Asymmetric binding between SecA and SecB two symmetric proteins: implications for function in export

J Mol Biol. 2005 Apr 29;348(2):479-89. doi: 10.1016/j.jmb.2005.02.036.


SecB, a small tetrameric chaperone in Escherichia coli, facilitates export of precursor polypeptides from the cytoplasm to the periplasmic space. During this process, SecB displays two modes of binding. As a chaperone, it binds promiscuously to precursors to maintain them in a non-native conformation. SecB also demonstrates specific recognition of, and binding to, SecA. SecB with the precursor tightly bound enters an export-active complex with SecA and must pass the ligand to SecA at the translocon in the membrane. Here we use variants of SecA and SecB to further probe these interactions. We show that, unexpectedly, the binding between the two symmetric molecules is asymmetric and that the C-terminal alpha-helices of SecB bind in the interfacial region of the SecA dimer. We suggest that disruption of this interface by SecB facilitates conformational changes of SecA that are crucial to the transfer of the precursor from SecB to SecA.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adenosine Triphosphatases / chemistry*
  • Adenosine Triphosphatases / metabolism*
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / metabolism*
  • Binding Sites
  • Dimerization
  • Escherichia coli / chemistry*
  • Escherichia coli / genetics
  • Escherichia coli / metabolism*
  • Ligands
  • Membrane Transport Proteins / chemistry*
  • Membrane Transport Proteins / metabolism*
  • Models, Molecular
  • Protein Binding
  • Protein Structure, Tertiary
  • Protein Subunits / chemistry
  • Protein Subunits / metabolism
  • Protein Transport
  • SEC Translocation Channels
  • SecA Proteins


  • Bacterial Proteins
  • Ligands
  • Membrane Transport Proteins
  • Protein Subunits
  • SEC Translocation Channels
  • SecB protein, Bacteria
  • Adenosine Triphosphatases
  • SecA Proteins