Targeting of an abundant cytosolic form of the protein import receptor at Toc159 to the outer chloroplast membrane

J Cell Biol. 2001 Jul 23;154(2):309-16. doi: 10.1083/jcb.200104022.


Chloroplast biogenesis requires the large-scale import of cytosolically synthesized precursor proteins. A trimeric translocon (Toc complex) containing two homologous GTP-binding proteins (atToc33 and atToc159) and a channel protein (atToc75) facilitates protein translocation across the outer envelope membrane. The mechanisms governing function and assembly of the Toc complex are not yet understood. This study demonstrates that atToc159 and its pea orthologue exist in an abundant, previously unrecognized soluble form, and partition between cytosol-containing soluble fractions and the chloroplast outer membrane. We show that soluble atToc159 binds directly to the cytosolic domain of atToc33 in a homotypic interaction, contributing to the integration of atToc159 into the chloroplast outer membrane. The data suggest that the function of the Toc complex involves switching of atToc159 between a soluble and an integral membrane form.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis
  • Arabidopsis Proteins*
  • Binding, Competitive / drug effects
  • Chloroplasts / metabolism*
  • Cytosol / metabolism*
  • Fluorescent Antibody Technique
  • GTP Phosphohydrolases / genetics*
  • GTP Phosphohydrolases / metabolism*
  • Intracellular Membranes / metabolism*
  • Macromolecular Substances
  • Membrane Proteins / genetics*
  • Membrane Proteins / metabolism*
  • Membrane Proteins / pharmacology
  • Peas
  • Plant Leaves / metabolism
  • Plant Proteins / genetics
  • Plant Proteins / metabolism
  • Protein Precursors / metabolism
  • Protein Transport / physiology
  • Subcellular Fractions / chemistry
  • Subcellular Fractions / metabolism


  • Arabidopsis Proteins
  • Macromolecular Substances
  • Membrane Proteins
  • OEP75 protein precursor, plant
  • Plant Proteins
  • Protein Precursors
  • TOC159 protein, Arabidopsis
  • TOC75 protein, Arabidopsis
  • Toc33 protein, Arabidopsis
  • GTP Phosphohydrolases