A 1-megadalton translocation complex containing Tic20 and Tic21 mediates chloroplast protein import at the inner envelope membrane

Plant Cell. 2009 Jun;21(6):1781-97. doi: 10.1105/tpc.108.063552. Epub 2009 Jun 16.

Abstract

Chloroplast protein import is mediated by two hetero-oligomeric protein complexes, the Tic and Toc translocons, which are located in the inner and outer envelope membranes. At the inner membrane, many Tic components have been identified and characterized, but it remains unclear how these Tic proteins are organized to form a protein-conducting channel or whether a stable Tic core complex that binds translocating preproteins exists. Here, we report the identification of a 1-megadalton (MD) translocation complex as an intermediate during protein translocation across the inner membrane in Arabidopsis thaliana and pea (Pisum sativum). This complex can be detected by blue native PAGE using the mild detergent digitonin without any chemical cross-linkers. The preprotein arrested in the 1-MD complex can be chased into its fully translocated form after a subsequent incubation. While Tic20 and Tic21 appear to be involved in the 1-MD complex, Tic110, a well-characterized Tic component, exists as a distinct entity from the complex. Several lines of evidence suggest that the 1-MD complex functions in between the Toc and Tic110-containing complexes, most likely as a protein-conducting channel at the inner envelope.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Triphosphate / metabolism
  • Arabidopsis / drug effects
  • Arabidopsis / metabolism
  • Arabidopsis / ultrastructure
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism
  • Arabidopsis Proteins / physiology
  • Cation Transport Proteins / genetics
  • Cation Transport Proteins / metabolism
  • Cation Transport Proteins / physiology
  • Cell Membrane / drug effects
  • Cell Membrane / metabolism*
  • Electrophoresis, Polyacrylamide Gel
  • Guanosine Triphosphate / metabolism
  • Homeostasis / genetics
  • Iron / metabolism
  • Membrane Transport Proteins / genetics
  • Membrane Transport Proteins / metabolism
  • Membrane Transport Proteins / physiology*
  • Mutation
  • Pisum sativum / drug effects
  • Pisum sativum / metabolism
  • Pisum sativum / ultrastructure
  • Plant Proteins / genetics
  • Plant Proteins / metabolism
  • Plant Proteins / physiology*
  • Protein Transport / genetics
  • Thermolysin / pharmacology
  • Trypsin / pharmacology

Substances

  • Arabidopsis Proteins
  • Cation Transport Proteins
  • Membrane Transport Proteins
  • Plant Proteins
  • Tic20 protein, Arabidopsis
  • Tic20 protein, Pisum sativum
  • Tic21 protein, Arabidopsis
  • Tic22 protein, Pisum sativum
  • Guanosine Triphosphate
  • Adenosine Triphosphate
  • Iron
  • Trypsin
  • Thermolysin