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Translocation of proteins across membranes is an ancient process that occurs even in bacteria and archaebacteria. In fact, the components of the translocation machinery are still conserved between prokaryotes and eukaryotes.
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Molecular code for transmembrane-helix recognition by the Sec61 translocon.

Tara Hessa1, Nadja M Meindl-Beinker, Andreas Bernsel

  • 1Center for Biomembrane Research, Department of Biochemistry and Biophysics, Stockholm University, SE-106 91 Stockholm, Sweden.

Nature
|December 14, 2007
PubMed
Summary

This study quantifies how amino acid sequences and properties influence the insertion of transmembrane helices into the endoplasmic reticulum membrane via the Sec61 translocon, revealing simple sequence characteristics related to lipid bilayer physics.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Protein Structure

Background:

  • Integral membrane proteins feature transmembrane alpha-helices essential for cellular function.
  • The Sec61 translocon facilitates co-translational insertion of these helices into the endoplasmic reticulum membrane.
  • A quantitative understanding linking amino acid sequence to membrane insertion efficiency is currently lacking.

Purpose of the Study:

  • To quantitatively analyze the contribution of individual amino acids to transmembrane helix insertion efficiency.
  • To investigate the impact of transmembrane segment length and flanking amino acids on membrane insertion.
  • To develop a predictive model for translocon-mediated transmembrane helix assembly.

Main Methods:

  • In vitro translation of a model protein using dog pancreas rough microsomes.
  • Systematic design and analysis of numerous hydrophobic segments.
  • Quantitative assessment of membrane insertion efficiency based on amino acid sequence.

Main Results:

  • Detailed position-dependent contribution of all 20 amino acids to membrane insertion efficiency was determined.
  • The length of transmembrane segments and flanking amino acids were shown to significantly affect insertion.
  • Critical sequence characteristics governing insertion were found to mirror lipid bilayer physical properties.

Conclusions:

  • Transmembrane helix insertion by the Sec61 translocon is governed by predictable sequence-based rules.
  • The physical properties of the lipid bilayer are key determinants of transmembrane helix assembly.
  • This work provides a quantitative framework for understanding membrane protein biogenesis.