期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：13
页码：3943-3948
DOI：10.1073/pnas.1424453112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe universally conserved signal recognition particle (SRP) and its receptor (FtsY) deliver [~]30% of the proteome to the proper cellular membrane. To ensure proper timing and fidelity of targeting, SRP and FtsY adopt multiple conformations in a GTP-dependent manner. We solved the cryo-EM structure of the SRP-FtsY complex with a GTP analogue in the presence of a ribosome translating a signal sequence (the closed state) at 5.7 [IMG]f1.gif" ALT="A" BORDER="0"> resolution. We describe the structural basis of ribosome and signal sequence binding by the SRP M domain. We demonstrate that in the closed state the SRP-FtsY GTPase domains are moving away from the ribosomal tunnel exit, allowing for translocon-ribosome interactions to accomplish cotranslational targeting. The signal recognition particle (SRP)-dependent pathway is essential for correct targeting of proteins to the membrane and subsequent insertion in the membrane or secretion. In Escherichia coli, the SRP and its receptor FtsY bind to ribosome-nascent chain complexes with signal sequences and undergo a series of distinct conformational changes, which ensures accurate timing and fidelity of protein targeting. Initial recruitment of the SRP receptor FtsY to the SRP-RNC complex results in GTP-independent binding of the SRP-FtsY GTPases at the SRP RNA tetraloop. In the presence of GTP, a closed state is adopted by the SRP-FtsY complex. The cryo-EM structure of the closed state reveals an ordered SRP RNA and SRP M domain with a signal sequence-bound. Van der Waals interactions between the finger loop and ribosomal protein L24 lead to a constricted signal sequence-binding pocket possibly preventing premature release of the signal sequence. Conserved M-domain residues contact ribosomal RNA helices 24 and 59. The SRP-FtsY GTPases are detached from the RNA tetraloop and flexible, thus liberating the ribosomal exit site for binding of the translocation machinery.
关键词：protein targeting ; signal recognition particle ; signal sequence ; ribosome ; single-particle electron cryomicroscopy
