OMP biogenesis and preprotein translocation, a topology of Omp85 with the POTRA domain in the periplasm or intermembrane space is assumed. Using selfassembly GFP-based in vivo experiments and in situ topology studies by electron cryotomography, we show that the POTRA domains of both Toc75-III and Toc75-V are exposed to the cytoplasm. This unexpected finding explains many experimental observations and requires a reevaluation of current models of OMP biogenesis and TOC complex function.

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confidence: 99%

mentioning

confidence: 99%

Chloroplast Omp85 proteins change orientation during evolution

Sommer

Daum

Groß

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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“…Once delivered to the apicoplast, proteins have to cross three additional membranes. Recently, we demonstrated that a homolog of Tic20, a component of the translocon of the inner chloroplast membrane (Tic) 4 complex in plants, is likely required for protein import across the innermost membrane of the apicoplast (12). To date bioinformatics searches have failed to identify a matching translocon of the outer chloroplast membrane in apicomplexans (13).…”

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confidence: 99%

Genetic Evidence that an Endosymbiont-derived Endoplasmic Reticulum-associated Protein Degradation (ERAD) System Functions in Import of Apicoplast Proteins

Agrawal

Dooren

Beatty

et al. 2009

Journal of Biological Chemistry

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Most apicomplexan parasites harbor a relict chloroplast, the apicoplast, that is critical for their survival. Whereas the apicoplast maintains a small genome, the bulk of its proteins are nuclear encoded and imported into the organelle. Several models have been proposed to explain how proteins might cross the four membranes that surround the apicoplast; however, experimental data discriminating these models are largely missing. Here we present genetic evidence that apicoplast protein import depends on elements derived from the ER-associated protein degradation (ERAD) system of the endosymbiont. We identified two sets of ERAD components in Toxoplasma gondii, one associated with the ER and cytoplasm and one localized to the membranes of the apicoplast. We engineered a conditional null mutant in apicoplast Der1, the putative pore of the apicoplast ERAD complex, and found that loss of Der1 Ap results in loss of apicoplast protein import and subsequent death of the parasite.Apicomplexa are a phylum of obligate parasites that include the causative agents of malaria, toxoplasmosis, and cryptosporidiosis. Recent evidence suggests that apicomplexans evolved from a free-living photosynthetic ancestor (1). This ancestry is reflected in the presence of a chloroplast-like organelle, the apicoplast (2). While no longer engaged in photosynthesis, the apicoplast is essential to parasite survival and home to several critical biosynthetic pathways. Bioinformatic and experimental evidence suggests that the apicoplast is engaged in the synthesis of fatty acids, isoprenoids, and heme (3, 4). Genetic or pharmacological ablation of these pathways blocks parasite growth and the apicoplast, therefore, is currently considered a prime target for antiparasitic drug development (5-7). The organelle was derived by secondary endosymbiosis and reflects the successful union of a red alga and a heterotrophic eukaryote (8). A large number of algal genes were transferred to the host nucleus, and their products must now be routed back into the organelle. Trafficking occurs via the secretory pathway and is guided by an N-terminal bipartite targeting sequence (9). Transport from the ER to the apicoplast is believed to be vesicle mediated and to sidestep the Golgi apparatus (10, 11). Once delivered to the apicoplast, proteins have to cross three additional membranes. Recently, we demonstrated that a homolog of Tic20, a component of the translocon of the inner chloroplast membrane (Tic) 4 complex in plants, is likely required for protein import across the innermost membrane of the apicoplast (12). To date bioinformatics searches have failed to identify a matching translocon of the outer chloroplast membrane in apicomplexans (13). Analysis of the genome of the remnant nucleus of the algal endosymbiont in cryptomonads showed the presence of an ER-associated protein degradation (ERAD) system and offered a candidate for a translocon across the third and potentially the second membrane (14). Classically, ERAD functions in the homeostasis of the secretory ...

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“…Stromal proteins are transported across membranes three and four, presumably mediated by a Toc/Tic (translocon of the outer/inner chloroplast membrane)-like system as it is known from primary plastids. Core components like a Toc75 homolog and several Tic factors were identified by in silico analyses and partially characterized in diatoms and related organisms (17)(18)(19).…”

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confidence: 99%

Evidence for glycoprotein transport into complex plastids

Peschke¹,

Moog

Klingl

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Diatoms are microalgae that possess so-called "complex plastids," which evolved by secondary endosymbiosis and are surrounded by four membranes. Thus, in contrast to primary plastids, which are surrounded by only two membranes, nucleus-encoded proteins of complex plastids face additional barriers, i.e., during evolution, mechanisms had to evolve to transport preproteins across all four membranes. This study reveals that there exist glycoproteins not only in primary but also in complex plastids, making transport issues even more complicated, as most translocation machineries are not believed to be able to transport bulky proteins. We show that plastidal reporter proteins with artificial N-glycosylation sites are indeed glycosylated during transport into the complex plastid of the diatom Phaeodactylum tricornutum. Additionally, we identified five endogenous glycoproteins, which are transported into different compartments of the complex plastid. These proteins get N-glycosylated during transport across the outermost plastid membrane and thereafter are transported across the second, third, and fourth plastid membranes in the case of stromal proteins. The results of this study provide insights into the evolutionary pressure on translocation mechanisms and pose unique questions on the operating mode of well-known transport machineries like the translocons of the outer/inner chloroplast membranes (Toc/Tic).

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Toxoplasma gondii Tic20 is essential for apicoplast protein import

Cited by 192 publications

References 27 publications

Chloroplast Omp85 proteins change orientation during evolution

Chloroplast Omp85 proteins change orientation during evolution

Genetic Evidence that an Endosymbiont-derived Endoplasmic Reticulum-associated Protein Degradation (ERAD) System Functions in Import of Apicoplast Proteins

Evidence for glycoprotein transport into complex plastids

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