Ribosome Binding of a Single Copy of the SecY Complex: Implications for Protein Translocation
The SecY complex associates with the ribosome to form a protein translocation channel in the bacterial plasma membrane. We have used cryo-electron microscopy and quantitative mass spectrometry to show that a nontranslating E. coli ribosome binds to a single SecY complex. The crystal structure of an archaeal SecY complex was then docked into the electron density maps. In the resulting model, two cytoplasmic loops of SecY extend into the exit tunnel near proteins L23, L29, and L24. The loop between transmembrane helices 8 and 9 interacts with helices H59 and H50 in the large subunit RNA, while the 6/7 loop interacts with H7. We also show that point mutations of basic residues within either loop abolish ribosome binding. We suggest that SecY binds to this primary site on the ribosome and subsequently captures and translocates the nascent chain.
© 2007 Elsevier. Under an Elsevier user license. Received 26 April 2007, Revised 3 August 2007, Accepted 4 October 2007, Available online 27 December 2007. We thank J. Rush (Cell Signaling Technology, Inc.) for synthesis of the AQUA peptides. J.S. was supported by the Boehringer Ingelheim Fonds, and W.M.C. was supported by a Damon Runyon Cancer Fellowship. Work in the T.A.R. and C.W.A. laboratories was supported by National Institutes of Health grants. T.A.R. is an investigator of the Howard Hughes Medical Institute. Accession Numbers: The modeled ribosome-SecY complexes have been deposited in the Protein Data Bank under ID codes 3BO0 and 3BO1, and the EM map has been deposited in the Electron Microscopy Database (EMD-5907).
Supplemental Material - 1-s2.0-S1097276507008258-mmc1.pdf