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The peripheral membrane subunits of the SAM complex function codependently in mitochondrial outer membrane biogenesis

Nickie C Chan1, Trevor Lithgow

  • 1Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Parkville, Victoria 3010, Australia.

Molecular Biology of the Cell
|November 6, 2007
PubMed

Insights

The sorting and assembly machinery (SAM) complex

Area of Science:

  • Mitochondrial biology
  • Protein translocation
  • Molecular machinery

Background:

  • The sorting and assembly machinery (SAM) complex is crucial for inserting beta-barrel proteins into the mitochondrial outer membrane.
  • The eukaryotic SAM complex has unique peripheral subunits, Sam35 and Sam37, whose functions in protein assembly remain largely unknown.
  • These subunits are located on the cytosolic face of the SAM complex.

Purpose of the Study:

  • To elucidate the distinct roles of Sam35 and Sam37 within the mitochondrial SAM complex.
  • To understand how these subunits contribute to the assembly of beta-barrel proteins.

Main Methods:

  • Screening a library of sam35 mutants to identify suppressors.
  • Utilizing specific sam35 mutants (sam35-409 and sam35-424) to differentiate subunit functions.
  • Analyzing substrate binding and release by the SAM complex.

Main Results:

  • Overexpression of SAM37 suppressed 13 distinct sam35 mutant alleles.
  • Sam35 is essential for the SAM complex's ability to bind outer membrane substrate proteins, with its destabilization inhibiting Sam50-mediated binding.
  • Sam37 functions downstream of Sam35, potentially aiding in the release of substrates from the SAM complex.

Conclusions:

  • Sam35 and Sam37 play sequential and distinct roles in the mitochondrial SAM complex.
  • Sam35 facilitates substrate recognition and binding, while Sam37 assists in substrate release.
  • These findings provide insights into the specialized mechanisms of mitochondrial protein import compared to bacterial systems.

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