C OMMUNICATION Projection Structure of yidC: A Conserved Mediator of Membrane Protein Assembly Mirko Lotz, Winfried Haase, Werner Kühlbrandt and Ian Collinson⁎ Max Planck Institute of Biophysics, Max-von-Laue- Strasse 3, D-60438 Frankfurt am Main, Germany Received 24 July 2007; received in revised form 13 October 2007; accepted 17 October 2007 Available online 12 November 2007 Bacteria, mitochondria and chloroplasts harbour factors that facilitate the insertion, folding and assembly of membrane proteins. In Escherichia coli, yidC is required for membrane insertion, acting in both a Sec-dependent and a Sec-independent manner. There is an expanding volume of biochemical work on its role in this process, but none so far on its structure. We present the first of this class of membrane proteins determined by electron cryomicroscopy in the near-nativelike state of the membrane. yidC forms dimers in the membrane and each monomer has an area of low density that may be part of the path transmembrane segments follow during their insertion. Upon consideration of the structures of yidC and SecYEG, we speculate on the nature of the interfaces that facilitate the alternative pathways (Sec-dependent and -independent) of membrane protein insertion. © 2007 Elsevier Ltd. All rights reserved. Edited by W. Baumeister Keywords: protein translocation; membrane insertion; yidC; electron microscopy; projection map Membrane proteins require the assistance of several protein factors for their correct localisation, insertion and folding. They carry sequences (trans- membrane (TM) domains or signal sequences) that target them to specific membranes where they are recognised by protein assemblies responsible for their translocation. This reaction is best understood in a ubiquitous process that occurs in the endoplas- mic reticulum (ER) of eukaryotes and the plasma membrane of bacteria and archaea. The respective Sec61 and SecY complexes form gated protein channels in the membrane capable of transferring proteins either across the membrane or into it. This can be achieved by either post- or cotranslational pathways. In bacteria, the posttranslational translocation route relies on a chaperone SecB and an ATPase SecA, 1 while the cotranslational route utilises the signal recognition particle (SRP) and its receptor. 2 The pathways converge on the translocon and discriminate the substrates according to their signal sequences and hydrophobicity; membrane proteins tend to be directed to the SRP pathway, whereas secreted proteins require the SecB–SecA route. 3–5 Protein translocation occurs through one mono- mer of the two found in the active dimeric SecYEG complex. 6,7 Two crystallographic studies have described the structure of the protein channel. One of them visualised the Escherichia coli membrane- bound dimer 8 and the other a detergent-solubilised monomer from M. jannaschii in atomic detail. 9 The structures identify a lateral gate adjacent to the lipid phase for the entry of TM segments into the bilayer. 8–10 Several E. coli membrane proteins have been shown to require a protein called yidC for their in- sertion. It contains six predicted TM segments with a large 320-amino-acid domain exposed to the peri- plasm; it assists membrane protein insertion either with or without the help of SecYEG. 11–18 Both Sec- dependent and Sec-independent pathways seem to proceed cotranslationally and some studies (but not all) report a requirement for SRP. 15,16, 18–22 *Corresponding author. E-mail address: ian.collinson@bristol.ac.uk. Present address: I. Collinson, Department of Biochemistry, University of Bristol, Bristol BS8 1TD, UK. Abbreviations used: SRP, signal recognition particle; 2-D, two-dimensional; ER, endoplasmic reticulum; TM, trans-membrane. doi:10.1016/j.jmb.2007.10.089 J. Mol. Biol. (2008) 375, 901–907 Available online at www.sciencedirect.com 0022-2836/$ - see front matter © 2007 Elsevier Ltd. All rights reserved.