The preprotein conducting channel at the inner envelope membrane of plastids

Heins, Lisa; Mehrle, Alexander; Hemmler, Roland; Wagner, Richard; Küchler, Michael; Hörmann, Friederike; Sveshnikov, Dmitry; Soll, Jürgen

doi:10.1093/emboj/21.11.2616

Cited by 128 publications

(90 citation statements)

References 38 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…However, Toc75 as well as the Tic complex within the inner plastid membrane are generally not believed to be able to transport bulky proteins (26) and it has been shown that plastid preproteins necessarily have to be transported in an unfolded conformation (32). Whereas the pore diameter of Toc75 measures about 14-26 Å (38) and the translocons of the inner plastid membrane Tic110 and Tic20 are in a similar range with 15-31 Å and 7.8-14.1 Å, respectively (39,40), typical N-glycosylation residues comprising 12 glycans have a size of ∼10 × 10 × 30 Å (41, 42), and hence might be too big for translocation. However, as in primary plastids, there is no need for Toc75 to transport glycoproteins, because the addition of glycans occurs exclusively in the ER and Golgi and stromal glycoproteins necessarily use a vesiclemediated route (20,(22)(23)(24)(25), analyses on that issue are not available so far.…”

Section: Discussionmentioning

confidence: 99%

Evidence for glycoprotein transport into complex plastids

Peschke¹,

Moog

Klingl

et al. 2013

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

View full text Add to dashboard Cite

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).

show abstract

Section: Discussionmentioning

confidence: 99%

Evidence for glycoprotein transport into complex plastids

Peschke¹,

Moog

Klingl

et al. 2013

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

View full text Add to dashboard Cite

show abstract

“…These conclusions were based on the observation that a urea-denatured fragment of psTic110 gave rise to ion channels when inserted into proteoliposomes. However, the relevance of the reconstitution data are unclear because native psTic110 does not form similar channels in recon-stituted proteoliposomes, nor are similar channels observed in native chloroplast envelope membranes (23).…”

mentioning

confidence: 98%

“…Recently, this model has been challenged by a third hypothesis. This study proposed that Tic110 is a ␤-barrel membrane protein and functions as the protein-conducting channel of the Tic translocon (23). These conclusions were based on the observation that a urea-denatured fragment of psTic110 gave rise to ion channels when inserted into proteoliposomes.…”

mentioning

confidence: 99%

atTic110 Functions as a Scaffold for Coordinating the Stromal Events of Protein Import into Chloroplasts

Inaba

Alvarez-Huerta

et al. 2003

Journal of Biological Chemistry

117

148

View full text Add to dashboard Cite

The translocon of the inner envelope membrane of chloroplasts (Tic) mediates the late events in the translocation of nucleus-encoded preproteins into chloroplasts. Tic110 is a major integral membrane component of active Tic complexes and has been proposed to function as a docking site for translocation-associated stromal factors and as a component of the protein-conducting channel. To investigate the various proposed functions of Tic110, we have investigated the structure, topology, and activities of a 97.5-kDa fragment of Arabidopsis Tic110 (atTic110) lacking only the amino-terminal transmembrane segments. The protein was expressed both in Escherichia coli and Arabidopsis as a stable, soluble protein with a high ␣-helical content. Binding studies demonstrate that a region of the atTic110-soluble domain selectively associates with chloroplast preproteins at the late stages of membrane translocation. These data support the hypothesis that the bulk of Tic110 extends into the chloroplast stroma and suggest that the domain forms a docking site for preproteins as they emerge from the Tic translocon.Chloroplast biogenesis is dependent upon the import of ϳ3000 different nucleus-encoded proteins (1). The majority of these proteins are synthesized as preproteins carrying an amino-terminal transit peptide that serves as the essential signal for targeting to the organelle. The transit peptide is recognized by receptor components of the translocon at the outer envelope membrane of chloroplasts (Toc), and a GTP-regulated switch initiates translocation through the protein-conducting channel of the Toc complex. At this stage, the Toc complex associates with the translocon at the inner envelope membrane (Tic), and this Toc-Tic supercomplex mediates the direct transport of the preproteins from the cytoplasm into the chloroplast stroma (2).Although many mechanistic details remain to be defined, the activities of the Toc components have been extensively investigated. Two membrane-associated GTPases, Toc159 and Toc34/33, mediate transit peptide recognition and regulate the initiation of translocation (3-7). The Toc GTPases form a complex with Toc75, an integral membrane protein that, along with Toc159, constitutes a major component of the protein-conducting channel (1, 8 -10).In contrast to the Toc complex, the activities and functions of the Tic components are less well defined. The biochemical analysis of Tic function has been complicated by the fact that assembly of functional Tic complexes is dynamic and occurs in response to preprotein translocation (11). Therefore, the isolation of a stable Tic complex has thus far been elusive. Tic110 was the first Tic component identified and represents a major component of active Tic complexes (12, 13). It is an integral inner envelope membrane protein, and structural predictions suggest that it consists of two predicted transmembrane helices at its extreme amino terminus and a 97.5-kDa carboxyl-terminal region that is largely hydrophilic. Tic110 transiently associates with at least five o...

show abstract

“…The first model describes Tic110 with six transmembrane helices (herein referred to as TM1 to TM6; Figure 1a, model 1) located throughout the polypeptide. The first two transmembrane helices (TM1 and TM2) function as a signal-anchor sequence to target the protein to the inner membrane (L€ ubeck et al, 1997), and the rest of the polypeptide traverses the inner membrane four more times (TM3 to TM6) to form a Ca 2+ -sensitive and cation-selective channel (Heins et al, 2002;Balsera et al, 2009;Kovacs-Bogdan et al, 2011). Tic110 is therefore proposed to be the major channel for translocation of protein across the inner membrane.…”

Section: Introductionmentioning

confidence: 99%

Structural characterizations of chloroplast translocon protein Tic110

Hsiao¹,

Tsai²,

Chu³

et al. 2013

Acta Cryst Sect A

View full text Add to dashboard Cite

SUMMARYTic110 is a major component of the chloroplast protein import translocon. Two functions with mutually exclusive structures have been proposed for Tic110: a protein-conducting channel with six transmembrane domains and a scaffold with two N-terminal transmembrane domains followed by a large soluble domain for binding transit peptides and other stromal translocon components. To investigate the structure of Tic110, Tic110 from Cyanidioschyzon merolae (CmTic110) was characterized. We constructed three fragments, CmTic110 A , CmTic110 B and CmTic110 C , with increasing N-terminal truncations, to perform smallangle X-ray scattering (SAXS) and X-ray crystallography analyses and Dali structural comparison. Here we report the molecular envelope of CmTic110 B and CmTic110 C determined by SAXS, and the crystal structure of CmTic110 C at 4.2 A. Our data indicate that the C-terminal half of CmTic110 possesses a rod-shaped helixrepeat structure that is too flattened and elongated to be a channel. The structure is most similar to the HEAT-repeat motif that functions as scaffolds for protein-protein interactions.

show abstract

The preprotein conducting channel at the inner envelope membrane of plastids

Cited by 128 publications

References 38 publications

Evidence for glycoprotein transport into complex plastids

Evidence for glycoprotein transport into complex plastids

atTic110 Functions as a Scaffold for Coordinating the Stromal Events of Protein Import into Chloroplasts

Structural characterizations of chloroplast translocon protein Tic110

Contact Info

Product

Resources

About