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Vol. 17, Issue 12, 5063-5074, December 2006

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The Structure of Escherichia coli Signal Recognition Particle Revealed by Scanning Transmission Electron Microscopy

Iain L. Mainprize^*, Daniel R. Beniac, Elena Falkovskaia, Robert M. Cleverley, Lila M. Gierasch, F. Peter Ottensmeyer^||, and David W. Andrews^*

*Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton L8N 3Z5, Canada; National Microbiology Laboratory, Canadian Science Centre for Human and Animal Health, Winnipeg R3E 3R2, Canada; Departments of Biochemistry and Molecular Biology and Chemistry, University of Massachusetts, Amherst, MA 01003; Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, United Kingdom; and ^||Ontario Cancer Institute and Department of Medical Biophysics, University of Toronto, Toronto M5G 2M9, Canada

Submitted May 4, 2006; Revised August 30, 2006; Accepted September 7, 2006
Monitoring Editor: Peter Walter

Structural studies on various domains of the ribonucleoprotein signal recognition particle (SRP) have not converged on a single complete structure of bacterial SRP consistent with the biochemistry of the particle. We obtained a three-dimensional structure for Escherichia coli SRP by cryoscanning transmission electron microscopy and mapped the internal RNA by electron spectroscopic imaging. Crystallographic data were fit into the SRP reconstruction, and although the resulting model differed from previous models, they could be rationalized by movement through an interdomain linker of Ffh, the protein component of SRP. Fluorescence resonance energy transfer experiments determined interdomain distances that were consistent with our model of SRP. Docking our model onto the bacterial ribosome suggests a mechanism for signal recognition involving interdomain movement of Ffh into and out of the nascent chain exit site and suggests how SRP could interact and/or compete with the ribosome-bound chaperone, trigger factor, for a nascent chain during translation.

This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E06-05-0384) on September 20, 2006.

Address correspondence to: David W. Andrews (andrewsd{at}mcmaster.ca)

Abbreviations used: 3-D, three-dimensional; CPM, 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin, ; FRET, fluorescence resonance energy transfer; IQAD, iterative quaternion-based angular determination; SRP, signal recognition particle; STEM, scanning transmission electron microscopy; TF, trigger factor.

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