Cryo-Electron Microscopic Structure of SecA Bound to the 70S Ribosome [Protein Structure and Folding]

January 17th, 2014 by Singh, R., Kraft, C., Jaiswal, R., Se&jnodot;wal, K., Kasaragod, V. B., Kuper, J., Berger, J., Mielke, T., Luirink, J., Bhushan, S.

SecA is an ATP-dependent molecular motor pumping secretory and outer membrane proteins across the cytoplasmic membrane in bacteria. SecA associates with the protein conducting channel, the heterotrimeric SecYEG complex, in a so-called post-translational manner. A recent study further showed binding of a monomeric state of SecA to the ribosome. However, the true oligomeric state of SecA remains controversial, as SecA can also form functional dimers, and high-resolution crystal structures exist for both the monomer and the dimer. Here we present the cryo-electron microscopy structures of E. coli SecA bound to the ribosome. We show that not only a monomeric SecA binds to the ribosome, but also two copies of SecA can be observed which form an elongated dimer. Two copies of SecA completely surround the tunnel exit providing a unique environment to the nascent polypeptides emerging form the ribosome. We identified the N-terminal helix of SecA required for a stable association with the ribosome. The structures indicate a possible function of the dimeric form of SecA at the ribosome.