title: Signal recognition particle-ribosome binding is sensitive to nascent chain length.
abstract: The signal recognition particle (SRP) directs ribosome-nascent chain complexes (RNCs) displaying signal sequences to protein translocation channels in the plasma membrane of prokaryotes and endoplasmic reticulum of eukaryotes. It was initially proposed that SRP binds the signal sequence when it emerges from an RNC and that successful binding becomes impaired as translation extends the nascent chain, moving the signal sequence away from SRP on the ribosomal surface. Later studies drew this simple model into question, proposing that SRP binding is unaffected by nascent chain length. Here, we reinvestigate this issue using two novel and independent fluorescence resonance energy transfer assays. We show that the arrival and dissociation rates of SRP binding to RNCs vary according to nascent chain length, resulting in the highest affinity shortly after a functional signal sequence emerges from the ribosome. Moreover, we show that SRP binds RNCs in multiple and interconverting conformations, and that conversely, RNCs exist in two conformations distinguished by SRP interaction kinetics.
authors.affiliation: From the Howard Hughes Medical Institute, Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94158.
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Noriega
authors.affiliation: the Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, and the Department of Applied Physics, Stanford University, Stanford, California 94305.
authors.forename: Albert
authors.initials: A
authors.lastname: Tsai
authors.affiliation: From the Howard Hughes Medical Institute, Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94158.
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Elvekrog
authors.affiliation: the Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, and.
authors.forename: Alexey
authors.initials: A
authors.lastname: Petrov
authors.affiliation: From the Howard Hughes Medical Institute, Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94158.
authors.forename: Saskia B
authors.initials: SB
authors.lastname: Neher
authors.affiliation: the Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, and the Department of Applied Physics, Stanford University, Stanford, California 94305.
authors.forename: Jin
authors.initials: J
authors.lastname: Chen
authors.affiliation: From the Howard Hughes Medical Institute, Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94158.
authors.forename: Niels
authors.initials: N
authors.lastname: Bradshaw
authors.affiliation: the Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, and puglisi@stanford.edu.
authors.forename: Joseph D
authors.initials: JD
authors.lastname: Puglisi
authors.affiliation: From the Howard Hughes Medical Institute, Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94158, peter@walterlab.ucsf.edu.
authors.forename: Peter
authors.initials: P
authors.lastname: Walter
mesh_terms: D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D014176:Protein Biosynthesis / Q000502:physiology*; D012270:Ribosomes / Q000235:genetics / Q000378:metabolism*; D018271:Signal Recognition Particle / Q000235:genetics / Q000378:metabolism*
chemical_list: D018271:Signal Recognition Particle
keywords: Fluorescence Resonance Energy Transfer (FRET); Protein Targeting; Ribosome Function; Signal Recognition Particle (SRP); Single-Molecule Biophysics