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Mmm1p spans both the outer and inner mitochondrial membranes and contains distinct domains for targeting and foci

Noriko Kondo-Okamoto1, Janet M Shaw, Koji Okamoto

  • 1Department of Biology, University of Utah, Salt Lake City, Utah 84112, USA. kokamoto@bioscience.utah.edu

The Journal of Biological Chemistry
|September 16, 2003
PubMed

Insights

Mitochondrial protein Mmm1p spans both mitochondrial membranes. Its matrix-exposed N-terminus is not essential for mitochondrial DNA maintenance, but its transmembrane domain is crucial for tubular mitochondrial morphology.

Area of Science:

  • Cell Biology
  • Mitochondrial Biology
  • Molecular Genetics

Background:

  • Mmm1p is an integral mitochondrial protein essential for mitochondrial morphology and mitochondrial DNA (mtDNA) maintenance in Saccharomyces cerevisiae.
  • Mmm1p localizes to foci near mtDNA nucleoids, suggesting a role in mtDNA organization, replication, or segregation.
  • Previous studies indicated Mmm1p crosses the outer mitochondrial membrane with its C-terminus exposed to the cytoplasm.

Purpose of the Study:

  • To determine the complete topology of Mmm1p, specifically the location of its N-terminus.
  • To investigate the role of the N-terminal region in Mmm1p function, including mitochondrial morphology and mtDNA maintenance.
  • To identify domains of Mmm1p critical for its localization and function.

Main Methods:

  • Immunofluorescence microscopy to visualize Mmm1p localization.
  • Biochemical assays to determine protein levels and topology.
  • Functional assays assessing mitochondrial morphology and mtDNA retention.
  • Complementation studies using homologous genes from other species.

Main Results:

  • Mmm1p spans both the outer and inner mitochondrial membranes, with the N-terminus exposed to the mitochondrial matrix.
  • Deletion of the N-terminal extension reduced Mmm1p levels but did not impair mitochondrial morphology or mtDNA maintenance.
  • The transmembrane segment and C-terminal domain are essential for Mmm1p foci formation and mitochondrial targeting, respectively.
  • Expression of a homologous gene from Neurospora crassa lacking a long N-terminal extension could functionally complement the yeast Mmm1p deletion.

Conclusions:

  • The matrix-exposed N-terminal region of Mmm1p is dispensable for mtDNA nucleoid maintenance.
  • The double membrane-spanning topology of Mmm1p at mitochondrial contact sites is critical for establishing tubular mitochondrial morphology.
  • Specific domains of Mmm1p mediate distinct functions, including targeting, foci formation, and membrane topology.

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