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  • The EMBO Journal (2001) 20, 2462 - 2471
  • doi:10.1093/emboj/20.10.2462

Projection structure and oligomeric properties of a bacterial core protein translocase

Ian Collinson1, Cécile Breyton2, Franck Duong3, Christos Tziatzios4, Dieter Schubert4, Eran Or1, Tom Rapoport1 and Werner Kühlbrandt2

  1. Department of Cell Biology, Harvard Medical School/HHMI, Boston MA 02115, USA
  2. Department of Structural Biology, Max-Planck-Insitut für Biophysik, Frankfurt am Main 60596, Germany
  3. Laboratoire Transports et Signalisation Cellulaires, Université de Paris XI, Orsay, France
  4. Institut für Biophysik, JWG-Universität, Frankfurt am Main, Germany

Correspondence to:

Ian Collinson, E-mail: ian_collinson@hms.harvard.edu

Received 17 November 2000; Accepted 23 March 2001; Revised 9 March 2001

The major route for protein export or membrane integration in bacteria occurs via the Sec-dependent transport apparatus. The core complex in the inner membrane, consisting of SecYEG, forms a protein-conducting channel, while the ATPase SecA drives translocation of substrate across the membrane. The SecYEG complex from Escherichia coli was overexpressed, purified and crystallized in two dimensions. A 9 Å projection structure was calculated using electron cryo-microscopy. The structure exhibits P121 symmetry, having two asymmetric units inverted with respect to one another in the unit cell. The map shows elements of secondary structure that appear to be transmembrane helices. The crystallized form of SecYEG is too small to comprise the translocation channel and does not contain a large pore seen in other studies. In detergent solution, the SecYEG complex displays an equilibrium between monomeric and tetrameric forms. Our results therefore indicate that, unlike other known channels, the SecYEG complex can exist as both an assembled channel and an unassembled smaller unit, suggesting that transitions between the two states occur during a functional cycle.

  • Keywords:

    • electron crystallography,
    • oligomerization,
    • SecYEG,
    • translocation,
    • two-dimensional crystallization