The prokaryotic signal recognition particle (SRP) targets membrane proteins into the inner membrane1,2,3,4. It binds translating ribosomes and screens the emerging nascent chain for a hydrophobic signal sequence, such as the transmembrane helix of inner membrane proteins. If such a sequence emerges, the SRP binds tightly, allowing the SRP receptor to lock on. This assembly delivers the ribosome-nascent chain complex to the protein translocation machinery in the membrane. Using cryo-electron microscopy and single-particle reconstruction, we obtained a 16 Å structure of the Escherichia coli SRP in complex with a translating E. coli ribosome containing a nascent chain with a transmembrane helix anchor. We also obtained structural information on the SRP bound to an empty E. coli ribosome. The latter might share characteristics with a scanning SRP complex, whereas the former represents the next step: the targeting complex ready for receptor binding. High-resolution structures of the bacterial ribosome and of the bacterial SRP components are available, and their fitting explains our electron microscopic density. The structures reveal the regions that are involved in complex formation, provide insight into the conformation of the SRP on the ribosome and indicate the conformational changes that accompany high-affinity SRP binding to ribosome nascent chain complexes upon recognition of the signal sequence.
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The authors would like to thank all members of the Ban laboratory for discussions and help with programs, and C. Frick, L. Verschragen (deceased) and J. Onderwater for technical assistance. W. Wintermeyer provided pET24a_Ffh. T. Shaikh and J. Frank are acknowledged for a script for supervised classification. We thank the Electron Microscopy Center Zürich (EMEZ) for support. C.S. was supported by a postdoctoral fellowship from the Ernst Schering Research Foundation. R.I.K. was supported by a VENI grant from the Netherlands Organisation for Scientific Research (NWO). This work was supported by the Swiss National Science Foundation (SNSF), the NCCR Structural Biology program of the SNSF, and a Young Investigator grant from the Human Frontier Science Program (to N.B.).
This file contains the Supplementary Results and Supplementary Discussion, Supplementary Methods, Supplementary Figures and Legends 1–4, Supplementary Table 1 and additional references.
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Functional characterization of Vibrio cholerae O1 WbeW enzyme responsible for initial reaction in O antigen biosynthesis
Biotechnology and Bioprocess Engineering (2015)