Specific targeting of proteins to outer envelope membranes of endosymbiotic organelles, chloroplasts, and mitochondria

Lee, Jun-Ho; Kim, Dae Heon; Hwang, Inhwan

doi:10.3389/fpls.2014.00173

Cited by 59 publications

(52 citation statements)

References 87 publications

(152 reference statements)

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“…AKR2A may also have an interaction with protein factor(s) at the COM at the insertion step of its client proteins. Import channel Toc75 has been shown to assist insertion of proteins to the COM in vitro (Bae et al, 2008; Lee et al, 2014; Tu et al, 2004). The physiological significance of the PG and MGDG binding to the ARD is demonstrated in planta using the mgd1 and pgp1-1 mutants that have a defect in biosynthesis of MGDG and PG, respectively, and consequently exhibit the yellow leaf phenotype with greatly reduced levels of COM proteins.…”

Section: Discussionmentioning

confidence: 99%

An Ankyrin Repeat Domain of AKR2 Drives Chloroplast Targeting through Coincident Binding of Two Chloroplast Lipids

et al. 2014

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SUMMARY In organellogenesis of the chloroplast from endosymbiotic cyanobacterium, the establishment of protein targeting mechanisms to the chloroplast should have been pivotal. However, it is still mysterious how these mechanisms were established and how they work in plant cells. Here, we show that AKR2A, the cytosolic targeting factor for chloroplast outer membrane (COM) proteins, evolved from the ankyrin repeat domain (ARD) of the host cell by stepwise extensions of its N-terminal domain, and two lipids monogalactosyldiacylglycerol (MGDG) and phosphatidylglycerol (PG) of the endosymbiont were selected to function as the AKR2A receptor. Structural analysis, molecular modeling and mutational analysis of the ARD identified two adjacent sites for coincidental and synergistic binding of MGDG and PG. Based on these findings, we propose that the targeting mechanism of COM proteins was established using components from both the endosymbiont and host cell through a modification of the protein-protein interacting ARD into a lipid binding domain.

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Section: Discussionmentioning

confidence: 99%

An Ankyrin Repeat Domain of AKR2 Drives Chloroplast Targeting through Coincident Binding of Two Chloroplast Lipids

et al. 2014

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“…Consequently, one of the most critical processes in chloroplast proteome biogenesis is the specific, posttranslational delivery of these nuclearencoded proteins to chloroplasts (Jarvis, 2008;Li and Chiu, 2010;Lee et al, 2013aLee et al, , 2014. Delivery to chloroplasts requires a specific targeting signal whose form depends on the type of protein and its location in the chloroplast.…”

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confidence: 99%

Sequence Motifs in Transit Peptides Act as Independent Functional Units and Can Be Transferred to New Sequence Contexts

et al. 2015

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A large number of nuclear-encoded proteins are imported into chloroplasts after they are translated in the cytosol. Import is mediated by transit peptides (TPs) at the N termini of these proteins. TPs contain many small motifs, each of which is critical for a specific step in the process of chloroplast protein import; however, it remains unknown how these motifs are organized to give rise to TPs with diverse sequences. In this study, we generated various hybrid TPs by swapping domains between Rubisco small subunit (RbcS) and chlorophyll a/b-binding protein, which have highly divergent sequences, and examined the abilities of the resultant TPs to deliver proteins into chloroplasts. Subsequently, we compared the functionality of sequence motifs in the hybrid TPs with those of wild-type TPs. The sequence motifs in the hybrid TPs exhibited three different modes of functionality, depending on their domain composition, as follows: active in both wild-type and hybrid TPs, active in wild-type TPs but inactive in hybrid TPs, and inactive in wild-type TPs but active in hybrid TPs. Moreover, synthetic TPs, in which only three critical motifs from RbcS or chlorophyll a/bbinding protein TPs were incorporated into an unrelated sequence, were able to deliver clients to chloroplasts with a comparable efficiency to RbcS TP. Based on these results, we propose that diverse sequence motifs in TPs are independent functional units that interact with specific translocon components at various steps during protein import and can be transferred to new sequence contexts.

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“…Most OEM proteins [5, 23] and a few IEM Proteins [24–26] are encoded in the nucleus as a mature form without a transit peptide. A series of elegant studies have established that a subset of OEM proteins are co-translationally recognized in the cytosol at their TMD and a positively charged flanking region at its C terminus by an ankyrin repeat protein, which directs its client proteins specifically to the chloroplast OEM [27–29].…”

Section: Introductionmentioning

confidence: 99%

Polyglycine Acts as a Rejection Signal for Protein Transport at the Chloroplast Envelope

et al. 2016

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PolyGly is present in many proteins in various organisms. One example is found in a transmembrane β-barrel protein, translocon at the outer-envelope-membrane of chloroplasts 75 (Toc75). Toc75 requires its N-terminal extension (t75) for proper localization. t75 comprises signals for chloroplast import (n75) and envelope sorting (c75) in tandem. n75 and c75 are removed by stromal processing peptidase and plastidic type I signal peptidase 1, respectively. PolyGly is present within c75 and its deletion or substitution causes mistargeting of Toc75 to the stroma. Here we have examined the properties of polyGly-dependent protein targeting using two soluble passenger proteins, the mature portion of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (mSS) and enhanced green fluorescent protein (EGFP). Both t75-mSS and t75-EGFP were imported into isolated chloroplasts and their n75 removed. Resultant c75-mSS was associated with the envelope at the intermembrane space, whereas c75-EGFP was partially exposed outside the envelope. Deletion of polyGly or substitution of tri-Ala for the critical tri-Gly segment within polyGly caused each passenger to be targeted to the stroma. Transient expression of t75-EGFP in Nicotiana benthamiana resulted in accumulation of c75-EGFP exposed at the surface of the chloroplast, but the majority of the EGFP passenger was found free in the cytosol with most of its c75 attachment removed. Results of circular dichroism analyses suggest that polyGly within c75 may form an extended conformation, which is disrupted by tri-Ala substitution. These data suggest that polyGly is distinct from a canonical stop-transfer sequence and acts as a rejection signal at the chloroplast inner envelope.

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Specific targeting of proteins to outer envelope membranes of endosymbiotic organelles, chloroplasts, and mitochondria

Cited by 59 publications

References 87 publications

An Ankyrin Repeat Domain of AKR2 Drives Chloroplast Targeting through Coincident Binding of Two Chloroplast Lipids

An Ankyrin Repeat Domain of AKR2 Drives Chloroplast Targeting through Coincident Binding of Two Chloroplast Lipids

Sequence Motifs in Transit Peptides Act as Independent Functional Units and Can Be Transferred to New Sequence Contexts

Polyglycine Acts as a Rejection Signal for Protein Transport at the Chloroplast Envelope

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