Mechanism of protein import across the chloroplast envelope

2000 ◽  
Vol 28 (4) ◽  
pp. 485-491 ◽  
Author(s):  
K. Chen ◽  
X. Chen ◽  
D. J. Schnell
Keyword(s):  
Protein Import ◽  
Essential Elements ◽  
Chloroplast Envelope ◽  
Envelope Membrane ◽  
Outer Envelope ◽  
Double Membrane ◽  
Protein Subunits ◽  
Targeting Signals ◽  
Outer Envelope Membrane ◽  
Envelope Membranes

The development and maintenance of chloroplasts relies on the contribution of protein subunits from both plastid and nuclear genomes. Most chloroplast proteins are encoded by nuclear genes and are post-translationally imported into the organelle across the double membrane of the chloroplast envelope. Protein import into the chloroplast consists of two essential elements: the specific recognition of the targeting signals (transit sequences) of cytoplasmic preproteins by receptors at the outer envelope membrane and the subsequent translocation of preproteins simultaneously across the double membrane of the envelope. These processes are mediated via the co-ordinate action of protein translocon complexes in the outer (Toe apparatus) and inner (Tic apparatus) envelope membranes.

scholarly journals A new chloroplast protein import intermediate reveals distinct translocation machineries in the two envelope membranes: energetics and mechanistic implications.

1996 ◽  
Vol 132 (1) ◽  
pp. 63-75 ◽  
Author(s):  
S V Scott ◽  
S M Theg
Keyword(s):  
Membrane Transport ◽  
Protein Import ◽  
Chloroplast Envelope ◽  
Envelope Membrane ◽  
Outer Envelope ◽  
Stromal Compartment ◽  
Chloroplast Protein Import ◽  
Chloroplast Protein ◽  
Outer Envelope Membrane ◽  
Envelope Membranes

Chloroplast protein import presents a complex membrane traversal problem: precursor proteins must cross two envelope membranes to reach the stromal compartment. This work characterizes a new chloroplast protein import intermediate which has completely traversed the outer envelope membrane but has not yet reached the stroma. The existence of this intermediate demonstrates that distinct protein transport machineries are present in both envelope membranes, and that they are able to operate independently of one another under certain conditions. Energetic characterization of this pathway led to the identification of three independent energy-requiring steps: binding of the precursor to the outer envelope membrane, outer membrane transport, and inner membrane transport. Localization of the sites of energy utilization for each of these steps, as well as their respective nucleotide specificities, suggest that three different ATPases mediate chloroplast envelope transport.

scholarly journals Synthesis and transfer of galactolipids in the chloroplast envelope membranes of Arabidopsis thaliana

2016 ◽  
Vol 113 (38) ◽  
pp. 10714-10719 ◽  
Author(s):  
Amélie A. Kelly ◽  
Barbara Kalisch ◽  
Georg Hölzl ◽  
Sandra Schulze ◽  
Juliane Thiele ◽  
...  
Keyword(s):  
Fusion Proteins ◽  
Chloroplast Envelope ◽  
Envelope Membrane ◽  
Lipid Transfer ◽  
Terminal Sequence ◽  
Outer Envelope ◽  
Outer Envelope Membrane ◽  
Envelope Reconstruction ◽  
Envelope Membranes

Galactolipids [monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG)] are the hallmark lipids of photosynthetic membranes. The galactolipid synthases MGD1 and DGD1 catalyze consecutive galactosyltransfer reactions but localize to the inner and outer chloroplast envelopes, respectively, necessitating intermembrane lipid transfer. Here we show that the N-terminal sequence of DGD1 (NDGD1) is required for galactolipid transfer between the envelopes. Different diglycosyllipid synthases (DGD1, DGD2, and Chloroflexus glucosyltransferase) were introduced into the dgd1-1 mutant of Arabidopsis in fusion with N-terminal extensions (NDGD1 and NDGD2) targeting to the outer envelope. Reconstruction of DGDG synthesis in the outer envelope membrane was observed only with diglycosyllipid synthase fusion proteins carrying NDGD1, indicating that NDGD1 enables galactolipid translocation between envelopes. NDGD1 binds to phosphatidic acid (PA) in membranes and mediates PA-dependent membrane fusion in vitro. These findings provide a mechanism for the sorting and selective channeling of lipid precursors between the galactolipid pools of the two envelope membranes.

scholarly journals Insertion of plastidic β-barrel proteins into the outer envelopes of plastids involves an intermembrane space intermediate formed with Toc75-V/OEP80

2021 ◽  
Author(s):  
Lucia E Gross ◽  
Anna Klinger ◽  
Nicole Spies ◽  
Theresa Ernst ◽  
Nadine Flinner ◽  
...  
Keyword(s):  
Membrane Proteins ◽  
Protein Import ◽  
Membrane Insertion ◽  
Intermembrane Space ◽  
Envelope Membrane ◽  
Outer Envelope ◽  
Pea Proteins ◽  
Outer Envelope Membrane ◽  
Correct Topology ◽  
Shed Light

Abstract The insertion of organellar membrane proteins with the correct topology requires the following: First, the proteins must contain topogenic signals for translocation across and insertion into the membrane. Second, proteinaceous complexes in the cytoplasm, membrane, and lumen of organelles are required to drive this process. Many complexes required for the intracellular distribution of membrane proteins have been described, but the signals and components required for the insertion of plastidic β-barrel-type proteins into the outer membrane are largely unknown. The discovery of common principles is difficult, as only a few plastidic β-barrel proteins exist. Here, we provide evidence that the plastidic outer envelope β-barrel proteins OEP21, OEP24, and OEP37 from pea (Pisum sativum) and Arabidopsis thaliana contain information defining the topology of the protein. The information required for translocation of pea proteins across the outer envelope membrane is present within the six N-terminal β-strands. This process requires the action of TOC (translocon of the outer chloroplast membrane). After translocation into the intermembrane space, β-barrel proteins interact with TOC75-V, as exemplified by OEP37 and P39, and are integrated into the membrane. The membrane insertion of plastidic β-barrel proteins is affected by mutation of the last β-strand, suggesting that this strand contributes to the insertion signal. These findings shed light on the elements and complexes involved in plastidic β-barrel protein import.

scholarly journals Identification of Protein Transport Complexes in the Chloroplastic Envelope Membranes via Chemical Cross-Linking

1997 ◽  
Vol 136 (5) ◽  
pp. 983-994 ◽  
Author(s):  
Mitsuru Akita ◽  
Erik Nielsen ◽  
Kenneth Keegstra
Keyword(s):  
Membrane Proteins ◽  
Protein Transport ◽  
Protein Import ◽  
Cross Linking ◽  
Envelope Membrane ◽  
Precursor Proteins ◽  
Intact Chloroplasts ◽  
Outer Envelope Membrane ◽  
Envelope Membranes ◽  
Chemical Cross Linking

Transport of cytoplasmically synthesized proteins into chloroplasts uses an import machinery present in the envelope membranes. To identify the components of this machinery and to begin to examine how these components interact during transport, chemical cross-linking was performed on intact chloroplasts containing precursor proteins trapped at a particular stage of transport by ATP limitation. Large crosslinked complexes were observed using three different reversible homobifunctional cross-linkers. Three outer envelope membrane proteins (OEP86, OEP75, and OEP34) and one inner envelope membrane protein (IEP110), previously reported to be involved in protein import, were identified as components of these complexes. In addition to these membrane proteins, a stromal member of the hsp100 family, ClpC, was also present in the complexes. We propose that ClpC functions as a molecular chaperone, cooperating with other components to accomplish the transport of precursor proteins into chloroplasts. We also propose that each envelope membrane contains distinct translocation complexes and that a portion of these interact to form contact sites even in the absence of precursor proteins.

scholarly journals Analysis of the Interactions of Preproteins with the Import Machinery over the Course of Protein Import into Chloroplasts

1997 ◽  
Vol 139 (7) ◽  
pp. 1677-1685 ◽  
Author(s):  
Andrei Kouranov ◽  
Danny J. Schnell
Keyword(s):  
Outer Membrane ◽  
Protein Import ◽  
Protein Translocation ◽  
Envelope Membrane ◽  
Outer Envelope ◽  
Direct Role ◽  
Inner Membrane ◽  
Outer Envelope Membrane ◽  
Additional Support ◽  
Hydrolysis Of

We have investigated the interactions of two nuclear-encoded preproteins with the chloroplast protein import machinery at three stages in import using a label-transfer crosslinking approach. During energy-independent binding at the outer envelope membrane, preproteins interact with three known components of the outer membrane translocon complex, Toc34, Toc75, and Toc86. Although Toc75 and Toc86 are known to associate with preproteins during import, a role for Toc34 in preprotein binding previously had not been observed. The interaction of Toc34 with preproteins is regulated by the binding, but not hydrolysis of GTP. These data provide the first evidence for a direct role for Toc34 in import, and provide insights into the function of GTP as a regulator of preprotein recognition. Toc75 and Toc86 are the major targets of cross-linking upon insertion of preproteins across the outer envelope membrane, supporting the proposal that both proteins function in translocation at the outer membrane as well as preprotein recognition. The inner membrane proteins, Tic(21) and Tic22, and a previously unidentified protein of 14 kD are the major targets of crosslinking during the late stages in import. These data provide additional support for the roles of these components during protein translocation across the inner membrane. Our results suggest a defined sequence of molecular interactions that result in the transport of nuclear-encoded preproteins from the cytoplasm into the stroma of chloroplasts.

scholarly journals Targeting and assembly of components of the TOC protein import complex at the chloroplast outer envelope membrane

2014 ◽  
Vol 5 ◽  
Author(s):  
Lynn G. L. Richardson ◽  
Yamuna D. Paila ◽  
Steven R. Siman ◽  
Yi Chen ◽  
Matthew D. Smith ◽  
...  
Keyword(s):  
Protein Import ◽  
Envelope Membrane ◽  
Outer Envelope ◽  
Outer Envelope Membrane

scholarly journals Ubiquitin-dependent chloroplast-associated protein degradation in plants

Science ◽  
2019 ◽  
Vol 363 (6429) ◽  
pp. eaav4467 ◽  
Author(s):  
Qihua Ling ◽  
William Broad ◽  
Raphael Trösch ◽  
Mats Töpel ◽  
Tijen Demiral Sert ◽  
...  
Keyword(s):  
Protein Degradation ◽  
Forward Genetics ◽  
26S Proteasome ◽  
Chloroplast Envelope ◽  
Outer Envelope ◽  
Aaa Atpase ◽  
Ubiquitin E3 Ligase ◽  
Outer Envelope Membrane ◽  
Underlying Mechanisms ◽  
Envelope Membranes

Chloroplasts contain thousands of nucleus-encoded proteins that are imported from the cytosol by translocases in the chloroplast envelope membranes. Proteolytic regulation of the translocases is critically important, but little is known about the underlying mechanisms. We applied forward genetics and proteomics in Arabidopsis to identify factors required for chloroplast outer envelope membrane (OEM) protein degradation. We identified SP2, an Omp85-type β-barrel channel of the OEM, and CDC48, a cytosolic AAA+ (ATPase associated with diverse cellular activities) chaperone. Both proteins acted in the same pathway as the ubiquitin E3 ligase SP1, which regulates OEM translocase components. SP2 and CDC48 cooperated to bring about retrotranslocation of ubiquitinated substrates from the OEM (fulfilling conductance and motor functions, respectively), enabling degradation of the substrates by the 26S proteasome in the cytosol. Such chloroplast-associated protein degradation (CHLORAD) is vital for organellar functions and plant development.

Sign in / Sign up

Export Citation Format

Share Document