scholarly journals Chloroplast-localized translation for protein targeting in Chlamydomonas reinhardtii

2021 ◽  
Author(s):  
Yi Sun ◽  
Shiva Bakhtiari ◽  
Melissa Valente-Paterno ◽  
Yanxia Wu ◽  
Christopher Law ◽  
...  
Keyword(s):  
Chlamydomonas Reinhardtii ◽  
Protein Import ◽  
Protein Targeting ◽  
Electron Tomography ◽  
Chloroplast Envelope ◽  
Cytoplasmic Protein ◽  
Chloroplast Protein Import ◽  
Translation Systems ◽  
Bacterial Type ◽  
Dual Translation

Translation is localized within cells to target proteins to their proper locations. We asked whether translation occurs on the chloroplast surface in Chlamydomonas and, if so, whether it is involved in co-translational protein targeting, aligned spatially with localized translation by the bacterial-type ribosomes within this organelle, or both. Our results reveal a domain of the chloroplast envelope which is bound by translating ribosomes. Purified chloroplasts retained ribosomes and mRNAs encoding two chloroplast proteins specifically on this translation domain, but not a mRNA encoding a cytoplasmic protein. Ribosomes clusters were seen on this domain by electron tomography. Activity of the chloroplast-bound ribosomes is supported by results of the ribopuromycylation and puromycin-release assays. Co-translational chloroplast protein import is supported by nascent polypeptide dependency of the ribosome-chloroplast associations. This cytoplasmic translation domain aligns localized translation by organellar bacterial-type ribosomes in the chloroplast. This juxtaposition the dual translation systems facilitates the targeting and assembly of the polypeptide products.

scholarly journals Origins, function, and regulation of the TOC–TIC general protein import machinery of plastids

2019 ◽  
Vol 71 (4) ◽  
pp. 1226-1238 ◽  
Author(s):  
Lynn G L Richardson ◽  
Danny J Schnell
Keyword(s):  
Protein Import ◽  
Protein Targeting ◽  
Bacterial Genome ◽  
Hybrid Origin ◽  
Chloroplast Envelope ◽  
Terrestrial Plants ◽  
General Protein ◽  
Host Nucleus ◽  
Import System ◽  
Host Genes

Abstract The evolution of chloroplasts from the original endosymbiont involved the transfer of thousands of genes from the ancestral bacterial genome to the host nucleus, thereby combining the two genetic systems to facilitate coordination of gene expression and achieve integration of host and organelle functions. A key element of successful endosymbiosis was the evolution of a unique protein import system to selectively and efficiently target nuclear-encoded proteins to their site of function within the chloroplast after synthesis in the cytoplasm. The chloroplast TOC–TIC (translocon at the outer chloroplast envelope–translocon at the inner chloroplast envelope) general protein import system is conserved across the plant kingdom, and is a system of hybrid origin, with core membrane transport components adapted from bacterial protein targeting systems, and additional components adapted from host genes to confer the specificity and directionality of import. In vascular plants, the TOC–TIC system has diversified to mediate the import of specific, functionally related classes of plastid proteins. This functional diversification occurred as the plastid family expanded to fulfill cell- and tissue-specific functions in terrestrial plants. In addition, there is growing evidence that direct regulation of TOC–TIC activities plays an essential role in the dynamic remodeling of the organelle proteome that is required to coordinate plastid biogenesis with developmental and physiological events.

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.

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.

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