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Yeast mitochondrial outer membrane specifically binds cytoplasmically-synthesized precursors of mitochondrial

H Riezman1, R Hay, C Witte

  • 1Department of Biochemistry, Biocenter, University of Basel, CH-4056 Basel, Switzerland.

The EMBO Journal
|January 1, 1983
PubMed

Insights

Mitochondrial protein precursors bind to the outer membrane, independent of inner membrane energy. Import and processing occur upon energization, indicating a specific outer membrane receptor.

Area of Science:

  • Mitochondrial biogenesis and protein import
  • Cellular and molecular biology
  • Biochemistry of protein targeting

Background:

  • Mitochondrial proteins are synthesized in the cytoplasm and must be imported into the organelle.
  • The precise mechanisms of initial precursor binding to mitochondria are not fully understood.
  • Cytochrome b(2) and citrate synthase are examples of cytoplasmically synthesized mitochondrial proteins.

Purpose of the Study:

  • To investigate the initial binding step of cytoplasmically synthesized mitochondrial protein precursors to mitochondria.
  • To characterize the nature of the interaction between precursor proteins and mitochondrial membranes.
  • To identify the mitochondrial location and properties of the binding activity.

Main Methods:

  • Incubation of isolated mitochondria or outer membrane vesicles with radiolabeled precursor proteins.
  • Assays for precursor binding under energized and non-energized conditions.
  • Protease treatment (trypsin) to assess surface accessibility of binding sites.
  • Solubilization of membrane proteins and reconstitution into liposomes.

Main Results:

  • The precursor of cytochrome b(2) binds to isolated mitochondria and outer membrane vesicles.
  • Binding is independent of inner membrane energization and sensitive to trypsin, suggesting a surface receptor.
  • The intermediate form of cytochrome b(2) precursor and non-mitochondrial proteins do not bind.
  • Import and cleavage to the mature form occur upon inner membrane energization.
  • Similar binding characteristics were observed for the citrate synthase precursor.
  • Binding activity resides in the outer membrane, is solubilized by detergent, and can be reconstituted.

Conclusions:

  • Mitochondria possess a specific, receptor-like binding activity on their outer membrane for cytoplasmically synthesized protein precursors.
  • This initial binding is a prerequisite for subsequent import and processing, which requires inner membrane energization.
  • The outer membrane receptor is likely a proteinaceous component involved in initiating mitochondrial protein import.

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