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Toc, Tic, and chloroplast protein import

P Jarvis1, J Soll

  • 1Department of Biology, University of Leicester, UK. rpj3@le.ac.uk

Biochimica Et Biophysica Acta
|December 26, 2001
PubMed

Insights

Most chloroplast proteins are made in the cytoplasm and imported into chloroplasts via a process involving transit peptides and the Toc/Tic translocon machinery. This machinery, with prokaryotic origins, requires energy for protein translocation.

Area of Science:

  • Plant Biology
  • Molecular Biology
  • Cell Biology

Background:

  • Chloroplast proteins are synthesized as precursors in the cytosol.
  • These precursors possess N-terminal transit peptides for targeting to chloroplasts.
  • Protein import into chloroplasts is a complex, multi-step process.

Purpose of the Study:

  • To elucidate the mechanism of chloroplast protein import.
  • To identify the key molecular players involved in protein translocation.
  • To understand the evolutionary origins of the chloroplast import machinery.

Main Methods:

  • Characterization of precursor proteins and their transit peptides.
  • Investigation of cytosolic guidance complexes.
  • Analysis of the Toc (translocon at the outer envelope membrane of chloroplasts) and Tic (translocon at the inner envelope membrane of chloroplasts) complexes.
  • In vivo, in organello, and in vitro biochemical assays.
  • Phylogenetic analysis of translocon subunits.

Main Results:

  • Transit peptides mediate organelle-specific targeting and can be phosphorylated.
  • A cytosolic guidance complex involving Hsp70 and 14-3-3 proteins docks at the outer envelope.
  • The Toc/Tic apparatus facilitates protein translocation across envelope membranes.
  • ATP and GTP hydrolysis are essential for the import process.
  • Key Toc and Tic subunits have been identified and characterized.
  • Phylogenetic data indicate a cyanobacterial origin for several translocon subunits.

Conclusions:

  • Chloroplast protein import is a regulated process involving cytosolic factors and the Toc/Tic translocon.
  • The import machinery possesses a prokaryotic core, reflecting the endosymbiotic origin of chloroplasts.
  • Understanding this process is crucial for chloroplast biogenesis and function.

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